Prof. Dr.-Ing. Werner Theisen

Institute for Materials
Ruhr-Universität Bochum

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  • Evaluation of antithrombogenic pHPC on CoCr substrates for biomedical applications
    Bannewitz, C. and Lenz-Habijan, T. and Lentz, J. and Peters, M. and Trösken, V. and Siebert, S. and Weber, S. and Theisen, W. and Henkes, H. and Monstadt, H.
    Coatings 11 (2021)
    Bare metal endovascular implants pose a significant risk of causing thrombogenic complications. Antithrombogenic surface modifications, such as phenox’s “Hydrophilic Polymer Coating” (pHPC), which was originally developed for NiTi implants, decrease the thrombogenicity of metal surfaces. In this study, the transferability of pHPC onto biomedical CoCr-based alloys is examined. Coated surfaces were characterized via contact-angle measurement and atomic force microscopy. The equivalence of the antithrombogenic effect in contact with whole human blood was demonstrated in vitro for CoCr plates compared to NiTi plates on a platform shaker and for braided devices in a Chandler loop. Platelet adhesion was assessed via scanning electron microscopy and fluorescence microscopy. The coating efficiency of pHPC on CoCr plates was confirmed by a reduction of the contact angle from 84.4° ± 5.1° to 36.2° - 5.2°. The surface roughness was not affected by the application of pHPC. Platelet adhesion was significantly reduced on pHPC-coated specimens. The platelet covered area was reduced by 85% for coated CoCr plates compared to uncoated samples. Uncoated braided devices were completely covered by platelets, while on the pHPC-coated samples, very few platelets were visible. In conclusion, the antithrombogenic effect of pHPC coating can be successfully applied on CoCr plates as well as stent-like CoCr braids. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/coatings11010093
  • Influence of Nb as Microalloying Element on the Recovery and Recrystallization of Fe–25Mn–12Cr–C–N Twinning-Induced Plasticity Steels
    Suárez Sierra, A. and Rodríguez Baracaldo, R. and Mujica Roncery, L. and Egels, G. and Theisen, W.
    Steel Research International (2021)
    The influence of Nb on the microstructure during annealing at 950, 1000, and 1100 °C is analyzed in two types of twinning-induced plasticity (TWIP) steels, Fe–25Mn–12Cr–C–N (TWIP-0) and Fe–25Mn–12Cr–C–N–Nb (TWIP-Nb). The addition of Nb as a microalloying element affects various phenomena taking place during annealing, namely, recrystallization, grain coarsening, and recovery processes. Microstructural characterization is conducted via light microscopy, scanning electron microscopy, and electron back scattering diffraction (EBSD). Recovery takes place after annealing at 950 °C, where remaining deformation and grain nucleation can be seen. Microstructural analyses indicate that the location of the recrystallization nuclei in the recovered structure is associated with the local chemical segregation of Mn and Cr, which leads to differences in the driving force for the martensitic transformation at microscale, and therefore local deformation mechanisms. The presence of Nb as a microalloying element decelerates recovery and recrystallization kinetics. At 1100 °C/10 min, both steels exhibit complete recrystallization; moreover, abnormal grain growth starts. © 2021 Wiley-VCH GmbH
    view abstract10.1002/srin.202000417
  • Path to single-crystalline repair and manufacture of Ni-based superalloy using directional annealing
    Kalfhaus, T. and Schaar, H. and Thaler, F. and Ruttert, B. and Sebold, D. and Frenzel, J. and Steinbach, I. and Theisen, W. and Guillon, O. and Clyne, T.W. and Vassen, R.
    Surface and Coatings Technology 405 (2021)
    Advanced methods for the repair of single-crystalline (SX) Ni-based superalloys are of special interest for the gas turbine industry. Polycrystalline repair approaches show promising results, while the repair of SX materials is still challenging. Directional annealing experiments resulted in large columnar grains by imposing thermal gradients at the abnormal grain growth temperature of a specific Ni-based superalloy. A numerical model of the Bridgman process is applied to provide an insight into the temperature evolution during zone annealing of the Vacuum-Plasma-Spray (VPS) repair coatings with the aim of promoting grain growth from the SX substrate. The results presented here suggest that this is a promising approach to repair or manufacture SX turbine blades. © 2020 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2020.126494
  • Subcritical crack growth in hard alloys under cyclic loading
    Brackmann, L. and Röttger, A. and Weber, S. and Theisen, W.
    Fatigue and Fracture of Engineering Materials and Structures 44 (2021)
    In this work, the subcritical crack growth in Fe-, Ni- and Co-base hard alloys was investigated. Specimens were cyclically loaded in the pressure threshold range until a ring crack resulted as a failure criterion. Crack propagation along with the individual microstructural constituents and the associated resistance of the individual materials to crack propagation was investigated by scanning electron microscopy and by the methods adapted to it. For the Ni-base alloys, the formation of a closed ring fracture occurred after the lowest load cycle number, followed by the Co- and Fe-base alloys. Almost no crack deflection by the hard phases was detected in the Ni-base alloys. The higher number of loading cycles to produce a closed crack ring in the Fe-base alloys is attributed to the pronounced crack deflection by the hard phases and to the higher matrix strength. Besides, phase transformations were registered in front of the crack tip of the Co- and the partially austenitic Fe-base alloy. This phase transformation counteracts crack formation in the case of the Fe-base matrix but promotes crack propagation in the Co-base alloy. © 2020 The Authors. Fatigue & Fracture of Engineering Materials & Structures published by John Wiley & Sons Ltd
    view abstract10.1111/ffe.13363
  • Surface metal matrix nano-composite of magnesium/hydroxyapatite produced by stir-centrifugal casting
    Khalili, V. and Moslemi, S. and Ruttert, B. and Frenzel, J. and Theisen, W. and Eggeler, G.
    Surface and Coatings Technology 406 (2021)
    The present study aims to investigate a liquid state method of stir and centrifugal casting as an in-situ and cost-attractive processing technology for the production of magnesium/hydroxyapatite surface metal matrix nano-composites (Mg/HA surface metal matrix nano-composite). The main contribution of this study is to design the best condition for achieving a uniform Mg/HA surface nano-composite as a potential bone implant. It was shown how casting parameters and the distribution of hydroxyapatite affect mechanical properties of nano-composites measured using nano-indentation, nano-scratch, and compression tests. Response surface method in Design Expert software was used to predict the best model and the optimum condition of casting based on the experimentally measured data. The surface metal matrix nano-composites, consisting of a magnesium matrix with different amounts of nano-sized hydroxyapatite and silicon-doped hydroxyapatite (0.75-3 wt%) particles, were prepared. Hot isostatic pressing was used to homogenize the nano-composites in terms of particle distribution and to reduce porosity. It was shown that the weight percent of hydroxyapatite reinforcement is the parameter which is best suited to tailor targeted strength values. The target values of maximum compression strength (187 MPa) and elastic modulus (33 GPa) were achieved with a combination of the following parameters: 1.83 wt% hydroxyapatite, 800 rpm mold rotational speed, and a propeller rotational time of 6.3 min. A specimen prepared under these conditions had a homogeneous distribution of nano-hydroxyapatite in magnesium metal matrix after hot isostatic pressing at 450 °C and 100 MPa for a holding time of 120 min. It indicated the best mechanical resistance in terms of hardness and material loss during the nano-scratch testing. Moreover, the XRD results show that there is no considerable chemical reaction between the reinforcement particles of n-HA and Mg metallic matrix during casting at 700 °C and thermo-mechanical treatment of HIP at 450 °C. © 2020
    view abstract10.1016/j.surfcoat.2020.126654
  • A Computational Approach to the Microstructural Design of High-Speed Steels
    Egels, G. and Wulbieter, N. and Weber, S. and Theisen, W.
    Steel Research International 91 (2020)
    Increasing requirements concerning the operational conditions and durability of tools create a demand for the optimization of tool steels. High-speed steels (HSSs), for example, contain high amounts of carbides embedded in a secondary hardenable martensitic matrix. The wear behavior and the mechanical properties of HSS can be optimized for a certain application by adjusting the type and amount of carbides, as well as their compositions and the composition of the matrix. Computational thermodynamics based on the calculation of phase diagrams method allow the estimation of arising phases as well as phase compositions during the solidification or the heat treatment of a steel. However, in complex alloy systems, for example, HSS, the relationships between the content of alloying elements and the stability and the composition of phases can be complicated and nonlinear. Therefore, it can be difficult to find alloy compositions that are suitable to achieve a desired microstructure with iterative calculations. To handle this difficulty, a computational tool is developed, which determines compositions to obtain predefined HSS microstructures. The computational tool is based on a neural network that was previously trained with a thermodynamically calculated database. The efficiency of this approach is experimentally verified by producing and investigating laboratory melts of different HSS. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/srin.201900455
  • An approach to use sub-surface characteristics for the prediction of process forces during cutting operations
    Bergmann, J.A. and Kimm, J. and Theisen, W. and Wiederkehr, P.
    Procedia CIRP 88 (2020)
    During machining operations, complex engagement situations between the tool and workpiece lead to varying amplitudes and directions of cutting forces. Correlations between material properties, process parameters and resulting forces have to be analyzed to ensure the predictability of machining processes. To model the material removal, a detailed analysis of the material behavior during the engagement is needed. In this work, a geometric physically-based simulation system is extended to take the material behavior into account for improving the prediction accuracy of process forces. A detailed analysis of the low-alloyed steel 42CrMo4 and validation experiments are presented. © 2020 The Authors.
    view abstract10.1016/j.procir.2020.05.048
  • An Integrated HIP Heat-Treatment of a Single Crystal Ni-Base Superalloy
    Ruttert, B. and Lopez-Galilea, I. and Theisen, W.
    Minerals, Metals and Materials Series (2020)
    The heat-treatment of a second generation single crystal Ni-base superalloy was implemented in a hot isostatic press providing fast quenching rates. Thus, it is possible to homogenize chemical heterogeneities, close porosity, and to set a fine and uniform γ/γ′-microstructure via fast quenching and subsequent aging in one processing step. The microstructural evolution in dependence of parameters such as temperature, pressure, and quenching is investigated on different length scales using diverse characterization methods. A virtually defect-free microstructure is the outcome of this unique integrated supersolvus HIP heat-treatment. © 2020, The Minerals, Metals & Materials Society.
    view abstract10.1007/978-3-030-51834-9_38
  • Comprehensive investigation of the microstructure-property relationship of differently manufactured Co–Cr–C alloys at room and elevated temperature
    Krell, J. and Röttger, A. and Theisen, W.
    Wear 444-445 (2020)
    The purpose of this study was to investigate the influence of the microstructure on sliding wear and hardness of four different Co–Cr–C alloys at room and elevated temperature. Different microstructures were produced by applying three different processes. The hardness, hot hardness and wear loss at room temperature of these alloys correlate strongly with the carbide volume content. In sliding wear tests against an Al2O3 ball, abrasive wear occurs at room temperature. The size or geometric arrangement of the carbides or metal matrix plays a minor role at room temperature. At 600 °C the wear behaviour changes due to the softening matrix. In alloys with small free matrix path lengths, the highest wear rates occur due to micro-fatigue and micro-cracking. In hypoeutectic alloys with a high free matrix path length, the carbides lose their effectiveness due to the lack of support by the matrix. In these alloys, wear is dominated by the properties of the matrix. A hypereutectic casting alloy with large primary carbides shows the best wear results, as the carbides support themselves due to their size and retain their wear-reducing effect. © 2019 Elsevier B.V.
    view abstract10.1016/j.wear.2019.203138
  • Effect of γ′ precipitate size on hardness and creep properties of Ni-base single crystal superalloys: Experiment and simulation
    Ali, M.A. and López-Galilea, I. and Gao, S. and Ruttert, B. and Amin, W. and Shchyglo, O. and Hartmaier, A. and Theisen, W. and Steinbach, I.
    Materialia 12 (2020)
    The role and effect of γ′ precipitate size on the mechanical properties of Ni-base single crystal superalloy is investigated. The underlying mechanisms are analyzed on the one hand with the help of experiments including hardness and creep tests, and on the other hand with the help of two different simulation approaches by taking the typical γ/γ′ microstructure into account. Simulations, based on the crystal plasticity finite element method (CPFEM) are carried out for the hardness tests, whereas simulations, based on the crystal plasticity coupled phase-field method (CPPFM) are carried out for the creep tests. The hardness test simulation results show that the hardness of material varies inversely with the size of γ′ precipitates for a given γ′ phase volume fraction and it varies directly with the volume fraction of γ′ precipitates for a given precipitate size. These results are qualitatively consistent with the experimental observations. The creep simulation results show that the refinement of γ′ precipitates with a certain volume fraction of precipitates leads to an improvement of creep resistance by delaying the plastic activity in the material. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.mtla.2020.100692
  • Electro-discharge sintering of nanocrystalline NdFeB magnets: process parameters, microstructure, and the resulting magnetic properties
    Leich, L. and Röttger, A. and Kuchenbecker, R. and Theisen, W.
    Journal of Materials Science: Materials in Electronics 31 (2020)
    This study investigates the compaction of nanocrystalline NdFeB magnet powder by electro-discharge sintering (EDS). On this account, process parameters, microstructure, and the associated magnetic properties of the EDS-densified nanocrystalline NdFeB specimens were investigated by varying the discharge energy EEDS and compression load pEDS. Although optimized process parameters could be evaluated, three different microstructures (fully densified zone, insufficiently densified zone, and melted zone) are present in the EDS-compacted specimens. Thereby, volume fractions of these formed three different microstructures determine the resulting mechanical and magnetic properties of the specimens. For all specimens, the intrinsic coercivity Hc,J deteriorates with increasing discharge energy, as the generated Joule heat leads to microstructural changes (grain growth, dissolution of magnetic phases), which reduces the magnetic properties. The compression load has less influence on the coercivity Hc,J, as it only affects the initial resistance of the pre-compacted powder loose. The residual induction Br deteriorates with increasing the discharge energy due to microstructural changes. An increase in the compression load pEDS results in an increase in the specimens’ density and thus promotes the residual induction Br. © 2020, The Author(s).
    view abstract10.1007/s10854-020-04562-6
  • Gas atomization and laser additive manufacturing of nitrogen-alloyed martensitic stainless steel
    Boes, J. and Röttger, A. and Theisen, W. and Cui, C. and Uhlenwinkel, V. and Schulz, A. and Zoch, H.-W. and Stern, F. and Tenkamp, J. and Walther, F.
    Additive Manufacturing 34 (2020)
    Nitrogen as an alloying element can improve the corrosion resistance and the mechanical properties of stainless steels. Therefore, nitrogen-alloyed martensitic stainless steels, such as X30CrMoN151, have been developed in recent decades and conventional processing of this steel by casting or powder metallurgy is well understood. However, only very few attempts to process nitrogen-alloyed martensitically hardenable stainless steels containing more than 0.2 mass-% of carbon by laser powder bed fusion (L-PBF) have been reported so far. In this study, X30CrMoN15-1 steel powder has been produced from quasi nitrogen-free X30CrMo15-1 steel by gas atomization using N2 as the process gas to introduce nitrogen into the steel. The gas-atomized powder was characterized in terms of nitrogen content, particle size distribution, particle morphology, and flow properties. The powder was then processed by L-PBF under an N2 gas atmosphere, and microstructural investigations were performed on the L-PBF-built samples using scanning electron microscopy and X-ray computed tomography. Additionally, a first impression of the mechanical properties of the L-PBF-built steel in the as-built and quenched and tempered condition was obtained by means of fatigue tests. It was shown that a nitrogen content of 0.16 mass-% could be introduced into the steel during gas atomization. The resulting powder was successfully processed by means of L-PBF, and specimens with a high density were produced. During fatigue testing, a large amount of retained austenite in the as-built condition resulted in a greater damage tolerance of the specimens compared to the heat-treated condition. © 2020 Elsevier B.V.
    view abstract10.1016/j.addma.2020.101379
  • Influence of Hot Hardness and Microstructure of High-Alloyed Powder Metallurgical Tool Steels on Abrasive Wear Behavior at Elevated Temperatures
    Wulbieter, N. and Pöhl, F. and Theisen, W.
    Steel Research International 91 (2020)
    Herein, the abrasive wear behavior of different high-alloyed powder metallurgical (PM) tool steels is investigated at elevated temperatures (400–600 °C) in a dry-pot wear tester containing Al2O3 particles. To identify the influence of the microstructure, PM tool steels with different hot hardnesses, carbide types, and carbide volume contents are selected. Wear tracks are analyzed by scanning electron microscopy (SEM) to clarify wear mechanisms. The results show that there is no direct correlation between wear resistance and only one material property such as hot hardness, carbide content, or carbide type. More important seems to be the best possible compromise between a sufficient hot hardness of the metallic matrix and a high volume content of carbides that are harder than the attacking abrasive particles at the respective temperature. When the test temperatures surpass the tempering temperature of the investigated steels, there is a pronounced change in wear behavior due to the stronger embedding of abrasive particles into the wear surface. It is thus necessary to discuss the microstructural properties as a function of temperature, considering interactions with the abrasive particles. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/srin.201900461
  • Influence of powder nitriding on the mechanical behavior of laser-powder bed fusion processed tool steel X30CrMo7-2
    Stern, F. and Grabienski, F. and Walther, F. and Boes, J. and Röttger, A. and Theisen, W.
    Materialpruefung/Materials Testing 62 (2020)
    Additive manufacturing allows for the production of highly complex structures due to its layer-wise local melting of powder material. For this reason, this technique has a high potential for manufacturing extremely lightweight components potential. However, laser based additive manufacturing is still restricted due to the limited amount of processable alloys, especially Fe-based materials. A main object in current research is to expand the varieties for steel that may be used. Additionally, the modification and optimization of steel powder is seen as an interesting aspect for improving the material properties of additively manufactured parts. In this work, secondary hardenable martensitic tool steel X30CrMo7-2 is investigated, starting from the raw powder which is enriched with nitrogen by gas nitriding and subsequently characterized to ensure the usability of the modified powder for laser-powder bed fusion. In a next step, the raw and nitrided powder are used to generate cylindrical specimens to allow for further analysis of the microstructure and for a mechanical characterization of compression behavior. Moreover, a variety of heat treatments is carried out. The higher content of nitrogen leads to an increase in porosity. However, the addition of nitrogen causes an increase in hardness and in the compressive yield point, especially after heat treatment. After tempering, compressive yield stress is increased from 1,111 MPa to 1990 MPa, while for conventional material it is slightly reduced from 1,316 MPa to 1225 MPa. © Carl Hanser Verlag GmbH & Co. KG
    view abstract10.3139/120.111446
  • Influence of precipitation hardening on the high-temperature sliding wear resistance of an aluminium alloyed iron-nickel base alloy
    Krell, J. and Röttger, A. and Ziesing, U. and Theisen, W.
    Tribology International 148 (2020)
    This work investigates the effects of precipitation hardening on hot hardness and high-temperature sliding wear resistance of an iron-nickel base alloy. Three variants of a carbide rich alloy containing 0, 2 and 3 wt.-% aluminium were manufactured and aged for 24 h at 650, 700 and 750 °C. Hot hardness (20–800 °C) and sliding wear tests (600 °C) were conducted for each condition. The addition of aluminium has little effect on the type or volume content of the carbides. Ageing caused the precipitation of NiAl in the aluminium-containing alloys. The precipitation-hardened samples show higher hot hardness and better hot wear resistance. The lower wear loss can mainly be attributed to the improved support of the carbides by the precipitation strengthened matrix. © 2020
    view abstract10.1016/j.triboint.2020.106342
  • Laser additive manufacturing of hot work tool steel by means of a starting powder containing partly spherical pure elements and ferroalloys
    Taruttis, A. and Hardes, C. and Röttger, A. and Uhlenwinkel, V. and Chehreh, A.B. and Theisen, W. and Walther, F. and Zoch, H.W.
    Procedia CIRP 94 (2020)
    Until now, additive manufacturing of high-performance materials such as martensitic hardenable tool steels is rarely investigated. This work addresses the introduction of an alternative alloying strategy for hot work tool steel powder, provided for laser powder bed fusion (L-PBF). The focus is on the question whether a powder mixture of spherical iron powder mixed with mechanically crushed ferroalloy particles can be processed by L-PBF, instead of using cost-intensive pre-alloyed gas-atomized powder, and to investigate the material properties associated with it. The particle morphology, packing density and flowability of this L-PBF powder feedstock is compared to gas-atomized spherical pre-alloyed steel powder and the results are correlated to the defect density, the resulting microstructure and the chemical homogeneity. Finally the resulting surface hardness is compared to a conventionally casted material as a reference state. It shows that the L-PBF fabrication of high-dense parts by means of both starting powders is technically feasible. Even though the alternative alloying concept promotes local chemical inhomogeneities within the microstructure, the overall porosity and the appearance of micro cracks are reduced. © 2020 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.procir.2020.09.010
  • Microstructure and mechanical properties of 316L austenitic stainless steel processed by different SLM devices
    Röttger, A. and Boes, J. and Theisen, W. and Thiele, M. and Esen, C. and Edelmann, A. and Hellmann, R.
    International Journal of Advanced Manufacturing Technology 108 (2020)
    In this work, we examined the influence of different types of selective laser melting (SLM) devices on the microstructure and the associated material properties of austenitic 316L stainless steel. Specimens were built using powder from the same powder batch on four different SLM machines. For the specimen build-up, optimized parameter sets were used, as provided by the manufacturers for each individual SLM machine. The resulting microstructure was investigated by means of scanning electron microscopy, which revealed that the different samples possess similar microstructures. Differences between the microstructures were found in terms of porosity, which significantly influences the material properties. Additionally, the build-up direction of the specimens was found to have a strong influence on the mechanical properties. Thus, the defect density defines the material’s properties so that the ascertained characteristic values were used to determine a Weibull modulus for the corresponding values in dependence on the build-up direction. Based on these findings, characteristic averages of the mechanical properties were determined for the SLM-manufactured samples, which can subsequently be used as reference parameters for designing industrially manufactured components. © 2020, The Author(s).
    view abstract10.1007/s00170-020-05371-1
  • Microstructure and properties of high-strength C + N austenitic stainless steel processed by laser powder bed fusion
    Boes, J. and Röttger, A. and Theisen, W.
    Additive Manufacturing 32 (2020)
    In the developing field of laser powder bed fusion (L-PBF), austenitic stainless steels, such as AISI 316L, have gained great importance owing to their excellent processability. However, the moderate strength of these steels limits their applicability. This can be counteracted by the use of nitrogen as an alloying element to improve both strength and corrosion resistance. In this work, nitrogen-alloyed high-strength austenitic stainless steel X40MnCrMoN19-18-1 was processed by L-PBF, and the resulting microstructural and mechanical properties were investigated. The same material was also processed by hot isostatic pressing (HIP), which was used as a reference state. In the L-PBF process, argon and nitrogen were used as process gases to investigate the influence of process atmosphere on the microstructure and on changes in the chemical composition during processing. The results show a minor decrease in the nitrogen content of the steel after L-PBF, independently of the process gas, whereby argon resulted in a slightly higher specimen density. The microstructure after L-PBF processing contained small precipitates that could be removed by a short solution-annealing treatment. The tensile properties of the L-PBF-built steel are comparable to those of the steel produced by hot isostatic pressing in terms of ultimate tensile strength, but had lower elongation to fracture values. The ductility of the material was enhanced by solution annealing without significant impairment of the ultimate tensile strength. This work demonstrates that nitrogen-alloyed stainless steels can be processed by means of L-PBF and can extend the variety of appropriate steels towards applications with high requirements for the material strength and chemical resistance. © 2020 Elsevier B.V.
    view abstract10.1016/j.addma.2020.101081
  • Microstructures, Heat Treatment, and Properties of Boron-Alloyed Tool Steels
    Lentz, J. and Röttger, A. and Theisen, W.
    Steel Research International 91 (2020)
    To enable the development of novel Fe–C–B–Cr and Fe–C–B–Cr–Mo cold work tool steels, the microstructures and hardness-tempering behaviors of hypoeutectic laboratory melts are investigated. The results show that increasing Cr content enhances the thermodynamic stability of the ultrahard M2B borides. The formation of carboborides is suppressed by adjusting the B/(C + B) ratio, Cr content, and austenitization temperature. A secondary hardenability at 500 °C is achieved by Mo addition. In addition, Mo stabilizes the M23(C,B)6 phase and at higher contents the M3B2 boride. Based on these investigations, Fe0.4C1B–Cr alloys are designed which, inspired by the microstructure of the steel X153CrMoV12-1, feature a α′-Fe hardenable matrix but 15 vol% of eutectic M2B borides instead of M7C3 for wear protection. The Fe0.4C1B–Cr steels are produced by casting and hot rolling as well as powder metallurgy and hot isostatic pressing. The (tribo-) mechanical properties are investigated and compared with X153CrMoV12-1. Fracture toughness, bending strength, wear resistance, and hardness of the novel Fe0.4C1B–Cr alloys are found to be similar or superior to the steel X153CrMoV12-1, at decreased material cost. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/srin.201900416
  • Processing of X65MoCrWV3-2 Cold Work Tool Steel by Laser Powder Bed Fusion
    Boes, J. and Röttger, A. and Theisen, W.
    Steel Research International 91 (2020)
    Laser powder bed fusion (L-PBF) of forming tools has become of major interest in the tooling industry because of the high geometrical flexibility of this process. During L-PBF, a metallic powder bed is melted selectively by a laser beam, enabling the layer-wise manufacturing of parts from 3D computer-aided design data. The process is characterized by a locally and temporally unsteady heat flow in the solidified part and in the melt pool, causing nonequilibrium solidification and phase transformations. In addition, rapid heating and cooling occur, promoting the formation of microstructural defects, cold cracks, and distortion. Because of the high tendency to form cold cracks, processing of martensitic tool steels is still a challenging task. Tool steel X65MoCrWV3-2 is processed by L-PBF and the resulting microstructure and the associated local properties are investigated by microhardness measurements, nanoindentation, and scanning electron microscopy. It is gathered from the investigations that regions of different microstructures and mechanical properties on both micro- and macroscale are present in the L-PBF-densified steel. The different microstructures and properties are the result of the alternating heat insert at different temperature regimes, forming heat-affected zones in which the tempering processes are triggered and strongly varying properties are generated. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/srin.201900445
  • Subsurface microstructure evolution and residual stress in 42CrMo4 steel after single-pass pendulum tests
    Kimm, J.S. and Pöhl, F. and Wiederkehr, P. and Theisen, W.
    International Journal of Mechanical Engineering and Robotics Research 9 (2020)
    This work investigates the effect of cutting on the microstructure and subsurface properties of a cut specimen, with particular focus on the influence of the sample's microstructure and its evolution. Single-pass pendulum tests were conducted with samples of 42CrMo4 steel that had been subjected to different heat treatments. The subsurface region after cutting was analyzed optically by SEM and EBSD. The strain hardened surface region was investigated by nanoindentation. It was found that a soft-annealed structure, in particular, was altered deeply by cutting, resulting in a hardness increase as well as an microstructure that is elongated and refined by high shear deformation microstructure in near-surface regions. The strain hardening behavior and potential of a microstructure had a striking influence on the subsurface alteration. Although some microstructures yielded the same forces under equal cutting conditions, the subsurface alteration was different. This type of change in microstructure was correlated with the micromechanical properties and residual stresses. General influences, such as cutting depth, on the alteration of microstructures by cutting were also taken into account. © 2020 Int. J. Mech. Eng. Rob. Res.
    view abstract10.18178/ijmerr.9.8.1072-1080
  • XRD measurement of stacking fault energy of Cr–Ni austenitic steels: influence of temperature and alloying elements
    Walter, M. and Mujica Roncery, L. and Weber, S. and Leich, L. and Theisen, W.
    Journal of Materials Science 55 (2020)
    In the present study, X-ray diffraction was applied to measure stacking fault energy of Cr–Ni austenitic steels containing different amounts of alloying elements. The results in austenitic steels show that the Ni content and Cr/Ni ratio have a strong effect on SFE. Cu, Si and N increase SFE, being the effect of nitrogen more pronounced; Mo has the opposite effect. In situ XRD experiments up to 300 °C were employed to determine experimentally the SFE and its temperature dependence in Ni and AISI 304. The microstructural parameters required to determine SFE, obtained by Rietveld refinement, made possible to determine experimentally an increase in the SFE with the temperature, related to a decrease in the accumulated deformation, a lower tendency to form stacking faults and a thermal expansion as the temperature increases. The accuracy was determined based on SFE measurements of Au, Cu and Ni pure metals, where the error of the applied method was carefully evaluated. © 2020, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstract10.1007/s10853-020-04953-4
  • A Concept for the Estimation of Soil-Tool Abrasive Wear Using ASTM-G65 Test Data
    Hoormazdi, G. and Küpferle, J. and Röttger, A. and Theisen, W. and Hackl, K.
    International Journal of Civil Engineering 17 (2019)
    In this paper, a new procedure for the prediction of soil-tool abrasive wear is presented which drastically reduces the duration and, therefore, the cost of simulations in comparison to conventional 3D wear modeling. The goal is to extend the experimental data from a single scratch test to the wear of mixtures by means of equations obtained from discrete element method (DEM) simulations and geometric relations. We are predicting abrasive wear with a combination of numerical and experimental approaches taking two shapes of particles into account. Single wear is quantified by measuring the width of scratch induced by a single quartz particle. Geometrical relations together with the particle’s microscopic picture are used to find the depth of scratch. DEM mixture simulations result in equations for the number of contacts and normal contact forces. Finally, the wear rate is calculated for a specific soil sample as an example to clarify the developed prediction procedure. The DEM simulations are performed using PFC 3 D code for both a homogeneous soil sample and a mixture of two different soils. We are specially investigating a relation to predict the abrasive wear caused by a mixture of particles. © 2018, Iran University of Science and Technology.
    view abstract10.1007/s40999-018-0333-9
  • Additive manufacturing of CMSX-4 Ni-base superalloy by selective laser melting: Influence of processing parameters and heat treatment
    Lopez-Galilea, I. and Ruttert, B. and He, J. and Hammerschmidt, T. and Drautz, R. and Gault, B. and Theisen, W.
    Additive Manufacturing 30 (2019)
    Selective laser melting (SLM) provides an economic approach to manufacturing Ni-base superalloy components for high-pressure gas turbines as well as repairing damaged blade sections during operation. In this study, two advanced processing routes are combined: SLM, to fabricate small specimens of the nonweldable CMSX-4, and hot isostatic pressing (HIP) with a rapid cooling rate as post-processing to heal defects while the target γ/γ´ microstructure is developed. An initial parametric study is carried out to investigate the influence of the SLM process parameters on the microstructure and defects occurring during SLM. Special emphasis is placed on understanding and characterizing the as-built SLM microstructures by means of high-resolution characterization techniques. The post-processing heat treatment is then optimized with respect to segregation and the γ/γ´ microstructure. © 2019 Elsevier B.V.
    view abstract10.1016/j.addma.2019.100874
  • Cavitation erosion resistance of 316L austenitic steel processed by selective laser melting (SLM)
    Hardes, C. and Pöhl, F. and Röttger, A. and Thiele, M. and Theisen, W. and Esen, C.
    Additive Manufacturing 29 (2019)
    Every SLM-fabricated component typically possesses a process-specific microstructure that fundamentally differs from any conventionally fabricated specimen. This publication addresses the evaluation of microstructure-related influencing factors on the resistance against cavitation erosion. We exemplarily compared the findings to a cast and hot rolled reference sample. Due to careful adjustment of the process parameters, the overall cavitation erosion resistance of both SLM-processed and conventionally fabricated 316L are very much alike in the investigated case. The incubation period of intact surface areas is improved by the greater hardness and yield strength of the SLM specimen, which is attributable to an increased dislocation density and a smaller grain size. Nevertheless, processing and powder feeding during SLM-fabrication occasionally results in microstructural defects, at which pronounced mass loss during cavitation was registered. X-ray measurements of the residual stresses reveal the development of severe compressive stresses that emerge after a few seconds of cavitation. This compressive stress state delays the immediate propagation of SLM-inherent micro cracks. Moreover, investigations of the microstructure in combination with examination of the ongoing surface deformation highlighted the emergence of coarse grains that grew towards the temperature gradient. This effect leads to a temporarily high surface roughness, local stress concentrations and an increased probability of cavitation impacts. Furthermore, parallel cracks appear perpendicular to the scan tracks that are traced back to formerly protruded slip bands. © 2019 Elsevier B.V.
    view abstract10.1016/j.addma.2019.100786
  • Chromium-nickel-alloys for wear application at elevated temperature
    Krell, J. and Röttger, A. and Theisen, W.
    Wear 432-433 (2019)
    Chromium alloys are a promising class of materials for high temperature application. In this work we investigated the microstructure, hot hardness and sliding wear resistance against an Al2O3 counter-body of three Chromium-Nickel (CrNi) alloys. The microstructure of the investigated alloys consists of a Ni-rich and a Cr-rich phase and can be manipulated by heat treatments. In the solid solution annealed state the room temperature (RT) hardness of CrNi alloys exceeds 800 HV10. The most potent strengthening mechanism in this system is the super saturation of the Cr-rich phase with Ni. Consequently, solid solution annealed specimen experience a strong decline in hardness with the onset of diffusion and precipitation of Ni-rich precipitates within the Cr-rich phase. Aged specimens display a lower hardness at RT but show a lower relative decline in hardness at temperatures above 600 °C. Alloys with higher contents of Ni show a better performance at high temperature sliding wear due to the formation of stable mechanically mixed layers (MML). © 2019 Elsevier B.V.
    view abstract10.1016/j.wear.2019.05.039
  • Deformation behavior and dominant abrasion micro mechanisms of tempering steel with varying carbon content under controlled scratch testing
    Pöhl, F. and Hardes, C. and Theisen, W.
    Wear 422-423 (2019)
    The objective of this paper is the investigation of the abrasive wear behavior of tempering steel by the analysis of its deformation behavior and dominant abrasion micro mechanisms under controlled single scratch testing. We analyzed single scratches induced by a sphero-conical diamond tip with constant and progressively increasing load in tempering steel with varying carbon content in the as quenched condition. Among scratch and hardness testing the analysis and characterization included the optical determination of the deformation behavior (SEM, CLSM, AFM). The results show that the deformation behavior strongly depends on applied normal load and strength (yield stress, ultimate tensile strength) as it varies with carbon content. It was shown that an increasing load leads to the transition of predominant abrasion micro mechanisms from ideal micro ploughing to wedge and chip formation. The scratch resistance increased with increasing carbon content until for higher carbon contents a saturation tendency due to the presence of retained austenite is observed. An increasing carbon content also shifted the aforementioned transition of dominant micro mechanisms not only to higher applied normal loads, but also to higher scratch depths. © 2019 Elsevier B.V.
    view abstract10.1016/j.wear.2019.01.073
  • Development of Multilayer Sinter Cladding of Cold Work Tool Steel on Hadfield Steel Plates for Wear-Resistant Applications
    Farayibi, P.K. and Blüm, M. and Theisen, W. and Weber, S.
    Journal of Materials Engineering and Performance 28 (2019)
    Machinery components used for mining and mineral processing activities are often subjected to high impact loads and wear which have placed demands for the development of materials with high resistance to dynamic loads and aggressive wear conditions. In this study, a multilayered cladding of high alloyed cold work tool steel (X245VCrMo9-4), interlayered with Hadfield steel (X120Mn12) plates, which was also used as substrate using super-solidus liquid-phase sintering technique was investigated. A stack of the cold work tool steel powder was prepared with interlayered X120Mn12 steel plates in an alumina crucible at tap density with the substrate placed on it and was sintered in a vacuum furnace at 1250 °C at a heating rate of 10 K/min, held for 30 min under a nitrogen atmosphere at 0.08 MPa and furnace-cooled. Sample from the as-sintered cladding was subjected to austenization at 1000 °C, quenched in oil and tempered at 150 °C for 2 h. Samples were subjected to microstructural examination using optical and scanning electron microscopy. The microstructural investigations were supplemented by hardness and impact wear tests. Computational thermodynamics was used to support experimental findings. The results revealed that a near-net densification of the sintered X245 was achieved with 99.93 ± 0.01% density. The sintered X245 was characterized by a dispersion of vanadium carbonitride precipitates, especially at the grain boundaries. The heat-treated X245 sample had the highest hardness of 680 ± 7 HV30 due to the matrix of tempered martensitic microstructure when compared to as-sintered with hardness of 554 ± 2 HV30. The X245/X120 interface was characterized by diffusion of Cr, Mo, Mn and C, which resulted in metallurgical bonding between the cladded materials. The impact wear resistance of the sintered X245 was eight times that of the X120; hence, a tough and wear-resistant tool is anticipated when the X120 work hardened in service. © 2019, ASM International.
    view abstract10.1007/s11665-019-03942-2
  • Effect of heat treatment on the high temperature fatigue life of single crystalline nickel base superalloy additively manufactured by means of selective electron beam melting
    Meid, C. and Dennstedt, A. and Ramsperger, M. and Pistor, J. and Ruttert, B. and Lopez-Galilea, I. and Theisen, W. and Körner, C. and Bartsch, M.
    Scripta Materialia 168 (2019)
    The high temperature low cycle fatigue behavior of specimens manufactured from a single crystalline nickel base superalloy processed by selective electron beam melting (SEBM) has been investigated with respect to the effect of different heat treatments. The fatigue lifetime of heat treated material was significantly higher than that of as-built material. Applying hot isostatic pressing (HIP) with an integrated heat treatment resulted in even longer fatigue life. Lifetime limiting crack initiation occurred at interfaces of melting layers, at micro-porosity generated during solidification or, in HIP treated samples, at precipitates which formed at the location of collapsed pores. © 2019 Elsevier Ltd
    view abstract10.1016/j.scriptamat.2019.05.002
  • Improvements on the recovery of 3D particle size distributions from 2D sections
    Benito, S. and Cuervo, C. and Pöhl, F. and Theisen, W.
    Materials Characterization 156 (2019)
    The recovery of underlying 3D particle size distributions by analyzing only 2D sections (such as thin layers or scanning electron microscopy images) has been thoroughly investigated in the last forty years. However, with the advent of increasingly more powerful computers, flexible programming languages and readily available source code, very interesting studies have recently been published on this topic. In this paper, we implement and discuss some key improvements to one particularly promising approach, which is based on the linear representation of the effect of each particle on the smaller apparent sizes that are measurable in the 2D plane. Two main improvements are examined: (i) the inclusion of either a parametric or a nonparametric fit to the measured data and (ii) the utilization of optimization tools to solve the resulting linear system. We endeavor to prove that the new method produces reliable results both in simulations and in an experimental validation example, while also reducing the number of required measurements. © 2019 Elsevier Inc.
    view abstract10.1016/j.matchar.2019.109872
  • Micromechanical characterization of hard phases by means of instrumented indentation and scratch testing
    Kimm, J. and Sander, M. and Pöhl, F. and Theisen, W.
    Materials Science and Engineering A 768 (2019)
    Several hard phases were examined by indentation testing and scratch testing to provide a consistently evaluated database on the mechanical properties and deformation characteristics of hard phases. The analysis of indentations was conducted carefully so that the same method is applicable for hard phases with differing properties. To check whether an influence by the indentation size effect (ISE) is present or not, the Nix-Gao method was used to compute a macroscopic, corrected hardness. Additionally, the possible influence of the surrounding matrix material on the measurements was evaluated. The topography of scratch grooves during scratch tests was determined using a confocal laser scanning microscope. Quantitative information for clearer insights on the deformation characteristics can be acquired with this technique. An indentation size effect was verified to some extent for metallically bonded hard phases. Although the ISE does not have the same magnitude as in metals, it has to be considered if instrumented indentation is used to obtain accurate parameters from hard phases. Matrix effects were also observed and were accounted for. The deformation during indentation and scratch testing showed good correlation between the mechanical parameters and the deformation behavior during scratching. With increasing hardness, the hard phases generally showed more microchipping and deformed elastically to a higher degree. Covalently bonded hard phases yielded the highest hardnesses, and thus the smallest groove areas during scratching, while exhibiting much elastic deformation. Microcracking was observed in scratch grooves of some noncovalent hard phases that could not withstand the stresses underneath the indenter and thus fractured. © 2019 Elsevier B.V.
    view abstract10.1016/j.msea.2019.138480
  • Microstructural Analysis of Powder Metallurgy Tool Steels in the Context of Abrasive Wear Behavior: A New Computerized Approach to Stereology
    Benito, S. and Wulbieter, N. and Pöhl, F. and Theisen, W.
    Journal of Materials Engineering and Performance 28 (2019)
    The present work describes a new methodology designed to characterize the microstructures of tool steels containing carbide hard phases, with the focus set on their abrasive wear resistance. A series of algorithms were designed and implemented in MATLAB® to (i) recognize each of the features of interest, (ii) measure relevant quantities and (iii) characterize each of the phases and the alloy in function of attributes usually neglected in wear description applications: size distribution, shape and contiguity of the hard phases. The new framework incorporates new parameters to describe each one of these attributes, as observed in SEM micrographs. All three aforementioned stages contain novel contributions that can be potentially beneficial to the field of materials design in general and to the field of alloy design for severely abrasive environments in particular. Models of known geometry and micrographs of different powder metallurgy steels were analyzed, and the obtained results were compared with the obtained by the linear intercept method. The relation between the new parameters and the ones available in the scientific literature is also discussed. © 2019, ASM International.
    view abstract10.1007/s11665-019-04036-9
  • Microstructure, mechanical, and tribological properties of M3:2 high-speed steel processed by selective laser melting, hot-isostatic pressing, and casting
    Geenen, K. and Röttger, A. and Feld, F. and Theisen, W.
    Additive Manufacturing 28 (2019)
    In this work, the influence of different manufacturing techniques of M3:2 high-speed steel on the resulting microstructure and the associated material properties was investigated. Therefore, microstructure as well as the mechanical and tribological properties of cast steel (with subsequent hot-forming) and steel powder processed by two techniques: hot-isostatic pressing (HIP) and selective laser melting (SLM) were compared. A detailed SLM parameter analysis revealed that the porosity of SLM specimens can be decreased towards a smaller point distance and a longer exposure time (high energy input). A rise in preheating temperature is associated with a reduction in the crack density or the complete avoidance of cracks. In this context, the high-speed steel showed outstanding densification behavior by SLM, even though this steel is considered to be hardly processable by SLM due to its high content of carbon and hard phase-forming elements. In addition, the reusability of steel powder for SLM processing was investigated. The results indicated that multiple reuse is possible, but only in combination with powder processing (mechanical sieving) after each SLM cycle. The microstructure of SLM-densified high-speed steel consists of a cellular, fine dendritic subgrain segregation structure (submicro level) that is not significantly affected by preheating the base plate. The mechanical and tribological properties were examined in relation to the manufacturing technique and the subsequent heat treatment. Our investigations revealed promising behavior with respect to hardness tempering (position of the secondary hardness peak) and tribology of the M3:2 steel processed by SLM compared to the HIP and cast conditions. © 2019
    view abstract10.1016/j.addma.2019.05.028
  • On the evolution of dislocation cell structures in two Al-alloys (Al-5Mg and Al-11Zn) during reciprocal sliding wear at high homologous temperatures
    Parsa, A.B. and Walter, M. and Theisen, W. and Bürger, D. and Eggeler, G.
    Wear (2019)
    The formation of dislocation substructures in up to 10 µm deep subsurface regions of two aluminium alloys, Al-5Mg and Al-11Zn, was investigated under conditions of high homologous temperature reciprocal sliding wear (HT-RSW). Under creep conditions, Al-5Mg shows a solid solution type of inverse primary creep. In contrast, Al-11Zn creeps obstacle controlled and exhibits normal primary creep. These two materials were subjected to reciprocal sliding wear at 200 and 300 °C for 100 and 1000 cycles. Flat polished disks were exposed to the 1 mm reciprocal movements of a spherical aluminium oxide counterbody under normal forces of 5 and 10 N at an oscillation frequency of 1 Hz. Using focused ion beam (FIB) micromachining thin electron transparent foils were prepared from the surface regions of the as received and worn material states. Transmission electron microscopy (TEM) was used to study the evolution of nano and micro grain sizes in the surface regions. Despite the different creep behavior, the two materials behave similar under conditions of reciprocal sliding wear. The results obtained in the present work show that subgrain sizes decrease with increasing numbers of wear cycles and increasing normal forces. Subgrain sizes also increase with increasing temperature. At 300 °C, dynamic recrystallization was observed in both Al-alloys. The results of the present work are discussed in the light of previous results reported in the literature. Areas in need of further work are highlighted. © 2018 Elsevier B.V.
    view abstract10.1016/j.wear.2018.10.018
  • On the rejuvenation of crept Ni-Base single crystal superalloys (SX)by hot isostatic pressing (HIP)
    Horst, O.M. and Ruttert, B. and Bürger, D. and Heep, L. and Wang, H. and Dlouhý, A. and Theisen, W. and Eggeler, G.
    Materials Science and Engineering A 758 (2019)
    In the present work, we study the effect of HIP rejuvenation treatments on the creep behavior and residual life of a pre-crept single crystal Ni-base superalloy of type CMSX-4. The present work combines miniature creep experiments of precisely oriented [001]tensile creep specimens with HIP treatments and quantitative analysis of scanning and transmission electron micrographs. A HIP-rejuvenation treatment after 5.0% creep pre-strain significantly improves creep strength. However, the microstructural damage induced by the creep pre-deformation could not be fully removed. In a series of sequential creep/HIP/creep-experiments, increasing levels of pre-deformation result in increasing levels of creep rates even after identical HIP-rejuvenation treatments. The memory effect, which causes this phenomenon, is related to an accumulation of permanent microstructural damage, which is not associated with rafting or cavitation. The mechanical results obtained in the present work are interpreted based on microstructural results on the γ/γʼ-microstructure (γ-channel widths and γʼ-size), on the pore population (number density of pores, pore size distributions and pore area fractions)and dislocation substructures which have formed during creep. The results are discussed in the light of previous results reported in the literature. © 2019 The Authors
    view abstract10.1016/j.msea.2019.04.078
  • On the role of nitrogen on hydrogen environment embrittlement of high-interstitial austenitic CrMnC(N) steels
    Egels, G. and Fussik, R. and Weber, S. and Theisen, W.
    International Journal of Hydrogen Energy 44 (2019)
    This work investigates the susceptibility of high-interstitial CrMn austenitic stainless steel CN0.96 to hydrogen environment embrittlement. In this context, an N-free model alloy of CN0.96 steel was designed, produced, and characterized. Both steels were subjected to tensile tests in air and in a high-pressure hydrogen gas atmosphere. Both steels undergo severe hydrogen embrittlement. The CN0.96 steel shows trans- and intergranular failure in hydrogen, whereas the N-free model alloy shows exclusively intergranular failure. The different failure modes could be related to different deformation modes that are induced by the presence or absence of N, respectively. In the CN0.96 steel, N promotes planar dislocation slip. Due to the absence of N in the model alloy, localized slip is less pronounced and mechanical twinning is a more preferred deformation mechanism. The embrittlement of the model alloy could therefore be related to mechanisms that are known from hydrogen embrittlement of twinning-induced plasticity steels. © 2019 Hydrogen Energy Publications LLC
    view abstract10.1016/j.ijhydene.2019.10.109
  • Phase Formation during Heat Treatment of Zn- and ZnAlMg-Coated Steels at 400 and 750 °C
    Windmann, M. and Barthen, M. and Opitz, T. and Hahn, I. and Röttger, A. and Theisen, W.
    Steel Research International (2019)
    In contrast to a cold-forming process, a tempered forming process is able to deform high-strength steel used for manufacturing automotive bodyworks in a more economic manner. Cold-formed steel sheets are commonly coated with a Zn or ZnAlMg layer for cathodic corrosion protection. The tempering process would lead to diffusion processes at the steel/coating interface, which is accompanied by the formation of new phases in the coatings. This publication focuses on phase formation in Zn and ZnAlMg coatings on steel sheets, which are heat-treated at 400 and 750 °C. the authors find that the pure Zn coating remains in the solid state and transforms into the intermetallic δ phase (FeZn 10 ) during heat treatment at 400 °C. The coating melts during heating to 750 °C, but remains in the solid state after transformation into the Γ phase (Fe 4 Zn 9 ) and α-Fe. In the ZnAlMg coating, minor iron diffusion occurs at a temperature of 400 °C. Within a dwell time of 600 s, intermetallic Fe–Zn phases are not formed. During heat treatment at 750 °C, phase formation in the ZnAlMg coating is very similar to that in the pure Zn coating, during which Γ (Fe 4 Zn 9 ) and α-Fe are formed. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/srin.201800588
  • Processing of gas-nitrided AISI 316L steel powder by laser powder bed fusion – Microstructure and properties
    Boes, J. and Röttger, A. and Becker, L. and Theisen, W.
    Additive Manufacturing 30 (2019)
    This work investigated the processing of high nitrogen-alloyed austenitic stainless steels by laser powder bed fusion (L-PBF). Prior to L-PBF processing, the AISI 316 L steel powder was nitrided at a temperature of 675°C in a 3 bar nitrogen atmosphere, thus achieving a N content of 0.58 mass-%. By mixing nitrided 316 L powder with untreated 316 L powder, two different powder mixtures were obtained with 0.065 mass-% and 0.27 mass-% nitrogen, respectively. After nitriding and mixing, the powder was characterized in terms of its flow properties and chemical composition. The nitrided steel powder was then processed by L-PBF, and the microstructure as well as the chemical composition were investigated by means of scanning electron microscopy and carrier gas hot extraction. It was shown that nitriding of steel powders in an N2 atmosphere can be used to significantly increase the nitrogen content of the powder without impairing its flow properties. With increasing nitrogen content of the powder, the porosity within the L-PBF built specimens increased. However, both the yield strength and the tensile strength were greatly improved without a marked reduction in the elongation at fracture of the respective steels. This work shows that nitrogen-alloyed austenitic stainless steels can be processed by L-PBF and the mechanical properties can be improved. © 2019 Elsevier B.V.
    view abstract10.1016/j.addma.2019.100836
  • Recycling of NdFeB Magnets by Electrodischarge Sintering—Microstructure, Magnetic, and Mechanical Properties
    Leich, L. and Röttger, A. and Krengel, M. and Theisen, W.
    Journal of Sustainable Metallurgy 5 (2019)
    In this work, we investigate the feasibility of recycling NdFeB magnets by means of electrodischarge sintering (EDS). We crushed, sintered, and hot-deformed NdFeB magnets in a jaw crusher, and the NdFeB fragments were further compacted to a round shape by EDS. The EDS technique is a fast and energy-saving compaction process for powders with sufficient electrical conductivity. The current is discharged from capacitors into a loose powder that has been precompacted by Cu punches into a ceramic die, thus resulting in fully dense magnets. In this study, we investigated the apparent density, particle size distribution, oxygen content, and morphology of the crushed powder. In addition, the microstructure, compressive strength, and the magnetic properties of the EDS-densified samples were examined. For all specimens, the energy product decreases with the increasing discharge energy during EDS processing and the increasing oxygen content of the initial powder. Furthermore, high apparent densities together with large particle sizes promote EDS densification of NdFeB magnets. The applied EDS parameters led to the formation of three different microstructures (insufficiently densified zone, fully densified zone, and remelted zone) along the cross section of the EDS-densified specimens. These volume fractions of the different microstructural constituents during the EDS process and the powder characteristics (oxygen content, morphology, etc.) determine the resulting mechanical and magnetic properties of the specimens. © 2019, The Minerals, Metals & Materials Society.
    view abstract10.1007/s40831-018-0204-2
  • Repair of Ni-based single-crystal superalloys using vacuum plasma spray
    Kalfhaus, T. and Schneider, M. and Ruttert, B. and Sebold, D. and Hammerschmidt, T. and Frenzel, J. and Drautz, R. and Theisen, W. and Eggeler, G. and Guillon, O. and Vassen, R.
    Materials and Design 168 (2019)
    Turbine blades in aviation engines and land based gas-turbines are exposed to extreme environments. They suffer damage accumulation associated with creep, oxidation and fatigue loading. Therefore, advanced repair methods are of special interest for the gas-turbine industry. In this study, CMSX-4 powder is sprayed by Vacuum Plasma Spray (VPS) on single-crystalline substrates with similar compositions. The influence of the substrate temperature is investigated altering the temperature of the heating stage between 850 °C to 1000 °C. Different spray parameters were explored to identify their influence on the microstructure. Hot isostatic pressing (HIP) featuring fast quenching rates was used to minimize porosity and to allow for well-defined heat-treatments of the coatings. The microstructure was analysed by orientation imaging scanning electron microscopy (SEM), using electron backscatter diffraction (EBSD). The effects of different processing parameters were analysed regarding their influence on porosity and grain size. The results show that optimized HIP heat-treatments can lead to dense coatings with optimum γ/γ′ microstructure. The interface between the coating and the substrate is oxide free and shows good mechanical integrity. The formation of fine crystalline regions as a result of fast cooling was observed at the single-crystal surface, which resulted in grain growth during heat-treatment in orientations determined by the crystallography of the substrate. © 2019
    view abstract10.1016/j.matdes.2019.107656
  • Time-Dependent Evolution of Microstructure and Mechanical Properties of Mortar
    Röttger, A. and Youn-Čale, B.-Y. and Küpferle, J. and Lentz, J. and Breitenbücher, R. and Theisen, W.
    International Journal of Civil Engineering 17 (2019)
    This study investigates the evolution of the microstructure and mechanical properties of mortar. Mortar samples consisting of Portland cement CEM I42.5 R (~ 60 vol% of quartz sand 0/2 mm, w/c-ratio of 0.5) were prepared and stored according to EN 1015. After 1, 2, 7, 14 and 28 days, the samples were oven-dried until constant weight as well as vacuum-dried. The microstructure of the mortar samples was investigated using scanning electron microscopy. Phase analysis was performed using X-ray diffraction, allowing the description of the crystalline phase evolution during hardening. Mechanical properties were evaluated using nanoindentation. Based on the nanoindentation results, the effective Young’s modulus was calculated using the model by Hashin and Shtrikman. The moduli calculated based on the values of the nanoindentation experiments were compared to the Young’s modulus determined in compression experiments. The results show that the Young’s modulus determined by the nanoindentation and compression test describes a degressive curve progression. The studies show a correlation between the results from nanoindentation tests and the mechanical properties obtained from the compression tests. Therefore, the microstructural evolution of mortar, including the influence of pores on Young’s modulus, must be taken into account to estimate the macroproperties from the nanoindentation tests. © 2018, Iran University of Science and Technology.
    view abstract10.1007/s40999-018-0305-0
  • Ultra-fast sintered functionally graded Fe/W composites for the first wall of future fusion reactors
    Heuer, S. and Lienig, T. and Mohr, A. and Weber, T. and Pintsuk, G. and Coenen, J.W. and Gormann, F. and Theisen, W. and Linsmeier, C.
    Composites Part B: Engineering 164 (2019)
    Aiming at the realisation of nuclear fusion reactors, the joining of W and steel parts is currently examined. Based on proposals to implement functionally graded steel/W materials (FGMs) in the joint to cope with thermal stresses, the present contribution introduces a novel fabrication method for steel/W FGMs. Electro Discharge Sintering (EDS) was used to consolidate Fe/W powders within milliseconds at atmosphere. Due to the short process time, the formation of detrimental intermetallic Fe-W precipitates is limited compared to established fabrication methods. The current work first presents results of the Fe/W powder processing, then a feasibility study regarding the fabrication of homogeneous and graded Fe/W composites with W volume fractions of 0, 25, 50 and 75 % via EDS is presented. Lastly, the composites are characterised microstructurally, thermo-physically, and mechanically in detail. © 2018
    view abstract10.1016/j.compositesb.2018.11.078
  • Densification of nanocrystalline NdFeB magnets processed by electro-discharge sintering – Microstructure, magnetic, and mechanical properties
    Leich, L. and Röttger, A. and Theisen, W. and Krengel, M.
    Journal of Magnetism and Magnetic Materials 460 (2018)
    This work investigates the densification process of nanocrystalline NdFeB powder by electro-discharge sintering (EDS) and the associated magnetic properties. The EDS technique is used as a fast and energy-saving compaction process for metal powders. A large current is discharged from capacitors into a pre-compacted loose powder, thus resulting in complete compaction. In this study, the microstructure, magnetic, and mechanical properties of the compacted, hard magnetic NdFeB specimens were investigated under variation of the energy EEDS and compression load pEDS. For all specimens, the intrinsic coercivity HcJ decreases on increasing the discharge energy. However, the compaction load has apparently no influence on the coercivity HcJ, whereas the residual induction Br decreases only with increasing discharge energy. An increase in the compression load pEDS causes an increase in the specimens’ density and thus promotes residual induction Br. The applied EDS parameters led to the formation of three different microstructures (insufficiently densified zone, fully densified zone, and remelted zone) along the cross-section of the EDS-densified specimens. Volume fractions of the three different microstructures that form during the EDS process determine the resulting mechanical and magnetic properties of the specimens. © 2018 Elsevier B.V.
    view abstract10.1016/j.jmmm.2018.04.035
  • Effect of porosity and eutectics on the high-temperature low-cycle fatigue performance of a nickel-base single-crystal superalloy
    Ruttert, B. and Meid, C. and Mujica Roncery, L. and Lopez-Galilea, I. and Bartsch, M. and Theisen, W.
    Scripta Materialia 155 (2018)
    This work investigates the separate influence of porosity and γ/γ′-eutectics on the low-cycle fatigue life of a single-crystal Ni-base superalloy at high temperatures. A conventional vacuum furnace heat-treatment but also integrated heat-treatments in a hot isostatic press are applied to produce different material variants of the same alloy. High-resolution electron microscopy revealed that both pores and γ/γ′-eutectics act as crack starters, thus initiating early failure. Moreover, the results indicate that remaining γ/γ′-eutectics can weaken the fatigue resistance even more than pores. Furthermore, the results confirm the beneficial effect of proper integrated hot isostatic pressing heat-treatments on the fatigue performance. © 2018
    view abstract10.1016/j.scriptamat.2018.06.036
  • Enhancement of hardness, modulus and fracture toughness of the tetragonal (Fe,Cr)2B and orthorhombic (Cr,Fe)2B phases with addition of Cr
    Lentz, J. and Röttger, A. and Großwendt, F. and Theisen, W.
    Materials and Design 156 (2018)
    This study analyzes the influence of Cr content on hardness H, elastic modulus E and fracture toughness KIC of the M2B boride by means of nanoindentation experiments. Additionally, properties of the Fe3(C,B) phase are determined. Samples of the M2B phase are casted and microstructurally characterized by means of scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. At a Cr content higher than 14.7 atom% the M2B phase transforms from tetragonal into orthorhombic structure. The tetragonal M2B type possesses an optimum of H (21 ± 1 GPa), E (373 ± 6) GPa and KIC (3.5 ± 0.7 MPam) at 4–5 atom% Cr. The hardness, modulus and toughness of the orthorhombic M2B phase increase with Cr content and reach values of H = 27 ± 0.7 GPa, E = 473 ± 9 of and KIC = 3.26 ± 0.8 MPam at maximal investigated Cr content of 55 atom%. The hardness of the M2B phases decreases around 2.3–3.2 GPa as a function of indentation depth, which is known as the indentation size effect. Hardness and fracture toughness of M2B phase outperform conventionally used M7C3 carbides and are similar to MC-carbides. Findings can be used in novel alloying approaches in order to optimize the performance and reduce cost of tool steels. © 2018 Elsevier Ltd
    view abstract10.1016/j.matdes.2018.06.040
  • Experimental and Numerical Investigations on Interdiffusion Profiles in Compounds Produced by Sinter-Cladding
    Blüm, M. and Theisen, W. and Weber, S.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 49 (2018)
    Tools used for mineral processing applications are affected by strong abrasive wear and high dynamic loads. This results in opposing demands on the mechanical properties of these tools. Therefore, modern concepts for the manufacturing of mineral processing tools include a composite tool concept consisting of a low-alloyed substrate and a high-alloyed, wear-resistant cladding material. These coatings can be applied using different production processes such as composite casting, deposit welding, and HIP cladding. During the deposition of the cladding, interdiffusion between the substrate and cladding material occurs. This interdiffusion may have a negative impact on the compound, since characteristics such as wear resistance, mechanical properties, and the local microstructure are influenced. This article is focused on the investigation and simulation of interdiffusion processes in supersolidus-sintered compounds using computational thermodynamics, diffusion calculations, optical emission spectrometry, hardness profiles, and microstructural investigations. It is shown that the interdiffusion processes between the solid substrate and the semisolid cladding can be simulated using a dispersed phase model to give results with a close concordance to optical emission spectrometry measurements. © 2018, The Minerals, Metals & Materials Society and ASM International.
    view abstract10.1007/s11661-018-4750-9
  • General investigations on processing tool steel X40CrMoV5-1 with selective laser melting
    Krell, J. and Röttger, A. and Geenen, K. and Theisen, W.
    Journal of Materials Processing Technology 255 (2018)
    The X40CrMoV5-1 (H13) hot work tool steel was densified by selective laser melting (SLM) using different laser parameters and preheating temperatures. The porosity and crack densities of the processed specimen were determined, the resulting microstructure characterized, tempering hardness diagrams recorded and the reusability of the powder assessed. The X40CrMoV5-1 steel showed a good densification behaviour. Relative densities of above 99.5% were obtained. After SLM densification, the specimen showed a fine-grained microstructure, with a cellular arrangement consisting of ferrite and austenite. Although the microstructure did not change with preheating temperature, a decrease in crack density could be observed for higher preheating temperatures. By combining microstructural observations with some simulations, a new model describing the microstructural evolution of SLM-densified X40CrMoV5-1 is suggested. The peak in secondary hardness after tempering SLM-densified X40CrMoV5-1 was observed at higher temperatures compared to the cast reference steel in the same heat treatment condition. © 2018 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2018.01.012
  • Hardness and modulus of Fe2B, Fe3(C,B), and Fe23(C,B)6 borides and carboborides in the Fe-C-B system
    Lentz, J. and Röttger, A. and Theisen, W.
    Materials Characterization 135 (2018)
    This work provides a comparative and comprehensive study of the indentation hardness and indentation modulus of iron-rich borides and carboborides of types Fe2B, Fe3(C,B), and Fe23(C,B)6. In addition, the hardness and elastic modulus of Cr-rich M7C are investigated for comparative purposes. We investigated the impact of increasing B content and indentation size effect (ISE). The phases of interest were stabilized in cast Fe-C-B alloys that varied with respect to the B / (B + C) ratio and heat treatment. The resulting microstructures were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and wavelength X-ray spectroscopy (WDS). Dynamic in-situ nanoindentation experiments based on the method of continuous stiffness measurement (CSM) were coupled to SEM and EBSD investigations to determine the mechanical properties of the individual borides and carboborides as a function of the indentation depth. The results were compared to values obtained for the Cr-rich M7C3 carbide. It was found that the hardness of the B-rich Fe3(C,B) phase is considerably higher than pure Fe3C and increases with increasing B content. The ISE was present in all investigated phases, and the hardness decreased as a function of indentation depth. The hardness at infinite indentation depth H0 was estimated according to the model of Nix and Gao. The Fe2B phase was found to be the hardest phase (H0 = 19.04 GPa), followed by M7C3 (H0 = 16.43 GPa), Fe3(C,B) (H0 = 11.18 to 12.24 GPa), and Fe23(C,B)6 (H0 = 10.39 GPa). © 2017 Elsevier Inc.
    view abstract10.1016/j.matchar.2017.11.012
  • Impact of chemical inhomogeneities on local material properties and hydrogen environment embrittlement in AISI 304L steels
    Egels, G. and Mujica Roncery, L. and Fussik, R. and Theisen, W. and Weber, S.
    International Journal of Hydrogen Energy (2018)
    This study investigated the influence of segregations on hydrogen environment embrittlement (HEE) of AISI 304L type austenitic stainless steels. The microstructure of tensile specimens, that were fabricated from commercially available AISI 304L steels and tested by means of small strain-rate tensile tests in air as well as hydrogen gas at room temperature, was investigated by means of combined EDS and EBSD measurements. It was shown that two different austenitic stainless steels having the same nominal alloy composition can exhibit different susceptibilities to HEE due to segregation effects resulting from different production routes (continuous casting/electroslag remelting). Local segregation-related variations of the austenite stability were evaluated by thermodynamic and empirical calculations. The alloying element Ni exhibits pronounced segregation bands parallel to the rolling direction of the material, which strongly influences the local austenite stability. The latter was revealed by generating and evaluating two-dimensional distribution maps for the austenite stability. The formation of deformation-induced martensite was shown to be restricted to segregation bands with a low Ni content. Furthermore, it was shown that the formation of hydrogen induced surface cracks is strongly coupled with the existence of surface regions of low Ni content and accordingly low austenite stability. In addition, the growth behavior of hydrogen-induced cracks was linked to the segregation-related local austenite stability. © 2018 The Author(s).
    view abstract10.1016/j.ijhydene.2018.01.062
  • Influence of the slurry-stabilized tunnel face on shield TBM tool wear regarding the soil mechanical changes – Experimental evidence of changes in the tribological system
    Küpferle, J. and Zizka, Z. and Schoesser, B. and Röttger, A. and Alber, M. and Thewes, M. and Theisen, W.
    Tunnelling and Underground Space Technology 74 (2018)
    The wear of slurry shield TBM excavation tools is a challenging topic that has been discussed over many years, Due to the complexity of the tribological system that defines the tool wear, no sufficient approach to determine the wear in a specific tunneling project has been developed up to now. The elementary step to display the application-oriented tribological system is one of the main issues regarding laboratory scale test methods. Therefore, the RUB Tunneling Device has been developed at the Ruhr-University Bochum (Germany). With this apparatus, several influencing factors regarding the tribological system of a TBM-tool can be analyzed. A unique feature of the device is the experimental simulation of a slurry shield excavation, including a realistic tunnel face support. This paper focuses on the experimental simulation of slurry shield excavation and the influence of face support on tool wear. Changes in the soil mechanical properties due to slurry penetration at the tunnel face are regarded and correlated with the tribological system of a slurry shield excavation. It is proven that the tribological system, and thus the tool wear, changes significantly due to slurry injection. © 2018 Elsevier Ltd
    view abstract10.1016/j.tust.2018.01.011
  • Interaction between laser radiation and metallic powder of 316L austenitic steel during selective laser melting
    Mutke, C. and Geenen, K. and Röttger, A. and Theisen, W.
    Materials Characterization 145 (2018)
    During selective laser melting (SLM), a complex heat state develops that leads to a characteristic crystallography and microstructure of the processed materials. Depending on the geometry of the processed part, most scan tracks of a new layer, so-called hatches, are located above a dense substrate or already solidified structures whereas others are located above loose powder. This is, inter alia, the case for overhanging structures. Attributable to the lower thermal conductivity of loose powder, temperature gradients and cooling rates of the melt pool differentiate in these areas, resulting in a different microstructural build-up. In this work, the microstructure and the crystallographic orientation of grade 316L austenitic stainless steel processed by SLM was investigated to understand the interaction between the laser radiation and the metallic powder during SLM-processing and to investigate the remelting of a track on a previous SLM-densified track. Single and multiple tracks on a loose bulk powder substrate, as well as single tracks on a dense substrate plate, were generated. A parameter study revealed that high energy densities are necessary to build continuous tracks on a loose bulk powder substrate. In addition, the amount of adhered particles, which are sintered on the fully melted and solidified tracks, is determined in comparison to the melted powder because the sintered particles strongly influence the surface roughness. To understand the microstructure development and, particularly, the influence of adjacent hatches during SLM-processing, investigations on the resulting microstructure and crystallographic orientation of a single track and two connected multiple tracks were carried out. During SLM processing of the tracks, the substrate plate and the solidified structures influence the temperature gradient and cooling rate of the melt pool, thus directionally solidified and elongated grains occur. Furthermore, the solidification is characterized by an epitaxial growth due to a distinct thermal gradient between the melt pool and the surrounding elements. © 2018 Elsevier Inc.
    view abstract10.1016/j.matchar.2018.08.061
  • Investigation of heat transfer in a copper-infiltrated tool steel based on measurement, microtomography, and numerical simulation
    Klein, S. and Weber, S. and Theisen, W.
    Materials and Design 156 (2018)
    Copper-infiltrated tool steels potentially combine the good mechanical properties of tool-steels and the superior electrical and thermal conductivity of copper. However, their effective properties greatly depend on the constitution of the components as well as their topology. In this work, the copper-infiltrated cold-work tool steel of type X245VCrMo9-4-4 is analyzed. The thermal conductivity (TC) of the composite and its components is measured and their topology is analyzed by means of X-ray microtomography (μCT). Using the digitized topology and the attained properties, numerical FE simulations were laid out, which allowed the detailed investigation of heat transfer in the material. The results indicate, that 1) the simulated thermal conductivity is very sensitive to the assumed thermal boundary conductance (TBC) 2) the TBC can be approximated by iteratively converging the simulation results to the measured TC 3) both components contribute to the effective thermal conductivity (1/6 steel + 5/6 copper) and act as a bypass for each other, preventing hot spots 4) small increases in the copper content increase the TC by shortening the effective heat conduction path. © 2018 Elsevier Ltd
    view abstract10.1016/j.matdes.2018.06.028
  • Micro-magnetic and microstructural characterization of wear progress on case-hardened 16MnCr5 gear wheels
    Knyazeva, M. and Vasquez, J.R. and Gondecki, L. and Weibring, M. and Pöhl, F. and Kipp, M. and Tenberge, P. and Theisen, W. and Walther, F. and Biermann, D.
    Materials 11 (2018)
    The evaluation of wear progress of gear tooth flanks made of 16MnCr5 was performed using non-destructive micro-magnetic testing, specifically Barkhausen noise (BN) and incremental permeability (IP). Based on the physical interaction of the microstructure with the magnetic field, the micro-magnetic characterization allowed the analysis of changes of microstructure caused by wear, including phase transformation and development of residual stresses. Due to wide parameter variation and application of bandpass filter frequencies of micro-magnetic signals, it was possible to indicate and separate the main damage mechanisms considering the wear development. It could be shown that the maximum amplitude of BN correlates directly with the profile form deviation and increases with the progress of wear. Surface investigations via optical and scanning electron microscopy indicated strong surface fatigue wear with micro-pitting and micro-cracks, evident in cross-section after 3 × 105 cycles. The result of fatigue on the surface layer was the decrease of residual compression stresses, which was indicated by means of coercivity by BN-analysis. The different topographies of the surfaces, characterized via confocal white light microscopy, were also reflected in maximum BN-amplitude. Using complementary microscopic characterization in the cross-section, a strong correlation between micro-magnetic parameters and microstructure was confirmed and wear progress was characterized in dependence of depth under the wear surface. The phase transformation of retained austenite into martensite according to wear development, measured by means of X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) was also detected by micro-magnetic testing by IP-analysis. © 2018 by the authors.
    view abstract10.3390/ma11112290
  • Microstructure and mechanical properties of X65MoCrWV3-2 cold-work tool steel produced by selective laser melting
    Boes, J. and Röttger, A. and Mutke, C. and Escher, C. and Theisen, W.
    Additive Manufacturing 23 (2018)
    In this study, martensitic cold-work tool steel X65MoCrWV3-2 was processed by selective laser melting (SLM) by varying the laser scanning parameters and baseplate preheating temperatures. Porosity as well as crack density of the SLM-densified steel were determined by quantitative image analysis. The resulting microstructure and the associated local mechanical properties were characterized, and the hardness-tempering behavior of the SLM-densified steel was compared to the behavior of the conventionally manufactured X65MoCrWV3-2 steel in the cast and hot-formed condition. Regardless of the preheating temperature, SLM-densified X65MoCrWV3-2 possesses a porosity of less than 0.5 vol.-%. The crack density was reduced significantly by means of a higher preheating temperature. The microstructure after SLM densification shows a fine, equiaxed cellular-dendritic subgrain structure, superimposed by lath- or needle-like martensite. The martensite morphology appeared to be finer at a lower preheating temperature, whereas the observed subgrain structure did not seem to be influenced by the preheating temperatures. Microhardness measurements indicated tempering effects in first solidified layers caused by the densification of subsequently deposited layers. Peak hardness after tempering of the SLM-densified steel was found to be higher compared to the maximum hardness in the X65MoCrWV3-2 steel in the cast condition. © 2018 Elsevier B.V.
    view abstract10.1016/j.addma.2018.08.005
  • Mn-Alloyed High-Strength Steels with a Reduced Austenitization Temperature: Thermodynamic Calculations and Experimental Investigations
    Windmann, M. and Opitz, T. and Klein, S. and Röttger, A. and Theisen, W.
    Steel Research International 89 (2018)
    High-strength steels (e.g., 1.5528–22MnB5), processed by direct press-hardening, are widely used for security-relevant structures in automotive bodyworks. In this study, the austenitization temperature AC3 of the steel 22MnB5 (approx. 840 °C) is decreased to enable a reduction in the heat-treatment temperature. Thermodynamic calculations using the CALPHAD method are used to assess the effect of alloying elements on the α–γ transformation temperatures. On this account, 22MnB5 steel is alloyed with 6 to 9.5 mass% manganese, which decreases the α–γ transformation temperature to 744 °C. Simultaneously, the martensite finish temperature decreases below room temperature, which is accompanied by the presence of retained austenite after hardening. Furthermore, ϵ-martensite is formed. High Mn-alloyed steel 22MnB5 (9.5 mass% Mn, AC3 = 744 °C) possesses a high strength of Rm = 1618 MPa, similar to the initial material 22MnB5. Elongation-to-fracture decreases to A5 = 3.5% due to the formation of ϵ-martensite. The material strength of the steel alloyed with 6 mass% manganese (AC3 = 808 °C) strongly increases to Rm = 1975 MPa as a result of α-martensite and solid-solution strengthening by the element manganese. This steel possesses a higher elongation-to-fracture of A5 = 7%. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/srin.201800166
  • Rejuvenation of Single-Crystal Ni-Base Superalloy Turbine Blades: Unlimited Service Life?
    Ruttert, B. and Horst, O. and Lopez-galilea, I. and Langenkämper, D. and Kostka, A. and Somsen, C. and Goerler, J.V. and Ali, M.A. and Shchyglo, O. and Steinbach, I. and Eggeler, G. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science (2018)
    Rejuvenation of the initially hot isostatic pressing (HIP) heat-treated single-crystal Ni-base superalloy (SX) ERBO/1 was examined experimentally and via phase field simulation to establish rejuvenation treatments as a cost-effective alternative for another interval of service life. Creep was performed at 950 °C and 350 MPa, and the specimens were crept to 0.6 pct (creep rate minimum) or 2 pct strain, respectively. A slight coarsening of the γ/γ′ microstructure was observed experimentally and via simulation at 0.6 pct and rafting at 2 pct strain. The damaged microstructures were rejuvenated in a novel hot isostatic press that provides fast quenching rates before the same specimens were recrept under the same initial creep conditions. High-resolution microscopy proves that the rejuvenation re-establishes the original γ/γ′ microstructure in the dendrite core of the precrept specimens (0.6 and 2 pct). However, the interdendritic areas of the 2 pct precrept and rejuvenated specimen still contain elongated γ′ particles enwrapped by interfacial dislocation networks that survived the applied rejuvenation. The subsequent experimental and simulated creep tests after rejuvenation demonstrated that the creep behavior is only reproducible by the proposed rejuvenation for specimens that had crept until the end of the primary creep regime. © 2018 The Minerals, Metals & Materials Society and ASM International
    view abstract10.1007/s11661-018-4745-6
  • Subsurface characterization of high-strength high-interstitial austenitic steels after impact wear
    Mujica Roncery, L. and Agudo Jácome, L. and Aghajani, A. and Theisen, W. and Weber, S.
    Wear 402-403 (2018)
    The microstructure of the subsurface after impact wear of three high-strength high interstitial austenitic steels has been analysed using transmission electron microscopy (TEM) in alloys with C and N as interstitial elements. In all cases, a nanocrystalline region followed by a transition zone and a cold-worked area are present. Additionally, microhardness and nano-scratching tests were conducted to study the wear-related properties of the impact subsurface and the base material. The results of the microstructural analysis reveal that the following mechanisms are involved during impact wear: abrasion (ploughing), microcrack formation associated with contact fatigue, entrapment and adhesion of SiO2 particles. The analysis of the wear-related properties indicates that the subsurface acts as a self-protective layer that prevents the deterioration of the substrate. © 2018 Elsevier B.V.
    view abstract10.1016/j.wear.2018.02.016
  • Tribological analysis of the TBM tool wear in soil from the view of material science: Tribologische Analyse des Abbauwerkzeugverschleißes von Tunnelvortriebsmaschinen in Lockergestein aus werkstofftechnischer Sicht
    Küpferle, J. and Röttger, A. and Theisen, W. and Alber, M.
    Geomechanik und Tunnelbau 11 (2018)
    Excessive wear to excavation tools leads to unplanned downtime, which results in additional costs. To predict tool wear during the planning phase of a project, an analysis of the acting tribological system is essential. The different influential factors and interactions have to be considered. The abrasive surface degradation of tools is investigated on a microstructural scale with nano-scratch experiments. Material-dependent variables like hard phase content and size are discussed in terms of the resulting wear-resistance. Using the RUB Tunnelling Device, abrasive surface degradation can be investigated on a more global scale and correlated with the acting tribological system, and the different tribological system components and influential factors can be considered. Copyright © 2018 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin
    view abstract10.1002/geot.201700066
  • Adaptation of TiC hard particles properties and morphology in metal matrix composites by refractory elements
    Mohr, A. and Röttger, A. and Theisen, W.
    Key Engineering Materials 742 KEM (2017)
    High mechanical loads, corrosion, and abrasion decrease the lifetime of tools. One way to increase the wear resistance of tool materials can be achieved by adding hard particles to the metal matrix such as titanium carbide, which protect the softer metal matrix against abrasive particles. This material concept is designated as metal matrix composite (MMC). Ferro-Titanit® is such MMC material, possessing high wear and a simultaneously high corrosion resistance, for which reason this material is used in the polymers industry. The material concept is based on a corrosion-resistant Fe-base matrix with up to 45 vol% titanium carbide (TiC) as a hard particle addition to improve the wear resistance against abrasion. These TiC hard particles must be adapted to the present tribological system in terms of hardness, size and morphology. This study shows how the size and morphology of TiC hard particles can be influenced by the refractory element niobium (Nb). Therefore, the element Nb was added with 2 and 4 mass% to the soft-martensitic Ferro-Titanit® Grade Nikro128. The investigated materials were compacted by sintering, and the densified microstructure was further characterized by scanning electron microscopy (SEM), energy dispersive spectrometry (EDX), and optical image analyses. Furthermore, microstructure and properties of the compacted Nb-alloyed samples were compared to the reference material Nikro128. The results show that the addition of Nb influences the morphology, size and chemical composition of the TiC hard particle. These changes in the hard phase characteristics also influence the materials properties. It was shown that the phase niobium carbide (NbC) is formed around the TiC during the densification process, leading to a change in morphology and size of the TiC. © 2017 Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.742.99
  • Characterization of recycled TiC and its influence on the microstructural, tribological, and corrosion properties of a TiC-reinforced metal matrix composites
    Mohr, A. and Röttger, A. and Theisen, W.
    Journal of Composite Materials 51 (2017)
    Ferro-Titanit® is a metal matrix composite (MMC) with a high wear and corrosion resistance. It contains TiC as hard particles on account of their high hardness, good corrosion resistance, and low density. This wear- and corrosion-resistant material is amenable to machining in the soft-annealed state, which gives rise to chips containing a large amount of the expensive TiC hard particles. Due to the cost of TiC, there is great interest in recycling the TiC from these chips so that it can be reused in the production of further Ferro-Titanit® materials. In this study, the recycled TiC [(Ti,X)C] is investigated with regard to morphology, particle size, chemical composition, and phases, and the results were compared to industrially produced TiC. In the next step, the (Ti,X)C was reused in the production of new Ferro-Titanit®. The Ferro-Titanit® reinforced with (Ti,X)C was also characterized with respect to microstructure, wear behavior, and corrosion resistance. Our investigations identified a change in the chemical composition of the TiC as a result of diffusion processes and a decrease in TiC particle size with respect to the initial state. The change in morphology and size of TiC during the recycling process influences the microstructure and the material behavior of the MMC containing recycled TiC. © 2017, © The Author(s) 2017.
    view abstract10.1177/0021998317692032
  • Correlation between cavitation erosion resistance and cyclic mechanical properties of different metallic materials
    Kaufhold, C. and Pöhl, F. and Theisen, W.
    Journal of Physics: Conference Series 843 (2017)
    Machine components in contact with flowing fluids are especially prone to cavitation erosion, where plastic deformation and material loss occur due to the repeated implosion of cavitation bubbles in the vicinity of a solid surface. Identifying a correlation between experimentally derivable material properties and resistance against cavitation erosion could help improve the lifetime of cavitation-affected components. Cavitation erosion is a predominantly fatigue-driven phenomenon. In this investigation, we conducted nanoindentation experiments to examine cyclic micromechanical material properties in response to an increasing number of cycles. The experiments were performed on pure iron and different steel grades, i.e., austenitic stainless CrMnCN steels, interstitially alloyed with carbon and nitrogen. We confirmed the view, also proposed in literature, that indentation hardness is inappropriate for ordering the investigated materials by incubation period or maximum erosion rate. We found that the percentage increase of nanoindentation contact stiffness, after an increasing number of cycles, is a promising indicator in terms of the overall ranking of cavitation erosion resistance among the considered materials. Although a single cavitation impact is associated with a significantly higher strain rate than nanoindentation experiments, it is shown that the plastically deformed area around each indent exhibits indications of deformation, such as the formation of slip lines that are also observable after cavitation-induced impacts. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/843/1/012037
  • Corrosion behavior of 316L austenitic steel processed by selective laser melting, hot-isostatic pressing, and casting
    Geenen, K. and Röttger, A. and Theisen, W.
    Materials and Corrosion 64 (2017)
    This study investigated the corrosion behavior of grade 316L austenitic steel processed by casting, hot isostatic pressing (HIP), selective laser melting (SLM), and SLM+HIP. Electrochemical results showed that the SLM-densified specimen exhibited poorer corrosion resistance than specimens processed by casting and hot isostatic pressing in solution-annealed condition. Microstructural investigations revealed that the SLM-densified specimen had a fine-grained microstructure but comparatively higher porosity, which negatively influenced corrosion resistance. Additional HIP treatment further worsened corrosion resistance. The HIP process does not significantly reduce porosity compared to the SLM process, which can be attributed to the argon atmosphere used when manufacturing the SLM samples. Nevertheless, it was possible to reduce the crack density via HIP treatment and the formerly lamellar oxides underwent spheroidization. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/maco.201609210
  • Corrosion resistance of high-alloyed stainless steel membrane supports under flue gas conditions of a lignite-fueled power plant
    Bram, M. and Seifert, M. and Kot, A. and Wilkner, K. and Wulbieter, N. and Theisen, W.
    Materials and Corrosion (2017)
    Metal-supported silica membranes are attractive candidates for CO2 capture from the exhaust of coal-fueled power plants. Compared to their full ceramic counterparts, the introduction of the metal support facilitates sealing of the membrane by established technologies, such as welding, and enhances the robustness of the membrane in the harsh environment of the power plant. As well-known from other steel components in flue gas desulfurization units, long-term corrosion resistance of the metal support is mandatory for the success of this new membrane concept. In the present work, a research concept is introduced enabling a systematic benchmark of stainless steels regarding their suitability to be used for the metal support of the CO2 selective silica membranes. The study combines field tests of porous samples in direct contact with the exhaust gas of a lignite-fueled power plant and standardized corrosion tests of dense and porous samples in the laboratory according to DIN 50918 using exhaust gas condensate as the corrosive medium. Preliminary results are achieved on austenitic steel (AISI 316L) as well as on two ferritic steels (Crofer22APU, Plansee ITM). Ferritic steels are chosen due to their availability as substrates with well-defined porosity and with adapted thermal expansion coefficient enabling successful coating of the CO2 selective silica membrane. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/maco.201709456
  • Diffusion processes during cementite precipitation and their impact on electrical and thermal conductivity of a heat-treatable steel
    Klein, S. and Mujica Roncery, L. and Walter, M. and Weber, S. and Theisen, W.
    Journal of Materials Science 52 (2017)
    The thermal conductivity of heat-treatable steels is highly dependent on their thermo-mechanical history and the alloying degree. Besides phase transformations like the martensitic γ → α ′ or the degree of deformation, the precipitation of carbides exerts a strong influence on the thermal conductivity of these steels. In the current work, thermal and electrical conductivity of a 0.45 mass% C steel is investigated during an isothermal heat treatment at 700 ∘C and correlated with the precipitation kinetics of cementite. To include processes in the short-term as well as in the long-term range, annealing times from 1 s to 200 h are applied. This investigation includes microstructural characterization, diffusion simulations, and electrical and thermal conductivity measurements. The precipitation of carbides is connected with various microstructural processes which separately influence the thermophysical properties of the steel from the solution state to the short-term and long-term annealing states. In the early stages of cementite growth, an interstitial-dominated diffusion reaction takes place (carbon diffusion in the metastable condition of local equilibrium non-partitioning). Afterwards, substitutional-dominated diffusion controls the kinetics of the reaction. The electrical and thermal conductivity increase differently during the two stages of the carbide precipitation. The increment is associated to the binding of alloying elements into the carbides and to the reduction of the distortion of the martensitic matrix. Both factors increase the electron density and reduce the electron and phonon scattering. The correlation of the precipitation kinetics and the thermophysical properties are of general interest for the design of heat-treatable steels. © 2016, The Author(s).
    view abstract10.1007/s10853-016-0338-1
  • Effect of matrix and hard phase properties on the scratch and compound behavior of wear resistant metallic materials containing coarse hard phases
    Pöhl, F. and Mohr, A. and Theisen, W.
    Wear 376-377 (2017)
    Modern abrasion resistant materials, such as cold work tool steels, consist of higher amounts of hard phases embedded in a softer metallic matrix. Hence, the wear behavior is controlled by the mechanical compound properties, which are dependent on the single phase properties (e.g. hard phase type and matrix condition), volume content, morphology, and distribution. For the development of materials with adapted tribological properties and for a better understanding of the wear processes these influencing factors and their complex interactions must be known precisely. In this study, the effect of matrix and hard phase properties and volume content on scratch behavior and the mechanical compound properties are experimentally and numerically investigated for a hot work tool steel containing coarse hard phases. Spark Plasma Consolidation (SPC) was used to produce microstructures consisting of hot work tool steel matrix and different embedded hard phases. The matrix condition was controlled by subsequent heat treatment (hard and softer matrix condition). To analyze the single phase properties nanoindentation, scratch testing, and atomic force microscope investigations were conducted. Numerical simulations (Finite-Element-Method) show that the mechanical compound properties can also be predicted on the basis of measured single phase properties. The results reveal the interaction between single phase and compound properties and their effects on the scratch behavior. © 2016 Elsevier B.V.
    view abstract10.1016/j.wear.2016.10.028
  • Energy and time saving processing: A combination of hot isostatic pressing and heat treatment
    Weddeling, A. and Theisen, W.
    Metal Powder Report 72 (2017)
    The technique of hot isostatic pressing (HIP) is used to simultaneously heat up and apply high isostatic pressure to a metallic or ceramic entity in order to reach compaction. The integration of heat treatment into the compaction process generates much curiosity in the HIP community. This paper resumes the different methods for accelerated cooling and gives an overview of the research done with integrated heat treatment. Results in the field of steels, lightweight alloys and superalloys are resumed. © 2016 Elsevier Ltd.
    view abstract10.1016/j.mprp.2016.04.001
  • Excavation tool concepts for TBMs – Understanding the material-dependent response to abrasive wear
    Küpferle, J. and Röttger, A. and Theisen, W.
    Tunnelling and Underground Space Technology 68 (2017)
    Wear of cutting tools for tunneling applications can lead to decreased advance rates and unscheduled downtimes that are associated with increased tunneling times and project costs. During the planning phase, wear of tools and their associated lifetime and replacement times are estimated on the basis of the ground that is to be excavated. However, from the viewpoint of materials technology, this procedure is insufficient because it is essential to take account of the interactions between tool material, ground, and the acting wear mechanisms on the microscopic scale, such as abrasion, fatigue, or forced fracture. The respective tool materials feature different tribomechanical properties and thus different wear mechanisms and rates that depend on the ground to be mined. Low wear rates can only be achieved using an optimized tool material concept that is adapted to the acting ground and the associated tribological system. This requires a comprehensive understanding of the wear behavior of the respective materials. This article focuses on the different, commonly used tool concepts and their microstructure. Interactions of the microstructure of these materials with the abrasive particles and the associated microwear mechanisms are analyzed. The results provide a deeper understanding of the wear process of excavation tools depending on the respective tool and the material concept. The discussed correlations are illustrated by results from the RUB Tunneling Device and nanoscratch tests, which are used to map the tribological TBM tool system on the macroscopic and microscopic scales. © 2017 Elsevier Ltd
    view abstract10.1016/j.tust.2017.05.013
  • Fatigue and surface spalling of cemented carbides under cyclic impact loading – Evaluation of the mechanical properties with respect to microstructural processes
    Küpferle, J. and Röttger, A. and Theisen, W.
    Wear 390-391 (2017)
    Cemented carbides used in excavation tools have to feature particular mechanical properties to withstand impact loads and abrasive degradation processes. It is assumed that a high fracture toughness is important to counteract brittle material failure due to forced rupture (supercritical loads) or material fatigue caused by subcritical impacts or cyclic loads. The fatigue-induced failure mechanism on a microstructural scale (evolution of fracture) in cemented carbides is discussed controversially in the literature. We are thus focusing on fatigue-initiated surface degradation of cemented carbide grades during ground excavation. Various cemented carbide grades were loaded with cyclic subcritical impact loads that lead to microstructural damage. The crack path was analyzed by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The number of endured load cycles is further correlated with the microstructural composition and the resulting mechanical properties of the tested grades. The results demonstrate that the commonly used correlation between resistance against fatigue load and fracture toughness KIC does not seem to be suitable for describing subcritical crack growth in cemented carbides due to cyclic loads that lead to surface spalling and microfatigue. The cemented carbide grades with the highest KIC values do not possess the highest resistance against surface spalling and thus the highest fatigue resistance. The influence of the microstructural properties (dWC, VCo, λCo) on the results has been shown. Furthermore, the correlation between fatigue sensitivity and crack path fractions was analyzed. © 2017 Elsevier B.V.
    view abstract10.1016/j.wear.2017.07.002
  • Hardness and Microstructure of a Newly Developed Stainless Steel after Ausforming
    Seifert, M. and Botzet, M. and Theisen, W.
    Steel Research International (2017)
    In this work, ausforming is applied to a newly developed stainless steel. This process consists of austenitisation, quenching to a deformation temperature above room temperature, deformation of the metastable austenitic microstructure without the formation of martensite, and subsequent quenching in liquid nitrogen. The investigated steel is explicitly developed to be processed by ausforming and manufactured as a laboratory size test melt. The aim is to achieve a steel having a high hardness as well as a high corrosion resistance. Instead of conventional quenching and tempering, conventional processing is followed by ausforming. A parameter study incorporating the austenitisation temperature and time, deformation temperature, deformation speed, and degree of deformation is performed to achieve maximum hardness. Furthermore, the influence of soft annealing prior to ausforming is also investigated. The hardness of ausformed specimens is measured and correlated to the parameters used for processing. The microstructure of selected specimens is also investigated. Surprisingly, small amounts of martensite are found after ausforming, although a hardness of about 600 HV10 is achieved. In fact, a highly deformed austenitic microstructure is found predominantly. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/srin.201700010
  • Influence of temperature and tempering condition on the thermal conductivity of hot work tool steels for hot stamping applications
    Hafenstein, S. and Werner, E. and Wilzer, J. and Theisen, W. and Weber, S. and Sunderkötter, C. and Bachmann, M.
    HTM - Journal of Heat Treatment and Materials 72 (2017)
    Press hardening is used to produce automotive blanks with high tensile strength. In order to gain optimal blank properties it is necessary to rapidly decrease the blank temperature during press hardening. The thermal conductivity of the hot work tool steel used for die material is an important factor that defines the process time and the resulting blank properties. This study investigates the influence of temperature, alloying composition and heat treatment on the thermal conductivity of hot work tool steels used for press hardening dies. The dynamic method is used to determine the thermal conductivity of the hot work tool steels via an indirect measurement. The thermal conductivity decreases with increasing amount of alloying elements. In the temperature range between 295 and 473 K, which is the relevant temperature range for press hardening applications, the thermal conductivity of the hot work tool steels increases with temperature independent of the thermal treatment. With further increase of the temperature above 473 K the thermal conductivity of the hot work tool steels decreases. Copyright © 2017 Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/105.110319
  • Investigation of the tribological behaviour of HS6-5-3 type tool steels during high-temperature sliding wear
    Walter, M. and Egels, G. and Boes, J. and Röttger, A. and Theisen, W.
    HTM - Journal of Heat Treatment and Materials 72 (2017)
    The wear behaviour of work roll materials is an important issue during the hot rolling process of metals. For this reason, the present study investigates the mechanical properties and the sliding wear behaviour of HS6-5-3 type high-speed steels (HSS) at elevated temperatures. Influences on the performance of HSS discussed are the microstructural constitution (as-cast and electro-slag remelted condition), the heat-treatment condition and the overall tribological system (C60 carbon steel and X5CrNi18-10 stainless steel counter-body materials). The results of the study show, how mechanical and tribological properties of HSS depend on these aspects and how a modification of the processing route can lead to improved high-temperature properties of HSS. As a main aspect the investigations show that, the formation of tribochemical wear layers during high-temperature sliding wear needs to be regarded. Tribochemical wear layers dominate the high-temperature wear behavior of steels. Therefore, the formation, characteristics and stability of tribochemical wear layers are analysed [1]. Copyright © 2017 Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/105.110315
  • Manufacturing of hard composite materials on Fe-base with oxide particles
    Brust, S. and Röttger, A. and Kimm, J. and Usta, E. and Theisen, W.
    Key Engineering Materials 742 KEM (2017)
    Metal matrix composites (MMC) are often applied to tool surfaces to increase resistance to wear and tear. However, some matrix and particle materials such as Ni, Co, WC or TiC are expensive and partly classified as critical elements. With respect to tribo-mechanical properties, Fe-alloys reinforced with oxide particles are promising compound materials to produce wear-resistant MMC with low-cost and readily available materials. However, thus far the technical application of such MMCs is limited due to poor wettability of the oxides by Fe-base melts and an associated weak bonding between the oxide particles and the metal matrix phases. In this work two novel production techniques (namely pre-metallization and active sintering) are introduced, which improve the wettability and interfacial reactions between both materials and therefore enable supersolidus liquid-phase sintering (SLPS) of the MMC. For the first technique the oxide particles are pre-metallized by depositing a thin film of TiN on the surfaces. The second technique is called active sintering. For this technique the alloy design is adapted from active brazing, so that wettability of the oxide particles by the alloy-melt is increased. The resulting effects of these techniques are investigated using wetting and sintering experiments, and are analyzed with respect to the developed microstructures and interfacial reactions between the oxide particles and the metallic phases. © 2017 Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.742.106
  • Mechanical properties of AlXFeY intermetallics in Al-base coatings on steel 22MnB5 and resulting wear mechanisms at press-hardening tool steel surfaces
    Windmann, M. and Röttger, A. and Hahn, I. and Theisen, W.
    Surface and Coatings Technology 321 (2017)
    Hard and brittle intermetallic AlXFeY phases formed in the Al-base coating applied on high-strength steel 22MnB5 promote strong wear of press-hardening tools during forming and quenching (approx. 800–100 °C). In this study, bulk materials of the intermetallic phases Al13Fe4, Al5Fe2, Al2Fe, and AlFe were produced by remelting stoichiometric powder mixtures. These were then used for mechanical and wear investigations. We found that the dominating wear mechanisms on the tool steel surfaces are strongly influenced by the temperature and depend on the mechanical properties of the respective intermetallic phases. Phases of type Al13Fe4, Al5Fe2, and Al2Fe possess a high hardness of 850–1090 HV0.5 and a low fracture toughness of 0.9–1.6 MPa √ m at room temperature, whereas the AlFe phase has a much lower hardness (520 HV0.5) and a higher fracture toughness (26 MPa √ m ). The hardness of all phases decreases with increasing temperature. At high temperatures (500–800 °C), the intermetallic phases lead mainly to adhesive wear of the tool steel surfaces. At lower temperatures, also abrasive wear occurs due to delamination of hard and brittle intermetallic particles. We found that abrasive wear of the tool steel surface could be decreased by adapting the phase composition in the Al-base coating. © 2017
    view abstract10.1016/j.surfcoat.2017.04.075
  • Mechanisms of severe sliding abrasion of single phase steels at elevated temperatures: Influence of lattice structure and microstructural parameters
    Walter, M. and Weber, S. and Boes, J. and Egels, G. and Theisen, W.
    Wear 376-377 (2017)
    Due to the complex influence of elevated temperatures on the characteristics of a tribological system, severe high temperature sliding abrasion of single phase metals is a unique type of wear. The mechanisms of high temperature sliding abrasion (indentation and grooving of metallic surfaces) are strongly governed by the temperature-dependent interaction between the bulk metal and the abrasive during the wear process. This interaction can be correlated with the metal physical and microstructural parameters of the worn metal, which consequently greatly influence abrasive wear processes. In this context, the present study deals with the influence of microstructural aspects of single phase steels on the mechanisms of high temperature abrasion. Investigations focus on the aspects of abrasion by performing high temperature hardness and sliding wear experiments (two-body, ceramic counter body) on bcc and fcc steels. Results confirm a clear lattice-structure dependence of the abrasion behavior of steels. Major differences exist in the stability of the mechanical and tribological properties of the bcc and fcc materials investigated. Hardness and work hardening of bcc steels decrease above 500 °C, leading to non-stationary wear. In contrast, fcc steels show a steady decrease of mechanical properties, avoiding instabilities. Accordingly, wear experiments and investigations of the wear scars (surface and subsurface regions) show a higher wear resistance and more favorable mechanisms of high temperature abrasion of fcc steels (e.g. pronounced micro-ploughing). Further, the microstructural elements of fcc steels high temperature abrasion resistance are investigated in more detail using X-ray diffraction. Microstructural analysis using diffraction-line broadening (Rietveld analysis) is used to determine the degree of plastic deformation (microstrain) and the phase fraction of α′-martensite of the austenitic wear scars. These parameters are related to the present mechanisms of abrasion, explaining the high temperature wear properties of fcc steels. © 2017 Elsevier B.V.
    view abstract10.1016/j.wear.2017.01.043
  • Microstructure and mechanical properties of the heat-affected zone in laser-welded/brazed steel 22MnB5–AA6016 aluminum/AZ31 magnesium alloy
    Windmann, M. and Röttger, A. and Kügler, H. and Theisen, W.
    Journal of Materials Processing Technology 247 (2017)
    The martensitic microstructure of the steel 22MnB5 was tempered during laser welding/brazing. The strength of the HAZ greatly decreased from 1500 MPa to 800–1100 MPa, depending on the heat input. The lowest strength always occurred in the area with the highest heat input directly beside the welding zone. The strength of the aluminum alloy was slightly reduced from 233 MPa to 212 MPa. The strongest decrease in the strength did occur in the area with a critical temperature range of 400–500 °C due to the coarsening of GP zones. The short heat input in the laser welding/brazing process did not lead to a significant change in the material strength and microstructure in the HAZ of the AZ31 magnesium alloy. © 2017 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2017.04.008
  • Numerical simulation of the deformation behavior of metallic materials under cavitation induced load in the incubation period
    Kaufhold, C. and Pöhl, F. and Mottyll, S. and Skoda, R. and Theisen, W.
    Wear 376-377 (2017)
    Numerical simulations are performed to improve the understanding of the pit formation in metallic materials due to cavitation-induced impacts. Under the influence of cavitation, metallic materials show plastically deformed surface regions, called pits, which are created due to the high impact pressure induced by a cavitation bubble collapse. Both the observation of the collapse and the measurement of the wall-load development require complex experiments. In this study, isolated wall-near bubble collapses are calculated with the help of CFD-simulations to evaluate the radial pressure distribution regarding the temporal maximum of pressure, further referred to as “wall-load profile”. The resulting wall-load profiles show a different number of local maxima, depending on the dominant hydrodynamic mechanism, i.e. liquid jet- or shock wave impact. With regard to the corresponding standoff-distance L0/R0, different cases of possible wall-load profiles are classified. These wall-loads are implemented into a Finite-Element (FE) software to visualize the elastic and plastic material response for three different model materials. The resulting pit geometries and the corresponding plastic strain distributions vary in accordance to the applied wall-load profile, which is directly affected by the dominant hydrodynamic mechanism. In certain cases, the resulting pits in the material show plateau-like displacements with crescent-shaped plastic strain distributions, which are related to the collapse of a toroidal bubble fragment. In addition to this, cavitation pits are generated in a copper specimen by short-term cavitation experiments. Selected pits are exemplarily measured by atomic force microscopy and then qualitatively compared to the numerically calculated pit geometries. © 2017 Elsevier B.V.
    view abstract10.1016/j.wear.2016.11.024
  • Rejuvenation of creep resistance of a Ni-base single-crystal superalloy by hot isostatic pressing
    Ruttert, B. and Bürger, D. and Roncery, L.M. and Parsa, A.B. and Wollgramm, P. and Eggeler, G. and Theisen, W.
    Materials and Design 134 (2017)
    Ni-base single-crystal turbine blades are exposed to a combination of high temperatures and high stresses during their service life in high-pressure turbines of aero engines or stationary gas turbines. This unavoidably leads to various internal microstructural changes such as rafting and the formation of cavities. This study introduces a creep-rejuvenation-creep test cycle using one miniature Ni-base single-crystal creep specimen. A novel hot isostatic press providing high quenching rates was applied to rejuvenate the damaged microstructure of the specimen after the first high-temperature creep degradation before the same specimen was repeatedly creep-tested under the same initial creep conditions. After rejuvenating, microstructural results obtained from high-resolution microscopy prove that the creep cavities were closed, dislocation densities were re-set, and the original but now slightly finer γ/γ′-microstructure was restored without any recrystallization. The subsequent creep test of the rejuvenated specimen demonstrated that the proposed rejuvenation procedure in this work is a suitable method to reproduce the initial creep behavior and to thus prolong the lifetime of an already crept Ni-base single-crystal specimen. © 2017 Elsevier Ltd
    view abstract10.1016/j.matdes.2017.08.059
  • The impact of cold work and hard phases on cavitation and corrosion resistance of high interstitial austenitic FeCrMnMoCN stainless steels
    Niederhofer, P. and Huth, S. and Theisen, W.
    Wear 376-377 (2017)
    High interstitial austenitic stainless steels have been shown to exhibit superior mechanical properties, which include a unique combination of high strength and high toughness, due to the positive effects of the combination of carbon and nitrogen in solid solution. By the addition of molybdenum, a significant improvement of their localized corrosion resistance has been achieved. Further strengthening is necessary in the case of application in environments featuring both high mechanical loads and corrosive attack, e.g. in bearings in sea water. It can be induced by cold work hardening as well as the addition of hard phases, which in turn can affect the wear and corrosion resistance. In this study, the impact of cold work and the precipitation of niobium carbonitrides on the resistance to cavitation erosion, localized, and general corrosion has been investigated. Two newly developed high interstitial FeCrMnMoCN steels were analyzed by vibratory cavitation testing in distilled water and potentiodynamic polarization measurements in sodium chloride solution and sulfuric acid after cold work strengthening. The latter was induced by cold rolling to different degrees. Microstructural characterization was performed by hardness testing, optical, and scanning electron microscopy. The results show improved strength but decreased cavitation erosion resistance caused by the hard phases. In contrast, neither the localized nor the general corrosion resistance seem severely affected. The cold rolling leads to intense work hardening and enhanced cavitation erosion resistance, while the corrosion behavior is not significantly influenced. In the case of cavitation erosion, the improved resistance of the cold work hardened steel matrix seems to dominate the negative effect of the hard phases. The combination of high wear and high corrosion resistance, even in severely cold work strengthened condition, makes the FeCrMnMoCN austenitic stainless steels promising candidates for application in harsh environments. © 2017 Elsevier B.V.
    view abstract10.1016/j.wear.2017.01.016
  • Topological phase inversion after long-term thermal exposure of nickel-base superalloys: Experiment and phase-field simulation
    Goerler, J.V. and Lopez-Galilea, I. and Mujica Roncery, L. and Shchyglo, O. and Theisen, W. and Steinbach, I.
    Acta Materialia 124 (2017)
    Ni-base superalloys are materials which are designed to resist extreme thermal and mechanical conditions. In this regard, an essential factor is their microstructure consisting of γ′ precipitates embedded in a γ matrix. The application of superalloys at high temperatures can however induce the topological phase inversion, where the γ′-phase topologically becomes the matrix phase, resulting in subpar material properties. In this work, the topological inversion is analyzed via experiment and phase-field simulation. The evolution of the microstructure has been quantified in the second generation single crystal Ni-base superalloy ERBO/1, which belongs to the family of CMSX-4, submitted to long-term aging at 1100° C for up to 250 h. Phase-field simulations carried out using a multi phase-field approach deliver insight into the microstructure evolution driven by the loss of coherency of the γ′ precipitates, which is induced by the accumulation of dislocations at the γ/γ′ interfaces. The obtained simulation results are in good agreement with the experimental results, and indicate that the mechanisms causing the topological inversion are linked to the accommodation of the lattice misfit, which enables coalescence and ripening of γ′ precipitates. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.10.059
  • An investigation of the tribological behaviour of high-speed tool steels at elevated temperatures
    Walter, M. and Egels, G. and Boes, J. and Röttger, A. and Theisen, W.
    HTM - Journal of Heat Treatment and Materials 71 (2016)
    The tribological behaviour of work roll materials is a key issue during hot rolling process of metals. The characteristics of the material (hardness and wear resistance) at elevated temperatures are of great interest for many industrial applications. The study investigates the mechanical properties and the sliding wear behaviour of HS 6-5-3-5 (HSS) high-speed tool steel, which is a common work roll material of the intermediate and finishing stands of hot rolling manufacturing lines. Experimental analysis focuses on the mechanical properties of steel HS 6-5-3-5 at elevated temperatures and on the microstructural surface changes of this material during metallic sliding wear. The results give an overview about the absolute hardness value of HS 6-5-3-5 at elevated temperatures and its evolution at constant operating temperatures. To conclude interdependencies between mechanical properties, microstructure and wear behaviour at elevated temperatures, results are discussed and connected with wear investigations. Findings reveal that high temperature wear behaviour is mainly dependent on the formation of a tribochemical wear layer on the wear bodies' surfaces. Layers suppress direct metallic contact and change the characteristics of the tribological system. Discussed issues of high temperature sliding wear are the formation and stability of tribochemical wear layers, their connection to and support by the bulk material, as well as the fracturing and damage of the layer-bulk-material compound. © Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/105.110290
  • Comparison of microstructure and mechanical properties of 316 L austenitic steel processed by selective laser melting with hot-isostatic pressed and cast material
    Röttger, A. and Geenen, K. and Windmann, M. and Binner, F. and Theisen, W.
    Materials Science and Engineering A 678 (2016)
    Besides the chemical composition, the manufacturing route primarily determines a material's properties. In this work, the influence of the manufacturing process of the 316 L grade austenitic steel on the microstructure and the resulting material properties were investigated. Thus, the microstructure and mechanical properties of cast and solution annealed, as well as steel powder densified by hot-isostatic pressing (HIP), selective laser melting (SLM) and SLM+HIP, were compared. A SLM parameter study illustrates that the porosity of SLM-densified specimens can be reduced with direction of a higher exposure time and a smaller point distance. With an additional treatment by HIP, the porosity scarcely changes, while cracks are reduced. The mechanical properties were investigated depending on the manufacturing process, and the influence of the sample build up by SLM was examined. High mechanical values have been obtained; in particular, the yield strength in the SLM-densified condition is much higher than in cast or HIP condition, as a result of the smaller grain size. © 2016 Elsevier B.V.
    view abstract10.1016/j.msea.2016.10.012
  • Densifying and hardening of martensitic steel powders in HIP units providing high cooling rates
    Weddeling, A. and Wulbieter, N. and Theisen, W.
    Powder Metallurgy 59 (2016)
    Hot isostatic pressing (HIP) units are worldwide used for the compaction of metal alloy powders. The cooling rate in a HIP unit is usually comparatively low. This lengthens cycle times and requires an additionally heat treatment for quenched and tempered steels. Novel cooling HIP concepts in HIP units feature high quenching rates. In this study, tool steels were investigated with respect to their time–temperature–transformation behaviour for different cooling parameters. The paper shows that encapsuled powdered tool steels can be compacted and hardened in the HIP unit. The examined steels exhibit a comparable or even a higher hardness and a finer microstructure. HIP units with high-quenching rates enable to compact and heat treat materials in one step. © 2016 Institute of Materials, Minerals and Mining.
    view abstract10.1080/00325899.2015.1109803
  • Detection of the indentation-size-effect (ISE) and surface hardening by analysis of the loading curvature C
    Pöhl, F. and Huth, S. and Theisen, W.
    International Journal of Solids and Structures 84 (2016)
    Numerous materials are affected by the Indentation-Size-Effect (ISE). Thus, the interpretation of ISE-influenced indentation data plays an important role. This paper presents a method which allows for the characterization of the ISE using the loading curvature C of a single load-displacement curve (P-h curve) from sharp indentation. The method is based on the analysis of the change of the parameter C as a function of the indentation depth as described by Kick's law (related to the universal hardness or also called Martens hardness HM). The intensity as well as the indentation depth where the ISE is saturated can be detected. Furthermore, it allows for the correction of ISE-affected loading curves with the use of the Vickers macro hardness. For example, this can enable the application of inverse methods for ISE-affected materials or phases. Mechanically polished samples show an additional increase in strength during nanoindentation due to hardened surface layers. The presented method also accounts for this influence and can correct affected loading curves. It was applied to the austenitic stainless steel X2CrNi18-9 (AISI 304L) which is heavily affected and the C45 carbon steel which is slightly affected by the ISE. The influence of hardened surface layers was investigated using electropolished and mechanically polished AISI 304L. © 2016 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijsolstr.2016.01.024
  • Development of a powder metallurgical corrosion-resistant bearing steel containing NbC and its transfer to industrial applications
    Seifert, M. and Siebert, S. and Theisen, W.
    Materials Science and Technology (United Kingdom) 32 (2016)
    In this work, a new powder metallurgical corrosion-resistant bearing steel containing NbC was developed based on thermodynamical equilibrium calculations using CALPHAD (CALculation of Phase Diagrams) methods. Starting with conventionally cast test melts, a processing route, known as diffusion alloying, was used to achieve a fine dispersion of the NbC. Two different alloys were used to reveal the influence of nickel and cobalt on the system. The resulting properties led to the choice of the alloy containing cobalt for industrial production. The complex processing route was continuously optimised for this purpose. In addition, the heat treatment was adjusted to simultaneously achieve a high hardness and a high corrosion resistance. The promising laboratory results led to the manufacture of bearings, which were investigated in bearing lifetime tests to estimate their usability for applications in seawater. © 2016 Institute of Materials, Minerals and Mining.
    view abstract10.1080/02670836.2015.1118789
  • High-temperature stability and interfacial reactions of Ti and TiN thin films on Al2O3 and ZrO2
    Brust, S. and Röttger, A. and Theisen, W.
    Surface and Coatings Technology 307 (2016)
    Metallic thin films are used in many applications to modify ceramic surfaces. However, during subsequent processing, chemical interactions may change the properties of the coating. In addition, differences in thermal expansion can lead to delamination of the coating. In this study, titanium and titanium nitride thin films were deposited via physical and chemical vapor deposition (PVD and CVD, respectively) on alumina- and yttria-stabilised zirconia substrates, before being heat-treated at 1200 °C or 1500 °C in static argon atmosphere and analysed via SEM, EDS and XRD to investigate the effect of temperature on the thin films. It was shown that the chemical interactions between TiN and both Al2O3 and ZrO2 are weak. However, partial delamination of the TiN coating on alumina was observed after both annealing temperatures. The TiN coating on zirconia remained adherent. In contrast, the Ti coatings underwent a transformation to cubic TiO on both oxide substrates. This was due to partial reduction of the ZrO2 to ZrO2 − x and dissolution of the Al2O3, which leads to a Ti3Al0.9O1.1 interlayer. The TiO coating which formed remained adherent on the alumina at both annealing temperatures, but delaminated from the ZrO2 substrate after annealing at 1500 °C. © 2016 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2016.08.060
  • Impact of hot isostatic pressing on microstructures of CMSX-4 Ni-base superalloy fabricated by selective electron beam melting
    Ruttert, B. and Ramsperger, M. and Mujica Roncery, L. and Lopez-Galilea, I. and Körner, C. and Theisen, W.
    Materials and Design 110 (2016)
    Selective electron beam melting (SEBM) is a powder-bed-based additive manufacturing process that was used to produce cylindrical and columnar-grained parts made of Ni-base superalloy CMSX-4 from pre-alloyed and atomized powder. SEBM is characterized by high temperature gradients during solidification, which results in a very fine microstructure that is several orders of magnitude smaller than in conventionally cast material. This opens up new perspectives regarding time-consuming solution heat treatment. Nevertheless, microstructural heterogeneities, such as segregation and porosity, are still present on a much smaller scale, and also the high susceptibility to cracking of this alloy class during welding has to be taken into account. Since the latest generation of hot isostatic presses (HIP) are able to simultaneously heat-treat and eliminate porosity owing to their quenching capability, such a HIP is used in this work. The influence of different HIPing-heat-treatment-strategies with variation of temperature and time at a constant pressure on the SEBM-microstructure was investigated with emphasis primarily on segregation and porosity. The results demonstrate that only a few minutes of solutioning are sufficient to dissolve segregations and to close pores. The initial degree of homogeneity of the SEBM-material is responsible for the short solutioning-time. © 2016
    view abstract10.1016/j.matdes.2016.08.041
  • Influence of conventional and powder-metallurgical manufacturing on the cavitation erosion and corrosion of high interstitial CrMnCN austenitic stainless steels
    Niederhofer, P. and Richrath, L. and Huth, S. and Theisen, W.
    Wear 360-361 (2016)
    High interstitial CrMnCN austenitic stainless steels combine superior mechanical properties with high resistance to corrosion. The first is caused by the strengthening effect of C and N and the low stacking fault energy leading to intense cold work hardening and e.g. increased resistance to fatigue, which implies a high resistance to cavitation erosion. Corrosion resistance is provided by the elements chromium and molybdenum. Usual manufacturing consists of casting, often followed by hot working. An alternative approach uses pre-alloyed, gas-atomized powders, which can be compacted either by hot isostatic pressing or supersolidus liquid phase sintering. The latter provides the possibility of adapting the nitrogen content via sintering atmosphere. This results in fully dense materials exhibiting comparable mechanical properties like the cast and hot worked alloys. In this study, high interstitial CrMnCN steels with different nitrogen contents were tested in an ultrasonic vibratory cavitation rig and analyzed by electrochemical polarization measurements using different electrolytes. The results indicate positive influences of increasing nitrogen content on both cavitation erosion and corrosion resistance. A comparison with cast and hot worked alloys is included. © 2016 Elsevier B.V.
    view abstract10.1016/j.wear.2016.04.017
  • Influence of crystallographic orientation on cavitation erosion resistance of high interstitial CrMnCN austenitic stainless steels
    Niederhofer, P. and Pöhl, F. and Geenen, K. and Huth, S. and Theisen, W.
    Tribology International 95 (2016)
    The excellent resistance of high interstitial CrMnCN austenitic stainless steels to cavitation erosion was ascribed to strengthening by C+N and the low stacking fault energy causing improved resistance to fatigue and superior mechanical properties. Previous investigations revealed correlation of crystallographic orientation and cavitation erosion damage. In this study, different CrMnCN steels were investigated by ultrasonic cavitation testing. Results were correlated with indentation-derived properties. Alteration of the surface during cavitation was examined by scanning electron microscopy and electron back scatter diffraction. The investigations show influences of crystallographic orientation on the cavitation resistance of individual grains. A relationship between cavitation resistance and hardness and elastic indentation energy was derived in earlier research work. This study shows similar relationship for individual grains. © 2015 Published by Elsevier Ltd.
    view abstract10.1016/j.triboint.2015.11.002
  • Influence of temperature, pressure, and cooling rate during hot isostatic pressing on the microstructure of an SX Ni-base superalloy
    Mujica Roncery, L. and Lopez-Galilea, I. and Ruttert, B. and Huth, S. and Theisen, W.
    Materials and Design 97 (2016)
    This work investigates the application of hot isostatic pressing for heat treatment of the single-crystal Ni-base superalloy ERBO/1. Recent progress regarding incorporation of quenching within hot isostatic pressing enables heat treatments to be performed so that the microstructures can be frozen at a desired point. The influence of the temperature, pressure, and cooling method (quenching, natural convection, and slow cooling) as well as the cooling rate after solutioning-HIP treatment on pore densification and γ/. γ'-morphology was measured. Temperatures above γ'-solvus resulted in a greater efficiency of the porosity reduction. At super-solvus temperatures, pressures above 75. MPa are sufficient enough to annihilate the porosity. The cooling rate after HIP-solutioning treatment has a major influence on the γ'-particle size and shape. Quenching with 45-20. K/s at 100. MPa leads to high number density and monomodal distribution of γ'-particles with sizes around 140. nm. In contrast, slow cooling rate of 0.33. K/s leads to γ'-precipitate sizes of 720. nm. Moreover, an integrated heat treatment at 100. MPa, which consisted of solutioning and aging in the HIP, was successfully applied. It led to smaller γ'-particle sizes and narrower γ-channels compared to the conventionally heat-treated material and also almost no porosity. © 2016 Elsevier Ltd.
    view abstract10.1016/j.matdes.2016.02.051
  • Laser beam welding of magnesium to coated high-strength steel 22MnB5
    Windmann, M. and Röttger, A. and Kügler, H. and Theisen, W.
    International Journal of Advanced Manufacturing Technology 87 (2016)
    Magnesium alloy AZ31 was laser-welded to AlSi10Fe3-coated high-strength steel 22MnB5. The surface of the 22MnB5 steel sheet was treated by sandblasting before welding. Laser welding of magnesium with steel was found to be a welding-brazing process, due to the large difference in the melting temperatures of steel and magnesium. The liquid magnesium was wetted on the solid steel surface, which thus was a brazing process on the steel side. AZ61 magnesium welding filler was used to improve adhesion of the liquid magnesium to the steel surface. Furthermore, the suitability of using a flux was determined. Welded steel/magnesium joints with a high tensile strength of 2680–3090 N (178–213 MPa) were produced using a flux and inductive preheating of the conditioned steel surface. Under tensile loading, the welded joints did not fail at the steel/magnesium interface, but in the AZ61 magnesium welding filler. Metallurgical bonding at the magnesium/steel interface was attributed to the formation of a thin Al-Fe-rich layer, which is due to the alloyed aluminum (6 mass%) in the magnesium welding filler. © 2016, Springer-Verlag London.
    view abstract10.1007/s00170-016-8682-5
  • Mechanism of the Fe3(B,C) and Fe23(C,B)6 solid-state transformation in the hypoeutectic region of the Fe-C-B system
    Lentz, J. and Röttger, A. and Theisen, W.
    Acta Materialia 119 (2016)
    This study investigates the microstructural mechanisms involved in the solid-state transformation of the Fe3(B,C) → Fe23(C,B)6 phases in the hypoeutectic region of the iron-carbon-boron (Fe-C-B) system. We analyzed the influence of different initial microstructural characteristics on the Fe3(B,C) → Fe23(C,B)6 transformation with regards to the matrix phase, matrix C content, B/(C + B) ratio, and agglomeration of the parental Fe3(B,C) phase. We performed thermodynamic calculations using the CALPHAD method, validated by laboratory melts with varying B/(B + C) ratios. These laboratory melts were then microstructurally characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and wavelength-dispersive X-ray spectroscopy (WDS). We particularly focused on solid-state transformation of borides and carboborides of type M3(C,B) and M23(C,B)6 in the hypoeutectic region of the ternary system Fe-C-B, investigated via both in situ and ex situ XRD measurements. It was found that the solid-state transformations are influenced by enriched B inside the eutectic structure, a result of solidification. This increased B content is not reduced in solid state due to the kinetic limitations of B and C inside the hard-phase structure. Thus phase stability is subject to local equilibria depending on the local C and B concentration of the hard-phase structure. In this process the Fe23(C,B)6 phase also forms a shell-like structure surrounding the Fe3(B,C) and Fe2B phases. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.08.009
  • Modeling Hardenable Stainless Steels Using Calculated Martensite Start Temperatures in Thermodynamic Equilibrium Calculations
    Seifert, M. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 47 (2016)
    In this work, martensite start temperatures of several martensitic stainless steels containing different amounts and types of carbides were calculated by means of thermodynamic equilibrium calculations. Two different equations were introduced into the Thermo-Calc® software. The calculations were performed for the respective compositions at austenitization temperature and compared to martensite start temperatures measured using a quenching dilatometer. The purpose was to estimate hardenability and hardness of newly developed steels. Even though the equations used were determined empirically for specific alloying systems, general trends for the investigated steels were found to be reproduced very well. Thus, the comparison of martensite start temperatures of different steels in comparable alloying systems is highly effective for modeling new steels and for predicting their hardenability. © 2016, The Minerals, Metals & Materials Society and ASM International.
    view abstract10.1007/s11661-016-3805-z
  • On the Effect of Hot Isostatic Pressing on the Creep Life of a Single Crystal Superalloys
    Mujica Roncery, L. and Lopez-Galilea, I. and Ruttert, B. and Bürger, D. and Wollgramm, P. and Eggeler, G. and Theisen, W.
    Advanced Engineering Materials 18 (2016)
    The creep behavior of a single-crystal Ni-base superalloy in two microstructural states is compared. One is obtained by casting followed by a conventional heat treatment. The other results from the same nominal heat treatment integrated into a hot isostatic pressing process. The microstructure after HIP differed from that in the conventional route in two respects. First, the γ′ particles are smaller and the γ channels are narrower. Second, after HIP, the number density of pores is lower and the pore sizes are smaller. The HIP microstructure improves creep in two respects: the finer γ/γ′-microstructure results in lower minimum creep rates. Moreover, the shrinkage of cast porosity during HIP delays the nucleation and growth of micro cracks and results in higher rupture strains in the low-temperature high stress regime. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adem.201600071
  • Removal of oxides and brittle coating constituents at the surface of coated hot-forming 22MnB5 steel for a laser welding process with aluminum alloys
    Windmann, M. and Röttger, A. and Kügler, H. and Theisen, W.
    Surface and Coatings Technology 285 (2016)
    The surface of a press-hardened steel 22MnB5 coated with Al-base (AlSi10Fe3) and Zn-base (ZnNi10) was conditioned by a pulse laser and by sandblasting to remove undesirable oxides and brittle phases. Oxides formed on coating surfaces counteract the wettability of welding filler during a welding or brazing process. Furthermore, welding and brazing joints of 22MnB5 coated with aluminum alloys failed along the brittle intermetallic phases in the coating under a low mechanical load. Treated 22MnB5 surfaces were analyzed microscopically, and the phase compositions were investigated by synchrotron diffraction measurements. It was found that brittle phases could be locally removed by laser ablation; however, high laser energies led to remelting and oxidation of the coating surface. In contrast, sandblasting homogenously removed oxides and brittle intermetallic phases. Surface-treated 22MnB5 steel sheets were joined to AA6016 aluminum sheets by laser welding, and the strength of the weldment was determined by tensile tests. The measured mechanical strength of the aluminum/steel joints was 210-230. MPa. Failure of the weldments under tensile loading occurred within the aluminum sheet, away from the steel surface/welding filler interface if brittle coating components and oxides were removed homogenously. © 2015 Published by Elsevier B.V.
    view abstract10.1016/j.surfcoat.2015.11.037
  • Scratch behavior of soft metallic materials
    Pohl, F. and Hardes, C. and Theisen, W.
    Aims Materials Science 3 (2016)
    This paper investigates the scratch resistance of metallic materials that include pure iron and the two steels AISI 1045 and AISI 304L. To investigate the deformation behavior under scratch load, tests were performed with a gradually increasing scratch load combined with subsequent analysis by scanning-electron microscopy and by atomic force microscopy. The fab value was determined to quantify the active micro-mechanisms of abrasion. In addition, tensile tests, hardness measurements, and nanoindentation experiments were conducted to correlate the scratch behavior with the mechanical properties. It was shown that there is no general correlation between the individual mechanical properties and the results of the scratch tests. However, the results revealed that work hardening of metallic materials plays a significant role, especially in the development of pile-up, and thus it greatly affects the active micro-mechanisms. The specific work of fracture at least correctly reproduces the order of scratch depth and the tangential force of the investigated materials.
    view abstract10.3934/matersci.2016.2.390
  • The RUB Tunneling Device - A newly developed test method to analyze and determine the wear of excavation tools in soils
    Küpferle, J. and Röttger, A. and Theisen, W. and Alber, M.
    Tunnelling and Underground Space Technology 59 (2016)
    The efficiency of a tunneling project is mainly associated with the abrasivity of the acting soil and the wear resistance of the cutting tools. Heavy wear can dull the cutting tool, which negatively affects the penetration rate and therefore the efficiency of a tunneling process. Completely worn tools with a short service life have to be replaced by newer ones. This circumstance results in unplanned machine shutdowns and higher maintenance costs. It is thus of high economic as well as technical interest to obtain a deeper understanding of soil/cutting tool interactions during tunneling. To meet this challenge, a large number of different testing devices to estimate the abrasivity of soils have been developed within the last two decades. An innovative and promising experimental setup is presented in this work. A horizontal implementation offers the possibility of simulating a tunneling process as well as the tribological system of a TBM tool. The interactions between all system components can be mapped and analyzed in detail. This method offers a unique setup, which allows wear prediction of TBM tools in a homogeneous soil with project-specific parameters (soil composition/condition, soil mechanics, tool material and machine/tunneling data). © 2016 Elsevier Ltd.
    view abstract10.1016/j.tust.2016.06.006
  • The thermal stability of topologically close-packed phases in the single-crystal Ni-base superalloy ERBO/1
    Lopez-Galilea, I. and Koßmann, J. and Kostka, A. and Drautz, R. and Mujica Roncery, L. and Hammerschmidt, T. and Huth, S. and Theisen, W.
    Journal of Materials Science 51 (2016)
    In Ni-base superalloys, the addition of refractory elements such as Cr, Mo, Co, W, and Re is necessary to increase the creep resistance. Nevertheless, these elements induce the formation of different kinds of intermetallic phases, namely, the topologically close-packed (TCP) phases. This work focuses on intermetallic phases present in the second-generation single-crystal (SX) Ni-base superalloy ERBO/1. In the as-cast condition, the typical γ/γ′ structure is accompanied by undesirable intermetallic phases located in the interdendritic regions. The nature of these precipitates as well as their thermal stability between 800 and 1200 °C has been investigated by isothermal heat treatments. The investigation techniques include DSC, SEM, EDX, and TEM. The experimental information is complemented by (1) comparison with a structure map to link the local chemical composition with phase stability, as well as (2) thermodynamic calculations based on the CALPHAD method to determine the occurrence and composition of phases during solidification and in equilibrium conditions. The TCP phases Laves, µ and σ were identified in various temperature/time ranges. © 2015, Springer Science+Business Media New York.
    view abstract10.1007/s10853-015-9579-7
  • Tool steel with improved properties for hot stamping tools
    Wilzer, J.J. and Escher, C. and Kotzian, M. and Weber, S. and Theisen, W.
    HTM - Journal of Heat Treatment and Materials 71 (2016)
    In the automotive industry, hot stamping of high-strength automotive body parts is a key technology to fulfill the requirements given by lightweight constructions and safety concepts. To produce such parts, the process and heat control as well as the design of hot stamping tools are of major importance. Stamping of the hot sheet metal blanks leads to high thermal, mechanical, and tribological impact on the tool which decreases its lifetime. Therefore, current research projects deal with the development of new tool steels which show better performance with respect to hot stamping tools. One of these developments is the new special steel grade CP2M® which offers high thermal conductivity and excellent wear resistance. © 2016 Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/105.110278
  • Assessment of the LCPC abrasiveness test from the view of material science
    Küpferle, J. and Röttger, A. and Alber, M. and Theisen, W.
    Geomechanik und Tunnelbau 8 (2015)
    The abrasiveness of crushed rock is determined in the design phase of a tunnelling project to estimate the wear on excavation and boring tools, using for example the LCPC abrasiveness test. Considered from the point of view of material science, there are problems with the validity of such an index value since important tribological factors, like for example the internal structure of the sample impeller (size, phase composition) are not considered in the current testing standard (AFNOR P18-579). In this investigation, LCPC tests were performed with impellers of various steels of the same and different hardnesses against two abrasives (Mohs hardness 7, 9) in order to determine the influence of the internal structure and the associated tribo-mechanical properties on the LCPC index value (A<inf>BR</inf>). In addition to the material hardness required by the standard (60 to 75 HRB ≈ 105 to 140 HV 10 according to DIN EN ISO 18265-2013), sample materials with higher hardness values were also used in order to extend the basis of the LCPC abrasiveness index to these practically applicable material groups. The results make clear that awareness has to be raised regarding the materials used for the LCPC test. An abrasive can be classified differently just by using various materials of the same hardness. In order to obtain reliable and reproducible index values for the abrasiveness of crushed rock, it is necessary to consider the influence of materials in future recommendations for the LCPC. © 2015 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.
    view abstract10.1002/geot.201500002
  • Boron-alloyed Fe-Cr-C-B tool steels - Thermodynamic calculations and experimental validation
    Röttger, A. and Lentz, J. and Theisen, W.
    Materials and Design 88 (2015)
    This study focuses on the development of boron-alloyed tool steels. The influence of Cr additions from 0 to 10mass% on microstructural changes were investigated for a constant metalloid content (C+B=2.4mass%). In the first step, thermodynamic calculations were performed to map the quaternary Fe-Cr-C-B system. In the second step, thermodynamic calculations were validated with laboratory melts that were investigated with respect to the microstructure and phase composition. The results of thermodynamic calculations correspond to real material behavior of Fe-Cr-C-B alloys. Furthermore, the influence of chromium on hard phase formation was investigated by means of phase analysis methods, X-ray diffraction (XRD), and energy dispersive spectrometry (EDS). Nanoindentation was used to determine hard phase properties (hardness, Young's modulus). It was shown that chromium promotes the formation of M2B-type borides. An increase in the Cr content within the M2B phase led to a transformation from the tetragonal structure into an orthorhombic structure. This transformation is accompanied by an increase in hardness and in the Young's modulus. In contrast, Cr also promotes the formation of Cr-rich carboborides of type M23(C,B)6. However, an increased Cr content within the M23(C,B)6 phase is not associated with an increase in hardness or elastic modulus. © 2015 Published by Elsevier Ltd.
    view abstract10.1016/j.matdes.2015.08.157
  • Cavitation erosion and hydroabrasion resistance of cold work tool steels produced by powder metallurgy
    Niederhofer, P. and Huth, S. and Theisen, W.
    Wear 332-333 (2015)
    Carbide-rich cold work tool steels produced by powder metallurgy are designed for applications that are exposed to heavy abrasive wear. Alloying with higher amounts of chromium and molybdenum additionally leads to corrosion resistance. This makes them excellent candidates for use in fluid flow systems with a high abrasive load, e.g. due to liquids containing sand particles. In this study, two different cold work tool steels were examined with focus on wear attack that may occur in combination with flowing liquids. On one hand, hydroabrasive attack results from fluids containing abrasives such as sand. This was simulated using a slurry pot containing a mixture of water and SiO2 particles. On the other hand, changes in flow speed may cause formation of cavitation bubbles that lead to erosive wear of the surfaces of nearby materials. Thus, despite the fact that resistance to surface spalling (e.g. by cavitation) was not a primary goal of alloy development, both alloys were also investigated by means of ultrasonic cavitation testing. The aim was to detect the influences of alloying system, processing route, and heat treatment condition on the wear resistance. The results indicate a positive influence of retained austenite on cavitation resistance, whereas the exact opposite holds true in the case of hydroabrasion. The damage mechanisms were analyzed by means of optical as well as scanning electron microscopy with special focus on the role of carbides during cavitation. Further attention was paid to the manufacturing route by comparing commonly used hot isostatic pressing (HIP) with supersolidus liquid phase sintering (SLPS). The latter permits consolidation of pre-alloyed powders to near net-shape parts or to thick protective layers on lower alloyed substrates. Compared to HIP, SLPS promises lower manufacturing costs combined with comparable mechanical properties. © 2014 Elsevier B.V.
    view abstract10.1016/j.wear.2014.12.016
  • Comprehensive investigation of phase transformation during diffusion alloying of Nb-rich powder metallurgical tool steels
    Weddeling, A. and Krell, J. and Huth, S. and Theisen, W.
    Powder Metallurgy 58 (2015)
    Stainless tool steels highly alloyed in niobium can be produced by powder metallurgy using diffusion alloying. Steel powder atomised without carbon is subsequently mixed with graphite and hot isostatically pressed. The atomised powder contains the intermetallic Laves phase NbFe<inf>2</inf>that transforms into MC-type carbides during HIP when graphite has been added. The obtained structure features a fine distribution of carbides to increase wear resistance and chromium fully dissolved in the matrix to provide corrosion resistance. X-ray diffraction (XRD) measurements and reflection position analysis with additional scanning electron microscopy (SEM) have been conducted to study the phase transition of NbFe<inf>2</inf>Laves phase into NbC carbides in two high Nb alloyed stainless tool steels. The results show that carburisation starts at 1000-1050°C and also confirm the correlation between oxide reduction and carburisation. The formed carbides are distinctly understoichiometric, which leads to an overestimation of the suitable quantitiy of added carbon in the thermodynamic calculations. © 2015 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute.
    view abstract10.1179/1743290115Y.0000000001
  • Effect of heat treatment on phase structure and thermal conductivity of a copper-infiltrated steel
    Klein, S. and Weber, S. and Theisen, W.
    Journal of Materials Science 50 (2015)
    Infiltration of tool steels with copper is a suitable and cheap method to create dense parts using powder metallurgy. In this work, it is shown that the copper network that forms inside the steel skeleton during infiltration enhances the thermal conductivity of the resulting composite. The level of enhancement is dependent on the thermal conductivity of the copper phase and the volume fraction of copper. Multiple heat treatments of this composite revealed a strong dependency between the thermal conductivity of the composite and the solution state of Fe in the copper network. The latter is highly dependent on the heat-treated condition of the multi-phase material. Using infiltration, the thermal and electrical conductivity was increased from (Formula presented.) respectively, for aged steel-copper composite in comparison with original X245VCrMo9-4-4 steel. In addition, a model alloy that represents the copper-phase network in the composite was manufactured. By measuring both, the thermal conductivity of this model alloy and the bulk steel, and comparing it to the data for the composite, different models for calculating the overall conductivity of the composite are discussed. © 2015, The Author(s).
    view abstract10.1007/s10853-015-8919-y
  • Evolution of the Laves Phase in Ferritic Heat-Resistant Steels During Long-term Annealing and its Influence on the High-Temperature Strength
    Nabiran, N. and Klein, S. and Weber, S. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 (2015)
    Heat-resistant ferritic steels containing Laves phase precipitates were designed supported by thermodynamic modeling. High-temperature compression tests at 1173.15 K (900 °C) and a detailed characterization of the microstructural evolution during annealing at 1173.15 K (900 °C) were carried out to investigate the effect of Laves phase formation on the high-temperature strength. Due to the addition of W/Mo and/or Nb, the high-temperature strength of the newly designed alloys is significantly higher than that of the reference steels. However, the high-temperature strength of all investigated steels decreases slightly as the annealing time is increased up to 1440 hours. To determine the influence of Laves phase formation and coarsening on the high-temperature strength during long-term annealing, the precipitates were extracted from the ferritic matrix in different annealing states. The phases in the powder residue were determined by XRD, and the chemical composition of the Laves phase in dependence of the annealing time was analyzed by EDS measurements. During annealing, steel Fe18CrMoW forms Nb(C,N), Ti(C,N), Laves phase (Fe2Nb) and Fe3Nb3C, whereas alloy Fe19CrWAl forms Nb(C,N), Ti(C,N), and Laves phase (Fe2Nb). The Laves phase within the alloys Fe18CrMoW and Fe19CrWAl differs in its morphology as well as its chemical composition. The Laves phase in steel Fe18CrMoW attains its chemical equilibrium after 192 hours, whereas alloy Fe19CrWAl required 24 hours. Overall, the formation of the Laves phase prevents significant grain growth during high-temperature annealing, thus preserving the high-temperature strength over a long time period. © 2014, The Minerals, Metals & Materials Society and ASM International.
    view abstract10.1007/s11661-014-2505-9
  • Impact of the microstructure and surface finishing on the ductility response of AISI type 304L steel tested in high pressure hydrogen gas atmosphere
    Martin, M. and Theisen, W.
    International Congress of Science and Technology of Metallurgy and Materials, Sam - Conamet 2014 9 (2015)
    The presence of hydrogen in iron alloys leads to an impairment of mechanical properties. This can be related to the production routes and material processing. Microsegregations, grain size and surface finishing play a very important role on the ductility response in hydrogen. Particularly, a more homogeneous distribution of alloying elements, a reduction in grain size and a proper surface finishing improve the material's ductility for a fixed composition. The study material is an AISI type 304L austenitic stainless steel subjected to tensile tests in air at ambient pressure and in a 40 MPa hydrogen gas atmosphere at 25 degrees C. (C) 2015 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.mspro.2015.05.009
  • Influence of Alloying Elements, Heat Treatment, and Temperature on the Thermal Conductivity of Heat Treatable Steels
    Wilzer, J. and Küpferle, J. and Weber, S. and Theisen, W.
    Steel Research International 86 (2015)
    The thermal conductivity of heat treatable martensitic steels plays an important role for many industrial applications. In case of hot stamping, the thermal conductivity of forging tools determines not only the product quality but also the productivity. Hence, the aim of ongoing developments is to increase the thermal conductivity of tool steels for several industrial applications. Therefore, it is highly beneficial to know how thermal conductivity is influenced by alloying elements, heat treatment, and temperature. This work deals with the thermal conductivity of non-alloyed heat treatable steel C45 and high-alloyed corrosion resistant steel X42Cr13 in the as-quenched and in the tempered condition. Additionally, the influence of dislocations on thermal conductivity is analyzed using commercially pure iron (Armco-Iron) in the annealed and in the cold-rolled condition. The results reveal that tempering affects both the electronic and the phononic contribution to thermal conductivity. Furthermore, due to the high chromium content, the thermal conductivity of steel X42Cr13 increases with temperature, which can be traced back to the electronic contribution. These results are useful for the development and improvement of tool steels, when the thermal conductivity of tools is not only a property but also a process parameter. This work deals with the thermal conductivity of non-alloyed steel C45 and corrosion resistant steel X42Cr13. Tempering affects both the electronic and the phononic contribution to thermal conductivity. Furthermore, thermal conductivity of steel X42Cr13 increases with temperature, which can be traced back to the electronic contribution. These results are useful for the development and improvement of tool steels. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/srin.201400294
  • Influence of Cold Work on Pitting Corrosion and Cavitation Erosion of High Interstitial FeCrMnCN Austenites
    Niederhofer, P. and Siebert, S. and Huth, S. and Theisen, W. and Berns, H.
    Steel Research International 86 (2015)
    High interstitial steels (HIS) are based on the joint addition of carbon and nitrogen, which resulted in an austenitic FeCrMn steel grade. In contrast to high nitrogen steels (HNS), they can be produced by conventional metallurgy and offer a unique combination of mechanical properties and corrosion resistance. This makes them promising candidates for the use in environments featuring corrosive or wear attack or even both. Corrosion resistance can be improved by the addition of molybdenum, particularly in the case of media containing chloride ions. In this study, different FeCrMnCN alloys were investigated by means of pitting corrosion testing in sodium chloride solution, as well as cavitation erosion resistance. These properties were examined depending on prestraining, since the latter is used to strengthen this kind of alloys. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/srin.201400381
  • Influence of Temperature and Tempering Conditions on Thermal Conductivity of Hot Work Tool Steels for Hot Stamping Applications
    Hafenstein, S. and Werner, E. and Wilzer, J. and Theisen, W. and Weber, S. and Sunderkötter, C. and Bachmann, M.
    Steel Research International 86 (2015)
    The knowledge of thermal conductivity is essential for improving and designing tools for hot working applications like hot stamping and high-pressure aluminum die casting. This study investigates the influence of alloying composition and heat treatment on thermal conductivity of two different hot work tool steels in the temperature range between 20 and 500 °C. Thermal conductivity was determined with an indirect measurement by using the dynamic method. The thermal conductivity of the two tool steels was found to be dependent on the amount of alloying elements, heat treatment condition, and operating temperature. In the regime of hot stamping applications, i.e for service temperatures below 200°C, thermal conductivity increases with temperature for both steels irrespective of their heat treatment condition. In applications in which tools are subjected to temperatures above 200°C (such as high-pressure die casting operations), thermal conductivity of the steels decreases as tool temperature increases. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/srin.201400597
  • Laser beam welding of aluminum to Al-base coated high-strength steel 22MnB5
    Windmann, M. and Röttger, A. and Kügler, H. and Theisen, W. and Vollertsen, F.
    Journal of Materials Processing Technology 217 (2015)
    The microstructure of aluminum-Al-coated steel laser beam welding joints was analyzed with respect to the welding energy. Quantitative and qualitative analysis of the welding microstructure were used to measure the weld width as well as the thickness of the resulting intermetallic layer at the 22MnB5/aluminum interface in relation to the welding parameters. Weldability of Al-coated steel could be improved by removing brittle coating particles and oxides on the steel surface by sandblasting. Adhesion of aluminum filler material to the 22MnB5 steel sheet could be enhanced by inductive preheating of the steel surface during laser welding. This produced welded 22MnB5/aluminum joints that exhibited a linear mechanical resistance of 220 MPa and which failed away from the brittle intermetallic layer on the aluminum side under a tensile load. The shear strength of the intermetallic layer on the 22MnB5/aluminum interface was evaluated to 74 ± 21 MPa. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2014.10.026
  • Microsegregation and precipitates of an as-cast Co-based superalloy—microstructural characterization and phase stability modelling
    Koßmann, J. and Zenk, C.H. and Lopez-Galilea, I. and Neumeier, S. and Kostka, A. and Huth, S. and Theisen, W. and Göken, M. and Drautz, R. and Hammerschmidt, T.
    Journal of Materials Science 50 (2015)
    The demand for increased efficiency of industrial gas turbines and aero engines drives the search for the next generation of materials. Promising candidates for such new materials are Co-based superalloys. We characterize the microsegregation and solidification of a multi-component Co-based superalloy and compare it to a ternary Co–Al–W compound and to two exemplary Ni-based superalloys by combining the experimental characterization of the as-cast microstructures with complementary modelling of phase stability. On the experimental side, we characterize the microstructure and precipitates by electron microscopy and energy-dispersive X-ray spectroscopy and determine the element distributions and microsegregation coefficients by electron probe microanalysis (EPMA). On the modelling side, we carry out solidification simulations and a structure map analysis in order to relate the local chemical composition with phase stability. We find that the microsegregation coefficients for the individual elements are very similar in the investigated Co-based and Ni-based superalloys. By interpreting the local chemical composition from EPMA with the structure map, we effectively unite the set of element distribution maps to compound maps with very good contrast of the dendritic microstructure. The resulting compound maps of the microstructure in terms of average band filling and atomic-size difference explain the formation of topologically close-packed phases in the interdendritic regions. We identify B2, C14, and D0<inf>24</inf> precipitates with chemical compositions that are in line with the structure map. © 2015, Springer Science+Business Media New York.
    view abstract10.1007/s10853-015-9177-8
  • New Developments in Martensitic Stainless Steels Containing C + N
    Seifert, M. and Siebert, S. and Huth, S. and Theisen, W. and Berns, H.
    Steel Research International 86 (2015)
    The use of nitrogen in martensitic stainless steels is limited by its solubility. Nitrogen solubility can be increased by alloying with elements such as Cr, Mn, and Mo and the use of pressure, such as in Pressurized ElectroSlag Remelting (PESR). Furthermore, the joint addition of C + N increases their solubility. Solid-state nitriding can be used for case hardening or N-enrichment of steel powders before sintering. However, the resulting stabilization of austenite can be a drawback for martensitic steels. Besides cryogenic treatment below the martensite finish temperature, ausforming, that is, metal working above Ms, could be promising. This contribution gives an overview about latest developments in N-rich martensitic stainless steels. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/srin.201400503
  • Nitrogen uptake of nickel free austenitic stainless steel powder during heat treatment-an XPS study
    Weddeling, A. and Lefor, K. and Hryha, E. and Huth, S. and Nyborg, L. and Weber, S. and Theisen, W.
    Surface and Interface Analysis 47 (2015)
    In austenitic stainless steel nitrogen stabilizes the austenitic phase improves the mechanical properties and increases the corrosion resistance. Nitrogen alloying enables to produce austenitic steels without the element nickel which is high priced and classified as allergy inducing. A novel production route is nitrogen alloying of CrMn-prealloyed steel powder via the gas phase. This is beneficial as the nitrogen content can be adjusted above the amount that is reached during conventional casting. A problem which has to be overcome is the oxide layer present on the powder surface which impedes both the sintering process and the uptake of nitrogen. This study focuses on whether heat treatment under pure nitrogen is an appropriate procedure to enable sintering and nitrogen uptake by reduction of surface oxides. X-ray photoelectron spectroscopy (XPS) in combination with scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) are used to investigate the surface of powdered FeMn19Cr17C0.4N heat treated under nitrogen atmosphere. The analyses showed reduction of iron oxides already at 500 °C leading to oxide-free metallic surface zones. Mn and Cr oxides are reduced at higher temperatures. Distinct nitrogen uptake was registered, and successful subsequent sintering was reached. Copyright © 2014 John Wiley & Sons, Ltd.
    view abstract10.1002/sia.5730
  • Reciprocating sliding wear of case-hardened spheroidal cast iron against 100Cr6 under boundary lubrication
    Stickel, D. and Goeke, S. and Geenen, K. and Huth, S. and Theisen, W. and Biermann, D. and Fischer, A.
    Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology 229 (2015)
    Today cast iron with spheroidal graphite is used in a wide range of applications with a high production capacity per year. Due to optimized and well-controlled casting technology, the production of ductile cast iron became economic in such way that ductile cast iron replaced cast or wrought steel in many machinery components like crankshafts, piston rods, and engine mounts. These examples represent technical tribosystems of the automobile industry. Here, current political, economic, and ecological guidelines also demand downsizing combined with high power densities in order to minimize internal friction and reduce fuel consumption and satisfying CO2-emission limits. These guidelines can change the tribological loads and, therefore, result in more severe conditions. One example is the shift of the lubrication regime from hydrodynamic to mixed or boundary lubrication for larger periods of time. In these regimes, the applied load is partially or fully carried by the asperities. Still the need for maintaining as low as possible wear towards the ultra-mild sliding wear regime an integral approach is needed, which has to regard contact conditions, surface topography, interface chemistry, and sub-surface properties. One way to low wear can aim at lowering the run-in phase by e.g. optimizing the topography by means of adjusted machining processes. For this study, reciprocating sliding wear tests were conducted with grinded, milled, polished, and finished samples of case-hardened spheroidal cast iron slid against a 100Cr6 ball of a 5mm radius. The boundary lubrication was provided by a commercial combustion engine lubricant at 80°C. After predefined test cycles, 3D surface topographies were measured by means of confocal white-light microscopy within each wear test in order to analyse the development of the contact conditions over time. In combination with the measured forces and displacements, the tribological loads are calculated by means of a 3D elastic-ideal plastic contact model. Additionally the wear mechanism was analyzed by means of scanning electron microscopy. The overall wear rates and the coefficients of friction depend strongly on the initial surface topography and, therefore, on the machining process. This is also true for the development of a reaction layer (tribomaterial) allowing for ultra-mild siding wear even under boundary lubrication. © IMechE 2015.
    view abstract10.1177/1350650115576245
  • Solidification and phase formation of alloys in the hypoeutectic region of the Fe-C-B system
    Lentz, J. and Röttger, A. and Theisen, W.
    Acta Materialia 99 (2015)
    In this work, alloys from the hypoeutectic iron-rich region of the iron-carbon-boron (Fe-C-B) system were investigated with respect to the solidification and the phase formation. Laboratory melts with a constant carbon content of 0.6 mass% and boron contents of 0.2 mass%, 0.6 mass%, and 1.8 mass% were fabricated and metallographically examined. In addition the microstructures were investigated by CALPHAD method in the state of equilibrium and by multiphase-field (MPF) method to reproduce the non-equilibrium process of the technical solidification. The results were analyzed with respect to the effect of boron on the solidification paths, microstructural crystallization processes as well as the morphological and chemical characteristics of the solidified phases. The investigated alloys undergo primary crystallization of austenite (γ-Fe). Due to the low solubility of B in the primary phase γ-Fe, B is strongly segregated in the melt and the solidification paths are deviated to high B contents. Therefore, as the B content increases, the eutectic solidification sequence starts with the B-rich Fe<inf>2</inf>B phase and continues with the formation of the B-rich Fe<inf>3</inf>(B,C) phase in the latter process. The B content of the melt thus decreases during the eutectic reaction, and the eutectic Fe<inf>3</inf>(B,C) phase exhibits a decreasing B gradient in the direction of growth. Consequently, the low-melting phase of the Fe-C-B system is the Fe<inf>3</inf>(B,C) phase with a low B content and a composition closest to its low-melting B content of 14.10 at.% B. Increasing B/(C + B) ratios of the alloy composition raise the average B content of the Fe<inf>3</inf>(B,C) phase (up to >20 at.% B) and hence at the same time increase the solidus temperature of the alloy. These findings revealed consistency with experimental results for chemical composition (WDX), phase analysis (diffraction with synchrotron radiation, EBSD), and thermal analysis (DTA). © 2015, Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.07.037
  • Solution Heat Treatment of the Single Crystal Nickel-Base Superalloy CMSX-4 Fabricated by Selective Electron Beam Melting
    Ramsperger, M. and Mújica Roncery, L. and Lopez-Galilea, I. and Singer, R.F. and Theisen, W. and Körner, C.
    Advanced Engineering Materials 17 (2015)
    Selective electron beam melting (SEBM), which belongs to the additive manufacturing processes, is applied to produce samples from the single crystalline nickel-base superalloy CMSX-4. The influence of the high solidification rates on the microstructure and element distribution is investigated by OM, SEM, DSC, and EMPA. Solution heat treatments at different temperatures and holding times are applied to demonstrate the difference between conventionally cast and SEBM material. The results demonstrate that SEBM is able to produce superalloys with a degree of homogeneity which cannot be realized in conventional processes. Selective electron beam melting (SEBM) manufacturing of CMSX-4 leads to a very fine solidification structure which is two orders of magnitude smaller than in conventional castings (e.g., Bridgman). Microprobe mappings of Re distribution in CMSX-4 can be used to show the differences in homogeneity (a and b). Due to the homogeneity, a solution heat treatment (HT) of 4 min (1320 °C) is already sufficient for homogenization of SEBM CMSX- 4 (c). © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201500037
  • The thermal stability of intermetallic compounds in an as-cast SX co-base superalloy
    Lopez-Galilea, I. and Zenk, C. and Neumeier, S. and Huth, S. and Theisen, W. and Göken, M.
    Advanced Engineering Materials 17 (2015)
    The nature and stability of intermetallic phases in a multi-component γ/γ′ Co-base superalloy is investigated. At least three kinds of unwanted intermetallic phases form due to the segregation of Al, Ta, Ti, Si, and Hf during casting in the interdendritic areas in addition to γ and γ′. One of the intermetallic phases that shows a blocky shape, contains high content of Ta, Hf, and Ti and has been identified as a topologically close-packed Laves phase. A B2 phase appears as relatively big pools and in some cases contains needle-shaped precipitates with high content of Al and Ti. The thermal stability of these intermetallic phases is studied in this work under defined heat treatments. The needle-shaped precipitates already dissolve at 1000°C, whereas the B2 phase is dissolved only at higher temperatures of 1200°C. Small amounts of Laves phases remained stable during aging at 1200°C for 25h. The prediction of phases as well as their stability is also checked by the CALPHAD method. The TCNi5 database, containing the description of Co<inf>3</inf>(Al,W), predicts the presence of the observed intermetallic phases; however, the predicted main transformation temperatures for these phases differ from the experimentally obtained values. This work studies the nature and stability of three intermetallic phases - Laves, B2 and needle-shaped precipitates - in a multi-component γ/γ′ Co-base superalloy, named ERBO-Co0. These phases are generated during casting at the interdendritic areas. Experimental characterization of intermetallic phases is contrasted with thermodynamic calculations, which predicts the presence of the identified phases. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201400249
  • Thermal conductivity of advanced TiC reinforced metal matrix composites for polymer processing applications
    Wilzer, J. and Windmann, M. and Weber, S. and Hill, H. and Bennekom, A.V. and Theisen, W.
    Journal of Composite Materials 49 (2015)
    Tools used for fabricating polymers are often required to have low thermal conductivities, e.g. for pelletizing, because this lowers the risk of the polymer nozzle being obstructed by molten polymer solidifying as it exits. Latterly, advanced corrosion and wear resistant metal matrix composites (MMCs) are used for pelletizing tools. Therefore, with respect to polymer processing it is important to know how the thermal conductivity of MMC gets influenced by hard phase and metal matrix contribution. In this study, the thermal conductivity of a TiC reinforced corrosion and wear resistant MMC gets analyzed. Especially the influence of chemical interdiffusion between TiC and metal matrix on the resulting thermal conductivity gets analyzed. It is shown that changes in the chemical composition lead to distinct decrease in thermal conductivity of the TiC which has to be considered when MMC thermal conductivities have to be examined. © The Author(s) 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav.
    view abstract10.1177/0021998313516143
  • Thermo-physical properties of heat-treatable steels in the temperature range relevant for hot-stamping applications
    Kuepferle, J. and Wilzer, J. and Weber, S. and Theisen, W.
    Journal of Materials Science 50 (2015)
    In many industrial processes, the resulting mechanical properties of produced steel parts are directly influenced by the thermo-physical properties, which affect the heat treatment significantly. The quality of application-oriented simulations is strongly dependent on the input quantities, which are often generated by regression analysis or simple extrapolations. The aim of this paper is to demonstrate the influence of the thermo-physical properties on such a process simulation referring to the hot stamping. Hot stamping is an established process in the automotive industry to produce ultra-high strength parts. A typical material used for this application is the low-alloyed steel 22MnB5. The thermal conductivity of this steel was investigated referring to the temperature-dependent microstructural changes during the hot-stamping process, particularly the γ to α′ transformation. In terms of the dynamic measuring method, the specific heat capacity, the thermal expansion coefficient, the density and the thermal diffusivity for the different temperature-dependent microstructures of the steel 22MnB5 were determined. The thermal conductivity for the complete temperature range of the hot-stamping process was generated, referring to measured and extrapolated data. To account for the fast γ–α′ transformation kinetics, a novel characterization and extrapolation method was applied. The heat capacity and the thermal diffusivity have a major impact on the thermal conductivity compared to the subordinated influence of the density. The metastable austenitic condition (T ≥ 900 °C) was compared to the martensitic condition (T ≤ 400 °C). The dependent thermal conductivity is significantly dependent on the crystallographic orientation of the lattice. The face-centred cubic lattice (austenite) has referring to the body-centred cubic lattice (martensite), a proportionally low thermal conductivity. During the transformation from austenite to martensite, the development is not linear but based on complex interactions. The results reveal that the temperature-dependent thermal conductivity has to be considered for reliable process simulations. © 2015, Springer Science+Business Media New York.
    view abstract10.1007/s10853-015-8829-z
  • Carbide precipitation during tempering of a tool steel subjected to deep cryogenic treatment
    Gavriljuk, V.G. and Sirosh, V.A. and Petrov, Y.N. and Tyshchenko, A.I. and Theisen, W. and Kortmann, A.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 45 (2014)
    Using transmission electron microscopy, Mössbauer spectroscopy, and measurements of hardness, the carbide precipitation during tempering of steel X153CrMoV12 containing (mass pct) 1.55C, 11.90Cr, 0.70V, and 0.86Mo is studied after three treatments: quenching at RT and deep cryogenic treatment, DCT, at 77 K or 123 K (-196 °C or -150 °C). In contrast to some previous studies, no fine carbide precipitation after long-time holding at cryogenic temperatures is detected. After quenching at room temperature, RT, the transient ε(ε) carbide is precipitated between 373 K and 473 K (100 °C and 200 °C) and transformed to cementite starting from 573 K (300 °C). In case of DCT at 123 K (-150 °C), only fine cementite particles are detected after tempering at 373 K (200 °C) with their delayed coarsening at higher temperatures. Dissolution of cementite and precipitation of alloying element carbides proceed at 773 K (500 °C) after quenching at RT, although some undissolved cementite plates can also be observed. After DCT at 123 K (-150 °C), the transient ε(ε) carbide is not precipitated during tempering, which is attributed to the intensive isothermal martensitic transformation accompanied by plastic deformation. In this case, cementite is the only carbide phase precipitated in the temperature range of 573 K to 773 K (300 °C to 500 °C). If DCT is carried out at 77 K (-196 °C), the ε(ε) carbide is found after tempering at 373 K to 473 K (100 °C to 200 °C). Coarse cementite particles and the absence of alloying element carbides constitute a feature of steel subjected to DCT and tempering at 773 K (500 °C). As a result, a decreased secondary hardness is obtained in comparison with the steel quenched at RT. According to Mössbauer studies, the structure after DCT and tempering at 773 K (500 °C) is characterized by the decreased fraction of the retained austenite and clustering of alloying elements in the α solid solution. It is suggested that a competition between the strain-induced transformation of the retained austenite and carbide precipitation during the wear can control the life of steel tools. © 2014 The Minerals, Metals & Materials Society and ASM International.
    view abstract10.1007/s11661-014-2202-8
  • Effect of the cooling rate during heat treatment and hot isostatic pressing on the microstructure of a SX Ni-superalloy
    Lopez-Galilea, I. and Huth, S. and Theisen, W.
    Eurosuperalloys 2014 - 2nd European Symposium on Superalloys and Their Applications 14 (2014)
    Single crystal Ni-based gas turbine blades show a combination of large casting pores and pores from the homogenization heat treatment. Both kinds of pores can only be reduced by HIP; however, HIP not only reduces porosity but also affects the size, number and morphology of gamma' particles. From the HIP parameters, pressure, temperature, holding time and cooling speed, the main effect on both, the porosity and the gamma/gamma' microstructure is due to temperature and cooling rate. HIP temperatures above the gamma' solvus temperature allow the fastest and most effective reduction of the porosity, because only the soft gamma. phase is present. The recent and novel possibility of cooling the samples from the maximum HIP temperature with a fast cooling of about 200 K/min, results in a fine and homogeneous distribution of gamma' particles, which requires no additional solution annealing treatment to dissolve the developed gamma' particles during the extremely short cooling time. Therefore, the application of HIP at super gamma' solvus temperature followed by fast cooling on homogenized samples seems to have the most promising results: no porosity and fine gamma/gamma' microstructure.
    view abstract10.1051/matecconf/20141413009
  • Enhancing the surface integrity of tribologically stressed contacting surfaces by an adjusted surface topography
    Goeke, S. and Biermann, D. and Stickel, D. and Stemmer, P. and Fischer, A. and Geenen, K. and Huth, S. and Theisen, W.
    Procedia CIRP 13 (2014)
    Tribological systems are more and more used under mixed and boundary friction conditions. Typically the running-in phase of a tribological system shows a wear rate which is significantly higher than the wear rate during steady state. Commonly it is assumed that smoother surfaces would lead to lower wear rates. A new approach aims at the shortening of the running-in phase by adjusting the surface topography. Specific topographies resulting from milling, grinding, and honing processes were generated and tribologically tested under lubricated sliding wear conditions. Wear tests showed a distinct influence of the surface topography regarding wear rate and running-in time. © 2014 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.procir.2014.04.037
  • Formation of intermetallic phases in Al-coated hot-stamped 22MnB5 sheets in terms of coating thickness and Si content
    Windmann, M. and Röttger, A. and Theisen, W.
    Surface and Coatings Technology 246 (2014)
    AlSiFe coatings with differing thicknesses and Si contents were applied to steel sheets by hot dipping. The steel sheets were austenitized at TAUS=920°C for different dwell times and then quenched in water. Phase formation as a function of coating thickness and Si content at the steel substrate/coating interface was investigated by ex-situ phase analysis with synchrotron radiation and by electron backscatter diffraction (EBSD). X-ray diffraction (XRD) and EBSD investigations confirmed the formation of AlFe-rich intermetallics at the steel/coating interface as a result of a strong diffusion of the elements Al and Fe. Within the first minute, Fe diffusion into the partially melted Al-base coatings promotes the formation of intermetallics of type Al8Fe2Si, Al13Fe4, and Al5Fe2. After the coating has transformed completely into Al-Fe intermetallics, Al diffusion into the steel substrate becomes more pronounced, thus reducing the Al content in the Al-Fe intermetallics and promoting formation of the phases of type Al2Fe and AlFe in the coating and formation of an Al-rich bcc layer in the steel substrate. The transformation kinetics of the resulting Al-, Fe-rich intermetallics are influenced by the coating thickness and the chemical composition of the Al-base coating. On the one hand, faster saturation of Fe in the Al-base coating is promoted by a shorter diffusion path and therefore by a thinner coating thickness. Otherwise, Si influences the diffusivity of the elements Al and Fe in the Al-, Fe-rich intermetallics and promotes the formation of Si-richer intermetallics, which then act as nuclei for Fe-richer intermetallics. © 2014 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2014.02.056
  • Indentation of self-similar indenters: An FEM-assisted energy-based analysis
    Pöhl, F. and Huth, S. and Theisen, W.
    Journal of the Mechanics and Physics of Solids 66 (2014)
    In this study, an energy-based approach is used to derive general relationships between two independent parameters of the load-displacement curve (C and Wel/Wtot) and the mechanical properties of Ludwik-power law materials (E, K, and n). The approach uses conventional continuum mechanics to describe the elastic and plastic zone including their mean strains and volumes induced by indentation. It does not account for the indentation size effect which is owed to the nucleation of dislocations within the plastic zone, as it is described by the model developed by Nix and Gao (1998). The energy-based approach in combination with FEM simulations give an insight into the complex deformation processes during indentation and the relationships between the material parameters and the indentation results. The results are discussed and interpreted in the context of solving the forward and inverse indentation problem for self-similar indenters. © 2014 Elsevier Ltd.
    view abstract10.1016/j.jmps.2014.02.002
  • Influence of formation and coarsening of the laves phase on the mechanical properties of heat-resistant ferritic steels
    Klein, S. and Nabiran, N. and Weber, S. and Theisen, W.
    Steel Research International 85 (2014)
    Three newly designed heat-resistant ferritic alloys containing the intermetallic Laves phase were investigated with respect to an annealing dwell time of up to 1440 h at 900C and were compared with commercially available steels. A detailed characterization of the microstructure evolution in dependence of the annealing dwell time was performed. In order to estimate the influence of Laves phase formation and coarsening on the strength, ductility and toughness, the results of the microstructural analysis were correlated with tensile tests at room temperature and with Charpy-V impact tests. Precipitates of the Laves phase were observed in the recrystallized state with a mean particle diameter about 0.25 μm. The Laves phase in all investigated alloys showed rapid growth and coarsening with increasing annealing time. In spite of this behavior, the strength and ductility of the newly designed alloys were conserved, even after annealing for 1440 h. However, the toughness decreased with coarsening of the Laves phase, which is expressed by a shift of the ductile-to-brittle transition temperature to a higher temperature. Overall, it was shown that the influence of grain growth on the mechanical properties is more significant than the presence of the Laves phase. Precipitation of Laves phase lowers the mobility of the grain boundaries so that grain growth can be avoided. To improve mechanical properties of ferritic heat-resistant steels at high temperature, steels were developed that contain a certain amount of Laves phase. In the present work, the effect of its precipitation and coarsening on mechanical properties at room temperature was investigated. It was found, that the Laves phase rises DBTT, but retards grain coarsening and therefore stabilizes mechanical properties during annealing. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/srin.201300257
  • Influence of solution nitriding of supersolidus-sintered cold work tool steels on their hardenability
    Blüm, M. and Conrads, J. and Weber, S. and Theisen, W.
    HTM - Journal of Heat Treatment and Materials 69 (2014)
    Powder metallurgical steel grades offer higher quality and performance compared to cast and forged steel grades due to their finer microstructure without segregations or textures. Because high-alloyed, cold work tool steels cannot be compacted by solid-state sintering, hot isostatic pressing is state of the art. This process, however, is comparatively expensive and there is thus a high demand for alternative densification processes. Supersolidus liquid-phase sintering represents an alternative to hot isostatic pressing for densification of these steels to theoretical density. During sintering, the steel powder interacts with the sintering atmosphere, which can be a vacuum, hydrogen, hydrogen plus nitrogen, or nitrogen. In a nitrogen atmosphere, there may be nitrogen uptake by the sintered material, which changes the chemical composition of the steel and thus results in a decrease in the sintering temperature. The aim of this work is to investigate the influence of nitrogen uptake on the hardenability of a high-alloyed and supersolidus-sintered cold work tool steel. Computational thermodynamics using the Calphad method were applied to calculate the optimal parameters for direct quenching from the sintering temperature. In addition, the tempering response was investigated as a function of the heat-treatment parameters. It was found that nitriding exerts a significant influence on the hardenability, which is also dependent on the cooling rate from the sintering temperature to the quenching temperature. Hardenability was predicted qualitatively on the basis of equilibrium carbon concentrations of the austenitic matrix calculated with the Calphad method. © 2014 Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/105.110234
  • Influence of the PM-Processing Route and Nitrogen Content on the Properties of Ni-Free Austenitic Stainless Steel
    Lefor, K. and Walter, M. and Weddeling, A. and Hryha, E. and Huth, S. and Weber, S. and Nyborg, L. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 (2014)
    Ni-free austenitic steels alloyed with Cr and Mn are an alternative to conventional Ni-containing steels. Nitrogen alloying of these steel grades is beneficial for several reasons such as increased strength and corrosion resistance. Low solubility in liquid and δ-ferrite restricts the maximal N-content that can be achieved via conventional metallurgy. Higher contents can be alloyed by powder-metallurgical (PM) production via gas–solid interaction. The performance of sintered parts is determined by appropriate sintering parameters. Three major PM-processing routes, hot isostatic pressing, supersolidus liquid phase sintering (SLPS), and solid-state sintering, were performed to study the influence of PM-processing route and N-content on densification, fracture, and mechanical properties. Sintering routes are designed with the assistance of thermodynamic calculations, differential thermal analysis, and residual gas analysis. Fracture surfaces were studied by X-ray photoelectron spectroscopy, secondary electron microscopy, and energy dispersive X-ray spectroscopy. Tensile tests and X-ray diffraction were performed to study mechanical properties and austenite stability. This study demonstrates that SLPS process reaches high densification of the high-Mn-containing powder material while the desired N-contents were successfully alloyed via gas–solid interaction. Produced specimens show tensile strengths >1000 MPa combined with strain to fracture of 60 pct and thus overcome the other tested production routes as well as conventional stainless austenitic or martensitic grades. © 2014, The Author(s).
    view abstract10.1007/s11661-014-2701-7
  • Material requirements for hot stamping tools using the example of tool steels X38CrMoV5-3, 30MoW33-7, and 60MoCrW28-8-4
    Wilzer, J. and Weber, S. and Escher, C. and Theisen, W.
    HTM - Journal of Heat Treatment and Materials 69 (2014)
    Press hardening of low-alloyed sheet steel gained in importance, in particular with respect to the production of body parts in the automotive industry. The press hardening process makes high demands on the tools by mechanical, thermal and tribological loads whereas the lifetime of a tool is limited mainly by abrasive wear. The cycle time of the press hardening process and thus its productivity are furthermore dependent on the thermal conductivity of the tool material. Therefore, a compromise has to be found between the mechanical and thermophysical properties. Industrially established solutions rely on high-alloyed hot work steels of which the grade X38CrMoV5-3 became widely accepted. In this context, hardening capacity, hardness penetration, abrasive wear resistance and thermal conductivity of two new tool steels are presented and discussed with respect to the reference steel X38CrMoV5-3. The authors show that an exclusive optimization of tool steels regarding high thermal conductivity is insufficient. Furthermore, it is shown that the influence of heat treatment must be considered for both, mechanical and thermophysical properties. © 2014 Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/105.110237
  • Recycling of metallic chips by electro-discharge sintering
    Mohr, A. and Röttger, A. and Windmann, M. and Theisen, W.
    Materialwissenschaft und Werkstofftechnik 45 (2014)
    Composite material Ferro-Titanit® is produced powder-metallurgical by Deutsche Edelstahlwerke GmbH (DEW) and is commonly used for wear and corrosion resistant component parts. Materials properties can be attributed to the microstructure which consists of a corrosion resistant metallic matrix and a huge amount of approx. 50 vol.% of hard Ti-monocarbides. Although Ferro-Titanit® possesses a high amount of hard particles, the material can be machined by turning and drilling in solution annealed condition. Due to the alloying content (Mo, Cr, TiC) of Ferro-Titanit®, there is a high motivation to recover those elements by a recycling process of the chips, thus expensive and limited resources can be saved. On idea of a recycling process can be found in the redensification of those chips by electro discharge sintering (EDS). In this work, chips of the material Ferro-Titanit® were densified by EDS technique and the resulting microstructure was investigated by optical and scanning electron microscopy. Furthermore, microstructure and hardness of the EDS densified specimens was discussed with regard to the microstructure of conventionally sintered Ferro-Titanit®-samples in laboratory conditions. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201400266
  • Temperature-dependent thermal conductivities of non-alloyed and high-alloyed heat-treatable steels in the temperature range between 20 and 500 °c
    Wilzer, J. and Küpferle, J. and Weber, S. and Theisen, W.
    Journal of Materials Science 49 (2014)
    This work investigates the temperature-dependent thermal conductivity of the heat-treatable steels C45, 40CrMnMo7, and X42Cr13 in a high-tempered condition. The results reveal that the temperature-dependent evolution of the thermal conductivity is strongly influenced by alloying composition. Furthermore, not only the thermal diffusivity but also the specific isobaric heat capacity has a major impact on the resulting thermal conductivity at higher temperatures. The results are discussed with respect to the resulting microstructures and under consideration of Calphad calculations. The results are relevant for the thermal design of tools, particularly those used for high-pressure die casting. © 2014 Springer Science+Business Media New York.
    view abstract10.1007/s10853-014-8183-6
  • A thermodynamic approach for the development of austenitic steels with a high resistance to hydrogen gas embrittlement
    Martín, M. and Weber, S. and Theisen, W.
    International Journal of Hydrogen Energy 38 (2013)
    The CALPHAD method was employed to assess the austenite stability of model alloys based on the Cr-Mn-Ni-Cu system. Stability was evaluated as the difference in Gibbs free energy between the austenite and ferrite phases. This energy difference represents the chemical driving force for the martensitic transformation and is employed as a design criterion. Six novel alloys featuring a lower driving force compared to the reference material AISI 316L were produced in laboratory. The susceptibility of all alloys to hydrogen gas embrittlement was evaluated by slow strain-rate tensile testing in air and hydrogen gas at 40 MPa and -50 C. The mechanical properties and ductility response of four of the six alloys exhibited an equivalent performance in air and hydrogen. Thermodynamic calculations were in agreement with the amount of α′-martensite formed during testing. Furthermore, a 4.5 wt.% reduction in the nickel content in comparison to 316L promises a cost benefit for the novel materials. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijhydene.2013.08.133
  • Development of a stable high-aluminum austenitic stainless steel for hydrogen applications
    Martin, M. and Weber, S. and Theisen, W. and Michler, T. and Naumann, J.
    International Journal of Hydrogen Energy 38 (2013)
    A novel high-aluminum austenitic stainless steel has been produced in the laboratory with the aim of developing a lean-alloyed material with a high resistance to hydrogen environment embrittlement. The susceptibility to hydrogen environment embrittlement was evaluated by means of tensile tests at a slow strain rate in pure hydrogen gas at a pressure of 40 MPa and a temperature of -50 C. Under these conditions, the yield strength, tensile strength and elongation to rupture are not affected by hydrogen in comparison to companion tests carried out in air. Moreover, a very high ductility in hydrogen is evidenced by a reduction of area of 70% in the high-pressure and low-temperature hydrogen environment. The lean degree of alloying is reflected in the molybdenum-free character of the material and a nickel content of 8.0 wt.%. With regard to the alloy concept, a combination of high-carbon, high-manganese, and high-aluminum contents confer an extremely high stability against the formation of strain-induced martensite. This aspect was investigated by means of in-situ magnetic measurements and ex-situ X-ray diffraction. The overall performance of the novel alloy was compared with two reference materials, 304L and 316L austenitic stainless steels, both industrially produced. Its capability of maintaining a fully austenitic structure during tensile testing has been identified as a key aspect to avoid hydrogen environment embrittlement. Copyright © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijhydene.2013.02.127
  • Effect of high pressure and high temperature on the microstructural evolution of a single crystal Ni-based superalloy
    Lopez-Galilea, I. and Huth, S. and Theisen, W. and Fockenberg, T. and Chakraborty, S.
    Journal of Materials Science 48 (2013)
    The application of high nearly hydrostatic pressures at elevated temperatures on the LEK94 single crystal (SX) nickel-based superalloy directly affects its microstructure. This is due to a combination of the effect of pressure on the Gibbs free energy, on the diffusion coefficients of the alloying elements, on the internal coherent stresses, and on the porosity distribution. The last effect depends at least on the first three. Therefore, based on the theoretical influences of the pressure, the main objective of this work is to understand, by means of an experimental work, the effect of high pressure at elevated temperature during annealing on the evolution of the phases morphology, and porosity of the high-temperature material LEK94. Specifically, pressures up to 4 GPa, temperatures up to 1180 C, and holding times up to 100 h were investigated. The main findings are that, porosity can be considerably reduced without affecting significantly the γ/γ′ microstructure by high pressure annealing and the verification that increasing the external pressure stabilizes the γ′-phase. © 2012 Springer Science+Business Media, LLC.
    view abstract10.1007/s10853-012-6752-0
  • Ferritic stainless steels for high-temperature applications: Stabilization of the microstructure by solid state precipitation of MX carbonitrides
    Nabiran, N. and Weber, S. and Theisen, W.
    High Temperature Materials and Processes 32 (2013)
    Ferritic heat-resistant steels are commonly used for automotive exhaust systems and have replaced cast iron, the traditional material for this application. Efforts to improve the efficiency of engines, reduce weight, and minimize toxic ingredients by increasing the gas temperature have shifted the requirement for ferritic heat-resistant steels to a higher hot strength. Methods of improving the high-temperature strength are solid-solution strengthening, precipitation hardening, and grain refinement. In this work, the influence of MX precipitates on the high-temperature mechanical properties of three different ferritic Fe-Cr stainless steels was investigated and compared to a reference material. Investigations were performed with uniaxial compression tests of samples aged isothermally at 900 °C for up to 1440 h. The most effective method of increasing the high-temperature strength is to alloy the steel with 2 mass% tungsten. Grain growth during annealing at 900 °C was decelerated by solid-state formation of MX carbonitrides. Microstructural investigations also revealed a slow coarsening rate of the MX precipitates. © [2013] by Walter de Gruyter Berlin Boston 2013.
    view abstract10.1515/htmp-2013-0004
  • Finite element method-assisted acquisition of the matrix influence on the indentation results of an embedded hard phase
    Pöhl, F. and Huth, S. and Theisen, W.
    Materials Science and Engineering A 559 (2013)
    FE-simulations were performed in order to quantify the matrix influence on the load-displacement curve and thus on the apparent hardness and Young's modulus of an embedded hard phase. The model system investigated in this study is the cold work tool steel X210Cr12 with an embedded spherical M 7C 3 carbide. In order to investigate the matrix influence on the indentation results, two different heat-treatment conditions were distinguished (soft-annealed and quenched+tempered). For each material combination, as well as several hard phase diameters, load-displacement curves and mechanical properties were calculated (via traditional Oliver and Pharr method) [1]. For low hard phase sizes or deep indentation depths, the surrounding matrix undergoes plastic deformation as the hard phase is pushed into it. This push-in event leads to significant errors in the calculated material parameters. A critical maximum indentation depth was determined depending on the hard phase diameter. It is shown that the ratio of indentation depth and hard phase diameter is the quantity of importance. © 2012 Elsevier B.V.
    view abstract10.1016/j.msea.2012.09.029
  • Low-temperature martensitic transformation in tool steels in relation to their deep cryogenic treatment
    Gavriljuk, V.G. and Theisen, W. and Sirosh, V.V. and Polshin, E.V. and Kortmann, A. and Mogilny, G.S. and Petrov, Yu.N. and Tarusin, Ye.V.
    Acta Materialia 61 (2013)
    The low-temperature martensitic transformation in steel X153CrMoV12 containing (mass%) 1.55C, 11.90Cr, 0.70V, 0.86Mo is studied using dilatometry, Mössbauer spectroscopy, X-ray diffraction, mechanical spectroscopy and transmission electron microscopy. Some additional measurements were carried out on steel X220CrMoV13-4. It is shown that, in contrast to the widely known absence of martensitic transformation during deep cryogenic treatment, this transformation occurs with isothermal kinetics within the temperature range of -100 down to -170 °C with its largest intensity near -150 °C. No transformation is observed at -196 °C. The remarkable features of the isothermal martensitic transformation are: (i) the plastic deformation, which is explained by the absence of ageing of martensite at low temperatures; and (ii) the abnormally low tetragonality of martensite. In contrast to existing interpretations, the abnormally low c/a ratio is interpreted in terms of the capture of immobile carbon atoms by gliding dislocations during plastic deformation at low temperatures. A recommendation is proposed for optimizing the deep cryogenic treatment of tool steels. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.11.045
  • Mössbauer studies of a martensitic transformation and of cryogenic treatments of a D2 tool steel
    Costa, B.F.O. and Blumers, M. and Kortmann, A. and Theisen, W. and Batista, A.C. and Klingelhöfer, G.
    Hyperfine Interactions 219 (2013)
    A D2 tool steel X153CrVMo12 with composition C1. 53 Cr12 V0. 95 Mo0. 80 Mn0. 40(wt% Fe balanced) was studied by use of Mössbauer spectroscopy and X-ray diffraction. It was observed that the study of carbides by X-ray diffraction was difficult while Mössbauer spectroscopy gives some light on the process occurring during cryogenic treatment. With the increase of the martensitic phase the carbides decrease and are dissolved in solid solution of martensite as well as the chromium element. © 2012 Springer Science+Business Media Dordrecht.
    view abstract10.1007/s10751-012-0684-6
  • Numerical modelling of powder metallurgical coatings on ring-shaped parts integrated with ring rolling
    Kebriaei, R. and Frischkorn, J. and Reese, S. and Husmann, T. and Meier, H. and Moll, H. and Theisen, W.
    Journal of Materials Processing Technology 213 (2013)
    Today's demands for flexible and economic production of ring-shaped work pieces coated by functional layers can only be met by new manufacturing techniques. These are suitably based on precise process modelling and high-performance control systems. The process-integrated powder coating by radial axial rolling of rings introduces a new hybrid production technique. It takes advantage of the high temperatures and high forces of the ring rolling process. This is not only to increase the ring's diameter, but also to integrate powder metallurgical multi-functional coatings within the same process. To improve the feasibility assessment of the proposed geometries and material combinations as well as to investigate important quantities such as the stress state in the rolling gaps and the residual porosity of the powder metallurgically produced layer, the versatile application of the finite element method (FEM) is crucial. Therefore, parameterized two-dimensional and three-dimensional finite element (FE) models are created. It will be shown that the implementation of a new control mechanism based on Apollonian mutually orthogonal circles and bipolar coordinates allows an efficient stabilization of the proposed systems. The paper is concluded by a detailed description of the process simulation and a comparison of its results with experimental data. © 2013 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2013.05.023
  • Phase formation at the interface between a boron alloyed steel substrate and an Al-rich coating
    Windmann, M. and Röttger, A. and Theisen, W.
    Surface and Coatings Technology 226 (2013)
    Al-base coating (AlSi10Fe3) was applied to a steel substrate (22MnB5) by hot dipping. The coated steel substrates were austenitized at 920. °C for several dwells, and phase formation at the steel/coating interface was investigated by means of ex-situ phase analysis with synchrotron radiation and EBSD. Phase identification by EBSD and XRD confirmed the formation of Al-rich intermetallics during austenitization. Increasing the dwell time led to Fe diffusion into the Al-base coating as well as Al diffusion into the substrate. As a result of the diffusion processes, Al-rich intermetallics in the coating transformed to more Fe-rich intermetallics. Simultaneously, Al diffusion into the substrate changed the microstructure of the steel substrate near the coating interface. Formation of FeAl intermetallics and thus the mechanical properties of the AlSi10Fe3 coating can be influenced by heat treatment. Higher austenitization temperatures and longer dwell times support the formation of more ductile FeAl intermetallics but also lead to grain growth; thus having a negative effect on the mechanical properties of the steel. © 2013 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2013.03.045
  • The influence of heat treatment and resulting microstructures on the thermophysical properties of martensitic steels
    Wilzer, J. and Lüdtke, F. and Weber, S. and Theisen, W.
    Journal of Materials Science 48 (2013)
    This research study investigates the influence of heat treatment on the thermal conductivities of three different tool steels at room temperature. The results reveal not only that tempering plays a decisive role in their thermophysical properties, but also that the changes in thermal conductivity due to heat treatment are dependent on the degree of alloying. Isobaric heat capacities cp are found to be less dependent on heat treatment than thermal diffusivities a. The results are discussed with respect to the resulting microstructures and, for high-tempered conditions, under consideration of Calphad calculations. The results are relevant for the thermal design of tools, in particular, for tailored tempering of tools used for press hardening. © 2013 The Author(s).
    view abstract10.1007/s10853-013-7665-2
  • Characterization of the surface of Fe-19Mn-18Cr-C-N during heat treatment in a high vacuum - An XPS study
    Zumsande, K. and Weddeling, A. and Hryha, E. and Huth, S. and Nyborg, L. and Weber, S. and Krasokha, N. and Theisen, W.
    Materials Characterization 71 (2012)
    Nitrogen-containing CrMn austenitic stainless steels offer evident benefits compared to CrNi-based grades. The production of high-quality parts by means of powder metallurgy could be an appropriate alternative to the standard molding process leading to improved properties. The powder metallurgical production of CrMn austenitic steel is challenging on account of the high oxygen affinity of Mn and Cr. Oxides hinder the densification processes and may lower the performance of the sintered part if they remain in the steel after sintering. Thus, in evaluating the sinterability of the steel Fe-19Mn-18Cr-C-N, characterization of the surface is of great interest. In this study, comprehensive investigations by means of X-ray photoelectron spectroscopy and scanning electron microscopy combined with energy dispersive X-ray spectroscopy were performed to characterize the surface during heat treatment in a high vacuum. The results show a shift of oxidation up to 600 °C, meaning transfer of oxygen from the iron oxide layer to Mn-based particulate oxides, followed by progressive reduction and transformation of the Mn oxides into stable Si-containing oxides at elevated temperatures. Mass loss caused by Mn evaporation was observed accompanied by Mn oxide decomposition starting at 700 °C. © 2012 Elsevier Inc. All rights reserved.
    view abstract10.1016/j.matchar.2012.06.002
  • Correlation of processing route and heat treatment with the abrasive wear resistance of a plastic mold steel
    Hill, H. and Kunze, M. and Heet, C. and Petsch, A. and Weber, S. and Reiter, R. and Giese, P. and Theisen, W. and Wielage, B. and Wesling, V.
    Materialwissenschaft und Werkstofftechnik 43 (2012)
    The processing of polymers necessitates the use of corrosion and wear resistant tool materials being in direct contact with the feedstock material. Corrosion resistant cold work tool steels, the so called plastic mold steels, are successfully applied here, offering both a good wear and corrosion resistance. The lifetime of this tool depends on the applied heat treatment but also the processing route has a distinct effect on the resulting properties. In this work, different powder metallurgical routes like hot isostatic pressing, build-up welding (plasma transfer arc (PTA)) and thermal spraying (high velocity oxy fuel (HVOF) and atmospheric plasma spraying (APS)) were applied to produce coatings on low-alloyed construction steel. Coatings are compared in relation to the changes in microstructure and the feasibility of an adequate heat treatment. This paper discusses strategies to maximize wear resistance in dependence of heat treatment and the microstructural changes arising from the processing. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201200865
  • Coupled thermo-mechanical analysis of process-integrated powder coating by means of hot rolling
    Kebriaei, R. and Frischkorn, J. and Reese, S. and Moll, H. and Theisen, W. and Husmann, T. and Meier, H.
    Key Engineering Materials 504-506 (2012)
    In this paper, a new hybrid production technique is presented. It exploits the advantage of high temperatures and high forces in the ring rolling process. This manufacturing technique is not only suitable to increase the ring's diameter but also to apply and compact powder metallurgical multi-functional coatings onto solid substrate rings with the same process. In order to design this new process parameterized 2D and 3D FE models are created in ABAQUS/EXPLICIT on the basis of a viscoplastic material model formulation. The control capability of the conventional control mechanisms are based on the assumption of volume consistency. However, this assumption is not well applicable for a ring furnished by multifunctional surfaces with non-isochoric plastic deformation behavior. Therefore, this paper deals with the implementation of a new control mechanism. Finally the paper is concluded with the integration of heat treatment of the rolled ring into the subsequent cooling process. © (2012) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.504-506.193
  • Development of lean alloyed austenitic stainless steels with reduced tendency to hydrogen environment embrittlement
    Weber, S. and Martin, M. and Theisen, W.
    Materials Science Forum 706-709 (2012)
    Hydrogen gas is believed to play a more important role for energy supply in future instationary and mobile applications. In most cases, metallic materials are embrittled when hydrogen atoms are dissolved interstitially into their lattice. Concerning steels, in particular the ductility of ferritic grades is degraded in the presence of hydrogen. In contrast, austenitic steels usually show a lower tendency to hydrogen embrittlement. However, the so-called "metastable" austenitic steels are prone to hydrogen environmental embrittlement (HEE), too. Here, AISI 304 type austenitic steel was tensile tested in air at ambient pressure and in a 400 bar hydrogen gas atmosphere at room temperature. The screening of different alloys in the compositional range of the AISI 304 standard was performed with the ambition to optimize alloying for hydrogen applications. The results of the mechanical tests reveal the influence of the alloying elements Cr, Ni, Mn and Si on HEE. Besides nickel, a positive influence of silicon and chromium was found. Experimental results are supported by thermodynamic equilibrium calculations concerning austenite stability and stacking fault energy. All in all, the results of this work are useful for alloy design for hydrogen applications. A concept for a lean alloyed austenitic stainless steel is finally presented. © 2012 Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/MSF.706-709.1041
  • Gas-solid interactions during nonisothermal heat treatment of a high-strength CrMnCN austenitic steel powder: Influence of atmospheric conditions and heating rate on the densification behavior
    Krasokha, N. and Weber, S. and Huth, S. and Zumsande, K. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 43 (2012)
    This work deals with gas-solid interactions between a high-alloyed steel powder and the surrounding atmosphere during continuous heating. It is motivated by the recently developed corrosion-resistant CrMnCN austenitic cast steels. Here, powder metallurgical processing would be desirable to manufacture highly homogeneous parts and/or novel corrosion-resistant metal-matrix composites. However, the successful use of this new production route calls for a comprehensive investigation of interactions between the sintering atmosphere and the metallic powder to prevent undesirable changes to the chemical composition, e.g., degassing of nitrogen or evaporation of manganese. In this study, dilatometric measurements combined with residual gas analysis, high-temperature X-ray diffraction (XRD) measurements, and thermodynamic equilibrium calculations provided detailed information about the influence of different atmospheric conditions on the microstructure, constitution, and densification behavior of a gasatomized CrMnCN steel powder during continuous heating. Intensive desorption of nitrogen led to the conclusion that a vacuum atmosphere is not suitable for powder metallurgical (PM) processing. Exposure to an N2-containing atmosphere resulted in the formation of nitrides and lattice expansion. Experimental findings have shown that the N content can be controlled by the nitrogen partial pressure. Furthermore, the reduction of surface oxides because of a carbothermal reaction at elevated temperatures and the resulting enhancement of the powder's densification behavior are discussed in this work. © The Minerals, Metals & Materials Society and ASM International 2012.
    view abstract10.1007/s11661-012-1234-1
  • HVOF spraying of Fe-based MMC coatings with in situ formation of hard particles by hot isostatic pressing
    Röttger, A. and Weber, S.L. and Theisen, W. and Rajasekaran, B. and Vaßen, R.
    Journal of Thermal Spray Technology 21 (2012)
    Thick (2-3 mm) Fe-base coatings with admixed ferrotitanium (Fe 30Ti 70) were applied to austenitic steel by a high-velocity oxy-fuel process (HVOF). Hot-isostatic pressing (HIP) was carried out to the decrease porosity and to increase the material strength, wear resistance, and adhesive bond strength of the deposited coating to the substrate material. SEM and XRD investigations confirmed the formation of hard titanium carbide (TiC) particles during HIP treatment as a result of strong carbon diffusion out of the metal matrix and into the Fe 30Ti 70 particles. The mechanical and wear properties of the densified coatings were investigated by means of shear tests, hardness measurements, and abrasive wear tests. A comparison of the coatings in the as-sprayed and the HIPed state showed a large increase in the wear resistance due to in situ TiC formation. © ASM International.
    view abstract10.1007/s11666-012-9736-3
  • Impact of heat treatment on the mechanical properties of AISI 304L austenitic stainless steel in high-pressure hydrogen gas
    Weber, S. and Martin, M. and Theisen, W.
    Journal of Materials Science 47 (2012)
    Hydrogen environment embrittlement of metastable austenitic stainless steels is a well-known phenomenon partially related to the formation of straininduced martensite. In the literature, hydrogen environment embrittlement is often discussed on the basis of nominal chemical compositions only and neglects effects of metallurgical production and processing. The aim of this study is to investigate the influence of the d-ferrite volume fraction and grain size on the mechanical properties of a standard grade 1.4307 (AISI 304L) tested in high-pressure hydrogen gas. A negligible influence was found for dferrite volume fractions between 2 and 10 %. This result is explained by the dominating influence of machininginduced a-martensite on the surface of the tensile samples. In contrast, the grain size was found to have a significant effect on hydrogen environment embrittlement. In particular, grain sizes smaller than 50 lm were found to have a higher ductility. The results are discussed with respect to stacking fault energy, formation of strain-induced a-martensite, trapping of hydrogen and microsegregations. The results are of particular interest for the materials selection and development of materials for hydrogen applications. © Springer Science+Business Media, LLC 2012.
    view abstract10.1007/s10853-012-6526-8
  • In situ investigation of the gas-solid interaction between high-alloyed steel powder and nitrogen by energy dispersive diffraction
    Zumsande, K. and Krasokha, N. and Huth, S. and Weber, S. and Theisen, W.
    Journal of Materials Science 47 (2012)
    This study investigates the temperature-dependent gas-solid interaction between gaseous nitrogen and two different high-alloyed steel powders: CrMn austenitic steel X40MnCrN19-17, and vanadium-rich cold-work tool steel X245VCrMo9-4-4. A time- and temperature-resolved synchrotron study was performed to investigate the phase composition of the powder materials during continuous heating. In addition, the nitrogen uptake of each phase was investigated by measuring the lattice constant of the respective phase. Thermodynamic calculations of the phase composition and the amount of dissolved interstitials in the calculated phases were performed and compared to the experimental results. This study reveals that heating in gaseous nitrogen leads to nitrogen uptake into both steels, which influences the phase composition. The first gas-solid interaction starts at 700 °C. Depending on the chemical composition of the steel powder, nitrogen is measured to be dissolved in different phases. Thermodynamic calculations were used to predict the phases in which nitrogen is dissolved. © 2011 Springer Science+Business Media, LLC.
    view abstract10.1007/s10853-011-6159-3
  • Influence of laves phase precipitation and coarsening on high-temperature strength of ferritic stainless steels
    Nabiran, N. and Weber, S. and Theisen, W.
    Steel Research International 83 (2012)
    Downsizing trends in the design of internal combustion engines require ferritic steels with greater strength at elevated temperatures. One method of improving the high-temperature strength is precipitation hardening with intermetallic phases such as the Laves phase. Thermodynamic calculations show, that the elements Nb and Si contribute to the Laves phase formation strongly. In this work, the influence of intermetallic precipitates on the mechanical properties of three different ferritic FeCr stainless steels was investigated and compared to a reference material. The three main hardening mechanisms - precipitation-hardening, grain refinement, and solid-solution strengthening - were studied with appropriate alloy compositions and thermo mechanical treatment. Investigations were performed with uniaxial compression tests of samples aged isothermally at 900°C for up to 1440h. It is shown that, the solid solution effect of Mo and W increases the high-temperature strength about 40%, also after long-term annealing. The contribution of the Laves phase precipitates on the high-temperature strength is rather small due to their rapid coarsening. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/srin.201200016
  • Influence of post-treatment on the microstructural evolution of thermally sprayed Fe-base MMC containing TiC and Cr 3C 2
    Röttger, A. and Weber, S.L. and Theisen, W.
    Surface and Coatings Technology 209 (2012)
    In this study, ledeburitic cold-work tool steel X220CrVMo13-4 (AISI D7) was admixed with TiC and Cr 3C 2 and the resulting feedstock powder was deposited on an austenitic substrate X5CrNiMo17-12-2 (AISI 316L) by the High Velocity Oxy Fuel (HVOF) process. The coatings exhibited low porosity and good adhesion to the substrate material. Further improvement of the material and tribological properties of the deposited coatings was achieved by hot isostatic pressing (HIP) and heat treatments. HIP treatment, which was carried out without encapsulation, led to a fully dense MMC coating. Further heat treatment to secondary peak hardness of the ledeburitic cold-work tool steel increased the hardness and thus the tribological properties. Diffusion reactions and the embedding behavior of the hard particles in the steel matrix were investigated with respect to the post-treatment processes by means of diffusion calculations and microstructural examinations. The results indicated that diffusion processes at the hard particle/metal matrix interface exert a strong influence on the embedding behavior of TiC and Cr 3C 2 particles in the coating microstructure. During HIP and heat treatment, new phases with a larger amount of metallic bonding were formed, thus improving particle adhesion to the metal matrix. © 2012 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2012.08.003
  • Influence of processing and heat treatment on corrosion resistance and properties of high alloyed steel coatings
    Hill, H. and Weber, S. and Raab, U. and Theisen, W. and Wagner, L.
    Journal of Thermal Spray Technology 21 (2012)
    Corrosion and abrasive wear are two important aspects to be considered in numerous engineering applications. Looking at steels, high-chromium high-carbon tool steels are proper and cost-efficient materials. They can either be put into service as bulk materials or used as comparatively thin coatings to protect lower alloyed construction or heat treatable steels from wear and corrosion. In this study, two different corrosion resistant tool steels were used for the production of coatings and bulk material. They were processed by thermal spraying and super solidus liquid phase sintering as both processes can generally be applied to produce coatings on low alloyed substrates. Thermally sprayed (high velocity oxygen fuel) coatings were investigated in the as-processed state, which is the most commonly used condition for technical applications, and after a quenching and tempering treatment. In comparison, sintered steels were analyzed in the quenched and tempered condition only. Significant influence of alloy chemistry, processing route, and heat treatment on tribological properties was found. Experimental investigations were supported by computational thermodynamics aiming at an improvement of tribological and corrosive resistance. © 2012 ASM International.
    view abstract10.1007/s11666-012-9788-4
  • Mechanical properties of (20-30)Mn12Cr(0.56-0.7)CN corrosion resistant austenitic TWIP steels
    Mújica Roncery, L. and Weber, S. and Theisen, W.
    Steel Research International 83 (2012)
    New developed (20-30)Mn12Cr(0.56-0.7)CN TWIP steels developed from thermodynamic calculations exhibit great mechanical properties, such as high strength (1800MPa UTS), deformability (80-100% elongation), toughness (300 J ISO-V), and impact wear resistance equivalent to that of Hadfield steel. In addition, they exhibit corrosion resistance by passivation in aqueous acidic media. Microstructure examination by SEM and EBSD at different degrees of deformation reveals that twinning takes place and is responsible for the high cold-work hardening of the steels. Stacking fault energy measurement of three different developed steels locates them in the range of 30-40mJm -2, being highly dependent on the N and Mn contents. Measurements carried out with digital image correlation indicate that at room temperature dynamic strain aging or Portevin-LeChatelier effect takes place. Measurements of impact toughness indicate that the steels have ductile to brittle transition at cryogenic temperatures as a consequence of the effect of nitrogen on the deformation mechanisms, resulting in a quasi-cleavage fracture along the {111} planes at -196°C. New Fe-Cr-Mn-C-N TWIP steels developed from thermodynamic calculations exhibit great mechanical properties, such as high strength (1800MPa UTS), deformability (80-100% elongation), toughness (300J ISO-V), high impact wear resistance, and corrosion resistance by passivation in aqueous acidic media. This work examines the microstructure, stacking fault energy, and dynamic strain aging to understand the tensile behavior and toughness of these materials. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/srin.201100316
  • Microsegregation and secondary phase formation during directional solidification of the single-crystal Ni-based superalloy LEK94
    Lopez-Galilea, I. and Huth, S. and Fries, S.G. and Warnken, N. and Steinbach, I. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 43 (2012)
    A multicomponent phase-field method coupled to thermodynamic calculations according to the CALPHAD method was used to simulate microstructural evolution during directional solidification of the LEK94 commercial single-crystal Ni-based superalloy using a two-dimensional unit cell approximation. We demonstrate quantitative agreement of calculated microsegregation profiles and profiles determined from casting experiments as well as calculated fraction solid curves with those determined in differential thermal analysis (DTA) measurements. Finally, the role of solidification rate on dendrite morphology and precipitation of the secondary phases is investigated and a new measure of the dendrite morphology is presented to quantify the effect of back diffusion on the amount of secondary phases. © 2012 The Minerals, Metals & Materials Society and ASM International.
    view abstract10.1007/s11661-012-1327-x
  • SintClad: A new approach for the production of wear-resistant tools
    Blüm, M. and Hill, H. and Moll, H. and Weber, S. and Theisen, W.
    Journal of Materials Engineering and Performance 21 (2012)
    Tools used in the mineral processing industry are required to feature high wear resistance to facilitate an adequate cost efficiency. These kinds of tools are made of composite materials based on a low-alloyed substrate material and a high-alloyed coating. The coatings can be applied in different ways using production processes like HIP cladding, deposit welding, and composite casting. The article is concerned with the problem of a novel and cost-effective coating alternative: sinter cladding, using the principle of supersolidus liquid-phase sintering (SLPS). Usually SLPS represents a sintering technique, which is used for the compaction of high-alloyed metal powders. However, no recognizable efforts were made to use the SLPSprocess for applying a PM-coating on a bulk substrate material. Sinter cladding for the first time uses SLPS to combine the process of powder compaction with the application of a coating to a solid steel substrate into one single step. Another advantage of the process is the possibility to produce massive bulk coatings with thicknesses exceeding 20 mm. This article is original in the scope of question and investigation methods in terms of microstructure, hardness profiles, EDX measurements, diffusion calculations, and computational thermodynamics. © ASM International.
    view abstract10.1007/s11665-012-0199-y
  • Supersolidus liquid-phase sintering of ultrahigh-boron high-carbon steels for wear-protection applications
    Röttger, A. and Weber, S. and Theisen, W.
    Materials Science and Engineering A 532 (2012)
    Powder metallurgy (PM) represents an alternative to conventional casting processes for the production of wear-resistant materials. PM hard alloys for wear-protection applications feature both higher strength and fracture toughness compared to cast hard alloys due to their more finely grained microstructure. However, densification by hot-isostatic pressing (HIP), the conventional PM-compaction method, is relatively expensive and thus partially counteracts low-cost processing. To increase the economic efficiency of the processing route, supersolidus liquid-phase sintering (SLPS) was investigated. In addition, expensive Ni- and Co-base hard alloys were substituted by boron-rich Fe-base hard-facing alloys.In this study, three ultrahigh-boron hard-facing alloy powders were densified by SLPS and HIP. The sintering temperatures were optimized by means of sintering experiments that were supported by thermodynamic calculations. Both densification states were investigated and compared with respect to the microstructure and the tribological and mechanical properties of the compacted hard-facing alloys. It was shown that the mechanical and tribological properties are strongly influenced by the microstructure. Although the microstructure is affected by the chemical composition, it can also be adapted by the densification process. SLPS-densified hard-facing alloys have a coarse microstructure that imparts not only a high wear resistance but also a detrimental effect on the mechanical properties. © 2011 Elsevier B.V.
    view abstract10.1016/j.msea.2011.10.118
  • The stacking fault energy and its dependence on the interstitial content in various austenitic steels
    Mujica, L. and Weber, S. and Theisen, W.
    Materials Science Forum 706-709 (2012)
    The stacking fault energy (SFE) is an intrinsic property of metals and is involved in the deformation mechanism of different kind of steels, such as TWIP (twinning induced plasticity), TRIP (transformation induced plasticity), HNS (high nitrogen), and high strength steels. The dependence of the SFE on the content of interstitial elements (C, N) is not yet fully understood, and different tendencies have been found by different authors. In order to study the influence of the interstitial elements on the SFE, experimental measurements extracted from literature were collected and analyzed to predict the individual and combined effect of carbon and nitrogen in different systems. The referenced austenitic steels are Fe-22Mn-C, Fe-30Ni-C, Fe-15Cr-17Mn-N, Fe-18Cr-16Ni-10Mn-N, Fe-18Cr-9Mn-C-N, Fe-18Mn-18Cr-C-N and Fe-(20-30)Mn-12Cr-C-N. The calculation of the SFE is based on the Gibbs free energy of the austenite to ε-martensite transformation (ΔG γ→ε), which is calculated by means of the Calphad method. The revision of the measured values reveals that on different ranges of interstitial contents the SFE behaves differently. At lower values (C, N or C+N up to 0.4%), a local minimum or maximum is found in most of the systems. At higher concentration levels, a proportional dependence seems to occur. These observations agree with the theory of the dependence of SFE on the free electron concentration. Alloying with Mn or Ni has a strong influence on the electronic configuration and magnetic properties of the austenite and therefore on the SFE. The results of this study provide valuable information for materials design, especially in the context of alloying with C, N or C+N. © 2012 Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/MSF.706-709.2193
  • Welding of twinning-induced plasticity steels
    Mújica Roncery, L. and Weber, S. and Theisen, W.
    Scripta Materialia 66 (2012)
    This work focuses on the technical and technological aspects of fusion welding of high-manganese steels exhibiting twinning-induced plasticity (TWIP) for both similar and dissimilar joints. Changes in the alloy chemistry resulting from evaporation and dilution are discussed with respect to stacking fault energy and austenite stability. The influence of fusion welding on grain size and strength is also discussed. Conclusions are drawn with respect to optimization processes for fusion welding of TWIP steel. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2011.11.041
  • Corrosion properties of a plastic mould steel with special focus on the processing route
    Hill, H. and Huth, S. and Weber, S. and Theisen, W.
    Materials and Corrosion 62 (2011)
    Applications in plastics processing bear increased requirements for the used materials, especially with respect to their corrosion and wear resistance. For this reason, special powder metallurgical tools steels were developed that fulfil these demands. The common processing route for their production is hot isostatic pressing (HIP) of pre-alloyed powders which is followed by hot working if semi-finished parts are to be produced. As an alternative to HIP, super solidus liquid phase sintering (SLPS) permits the consolidation of pre-alloyed tool steel powders to near net-shape parts. It can be performed in different sintering atmospheres. In this work, the plastic mould steel X190CrVMo20-4 was processed by SLPS in vacuum as well as under nitrogen atmosphere. The resulting materials were analysed with respect to their microstructure, tempering behaviour and corrosion resistance in 0.5 molar sulphuric acid in dependence of the heat treatment. As a reference, the HIPed and the HIPed and worked state were also investigated. The results show that different heat treatments alter the ranking of the sintered and the HIPed state with respect to corrosion resistance. As expected, a high tempering for maximum secondary hardness causes a significant loss of corrosion resistance. The experimental findings were supported by thermodynamic calculations based on slight alterations in chemical composition that result from the different manufacturing processes. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/maco.200905570
  • Critical investigation of high temperature gas nitriding of a PM tool steel
    Weber, S. and Theisen, W.
    International Journal of Materials Research 102 (2011)
    The nitrogen uptake of high-alloyed steels, especially tool steels, from gaseous atmospheres at elevated temperatures is a well known effect. Besides, for instance, the nitrogen-based case hardening, one application arises in the field of powder metallurgy. Here, introducing nitrogen containing atmospheres during sintering of high-alloyed powder metal-lurgically produced tool steels could lead to an optimization of the sintering process and an improvement of the materials properties. In this context, several works deal with the application of computational thermodynamics for considering the nitrogen uptake. The scope of this contribution is to investigate in detail the agreement of calculated and experimentally found nitrogen contents. Therefore, one typical gas-atomized powder of a ledeburitic cold work steel was sintered at a temperature of 1230 °C, varying the nitrogen partial pressure during sintering between 0.02 MPa and 0.2 MPa. The measured nitrogen contents are compared with values obtained from equilibrium calculations. Additionally, the distribution of nitrogen in the microstructure is investigated by time-resolved optical emission spectrometry and energy dispersive X-ray spectrometry measurements. The results exhibit good agreement between calculated and measured values for low partial pressures of less than 0.1 MPa and an increasing deviation for higher partial pressures. Furthermore, the transformation of vanadium-containing MC carbides to M(C, N) carbonitrides was verified. © Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/146.110447
  • Development and characterization of novel corrosion-resistant TWIP steels
    Mujica, L. and Weber, S. and Hunold, G. and Theisen, W.
    Steel Research International 82 (2011)
    Austenitic steels exhibiting twinning induced plasticity (TWIP) are materials with exceptional mechanical properties. In this work, the development of new grades of TWIP steels exhibiting corrosion resistance is presented. The alloy development was supported by thermodynamic and diffusion calculations within the (Fe-Mn-Cr)-(C-N) alloy system. For the calculations ambient pressure and primary austenitic solidification were considered as necessary to avoid nitrogen degassing in all processing steps. Manganese is used as an austenite stabilizer, chromium to increase nitrogen solubility and provide corrosion resistance, while carbon and nitrogen provide mechanical strength. Diffusion calculations were used in order to predict the extent of micro segregations and additionally to evaluate the effect of diffusion annealing treatments. The material was cast in a laboratory scale with a nominal composition of Fe-20Mn-12Cr-0.25C-0.3N. Diffusion annealing was followed by hot rolling and solution annealing resulting in a fully austenitic microstructure. Tensile tests at room temperature were performed, exhibiting yield strengths of 430 MPa and elongation to fracture of 93%. In addition, not only the mechanical properties but also the weldability was studied, focussing on the characterization of the microstructure of bead on plate welds obtained by laser and TIG welding. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/srin.201000219
  • Diffusion and phase transformation at the interface between an austenitic substrate and a thermally sprayed coating of ledeburitic cold-work tool steel
    Röttger, A. and Weber, S. and Theisen, W. and Rajasekeran, B. and Vaßen, R.
    Steel Research International 82 (2011)
    An overlay coating against wear or corrosion on components is required for various technical applications. Thermal spraying is a well-established and near-net-shape deposition method. In this work, high-velocity oxy-fuel spraying of two different ledeburitic cold-work tool steels was employed to produce wear-resistant Fe-base coatings on a stainless steel substrate. This work focuses on the investigation of diffusion processes across the coating/substrate interface. Specimens were heat-treated for different dwell times and then analyzed by means of EBSD, XRD, OM, as well as SEM. Results of phase formation and diffusion profiles were compared with equilibrium and diffusion calculations obtained with ThermoCALC® and DICTRA®. The influence of diffusion processes across the coating/substrate interface on the mechanical properties, such as adhesive bond strength and hardness, was investigated by shear tests and microhardness profiles. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/srin.201000199
  • Effect of alloying elements on hydrogen environment embrittlement of AISI type 304 austenitic stainless steel
    Martin, M. and Weber, S. and Theisen, W. and Michler, T. and Naumann, J.
    International Journal of Hydrogen Energy 36 (2011)
    The chemical composition of an AISI type 304 austenitic stainless was systematically modified in order to evaluate the influence of the elements Mo, Ni, Si, S, Cr and Mn on the material's susceptibility to hydrogen environment embrittlement (HEE). Mechanical properties were evaluated by tensile testing at room temperature in air at ambient pressure and in a 40 MPa hydrogen gas atmosphere. For every chemical composition, the corresponding austenite stability was evaluated by magnetic response measurements and thermodynamic calculations based on the Calphad method. Tensile test results show that yield and tensile strength are negligibly affected by the presence of hydrogen, whereas measurements of elongation to rupture and reduction of area indicate an increasing ductility loss with decreasing austenite stability. Concerning modifications of alloy composition, an increase in Si, Mn and Cr content showed a significant improvement of material's ductility compared to other alloying elements. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijhydene.2011.09.013
  • Effect of HIP parameters on the micro-structural evolution of a single crystal Ni-based superalloy
    Lopez-Galilea, I. and Huth, S. and Bartsch, M. and Theisen, W.
    Advanced Materials Research 278 (2011)
    For reducing the porosity of single crystal (SX) nickel-based superalloys, Hot Isostatic Pressing (HIP) is used. High pressures of about 100-170 MPa lead to local deformation, which close the pores. However, since HIP also requires high temperatures (1000-12007deg;C) it has a pronounced effect on the microstructure and the local distribution of elements. This contribution analyses the effect of different HIP treatments on both the microstructure and the segregation of the SX superalloy LEK94 in the as-precipitation-hardened state. In addition, the effects of rapid or slow cooling are analyzed. To distinguish the effect of pressure from those of temperature, the HIPed samples are compared with specimens annealed at atmospheric pressure. © (2011) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/AMR.278.72
  • Finite element modelling of process-integrated powder coating by radial axial rolling of rings
    Frischkorn, J. and Kebriaei, R. and Reese, S. and Moll, H. and Theisen, W. and Husmann, T. and Meier, H.
    AIP Conference Proceedings 1353 (2011)
    The process-integrated powder coating by radial axial rolling of rings represents a new hybrid production technique applied in the manufacturing of large ring-shaped work pieces with functional layers. It is thought to break some limitations that come along with the hot isostatic pressing (HIP) which is used nowadays to apply the powdery layer material onto the rolled substrate ring. Within the new process the compaction of the layer material is integrated into the ring rolling and HIP becomes dispensable. Following this approach the rolling of such compound rings brings up some new challenges. The volume of a solid ring stays nearly constant during the rolling. This behaviour can be exploited to determine the infeed of the rollers needed to reach the desired ring shape. Since volume consistency cannot be guaranteed for the rolling of a compound ring the choice of appropriate infeed of the rollers is still an open question. This paper deals with the finite element (FE) simulation of this new process. First, the material model that is used to describe the compaction of the layer material is shortly reviewed. The main focus of the paper is then put on a parameterized FE ring rolling model that incorporates a control system in order to stabilize the process. Also the differences in the behaviour during the rolling stage between a compound and a solid ring will be discussed by means of simulation results. © 2011 American Institute of Physics.
    view abstract10.1063/1.3589538
  • Hot Direct Extrusion of Abrasion Resistant Fe-Base Metal Matrix Composites-Interface Characterization and Mechanical Properties of Co-Extruded Layered Structures
    Silva, P.A. and Weber, S. and Karlsohn, M. and Röttger, A. and Theisen, W. and Reimers, W. and Pyzalla, A.R.
    Friction, Wear and Wear Protection: International Symposium on Friction, Wear and Wear Protection 2008 Aachen, Germany (2011)
    view abstract10.1002/9783527628513.ch90
  • Hot Direct Extrusion of Abrasion Resistant Fe-Base Metal Matrix Composites-Microstructure and Wear Properties
    Karlsohn, M. and Röttger, A. and Silva, P.A. and Weber, S. and Pyzalla, R. and Reimers, W. and Theisen, W.
    Friction, Wear and Wear Protection: International Symposium on Friction, Wear and Wear Protection 2008 Aachen, Germany (2011)
    This paper discusses hot direct extrusion as a novel method to produce long cylindrical rods of a metal-matrix composite (MMC) and co-extruded rods of a low alloyed steel substrate cladded with a MMC layer. The excellent abrasive wear resistance of MMC is based on hard phase and iron powder mixtures, which can be tailored to the respective abrasive environment. This production process combines sintering of capsuled powder blends and subsequent pressing of the capsules through an extrusion die to produce an (almost) completely densified MMC. In contrast to HIP cladding (hot isostatic pressing), the standard method of producing powder metallurgical components (PM) and PM-layers, the simpler capsule technique is less expensive. The extruded MMC were investigated with respect to their microstructure, wear properties and the microstructural and mechanical properties of the interface of substrate and MMC layer. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/9783527628513.ch17
  • Influence of heat treatment on the performance characteristics of a plastic mold steel
    Hill, H. and Raab, U. and Weber, S. and Theisen, W. and Wollmann, M. and Wagner, L.
    Steel Research International 82 (2011)
    Processing of polymers depends on corrosion- and wear resistant tool materials being in direct contact with the feedstock material. Plastic mold steels, a special group of tool steels, are successfully applied here, offering both a good wear and corrosion resistance. The lifetime of tools used for plastic processing not only depends on wear and corrosion resistance but also on the mechanical properties of the tool material for example, its flexural strength. In this work, mechanical tests of a commercial plastic mold steel were performed and related to the heat treatment. A close correlation of the amount of retained austenite and carbides and the mechanical properties was encountered. The paper discusses strategies to optimize the mechanical properties by an adequate heat treatment with regard to the resulting corrosion and wear resistance. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/srin.201100098
  • Influence of intermetallic precipitates and heat treatment on the mechanical properties of high-temperature corrosion resistant ferritic steels
    Nabiran, N. and Weber, S. and Theisen, W.
    Procedia Engineering 10 (2011)
    High-temperature corrosion resistant ferritic steels are commonly used in heat exchangers for auxiliary power units (APU), automotive exhaust systems and structural parts of solid oxide fuel cells (SOFC) due to their excellent thermal fatigue resistance. As the process temperatures in these applications are primarily limited by the materials high temperature strength, the main focus of this work is on the improvement of this property by adjustments in the material design. Generally, two mechanisms were used to increase the high temperature strength, solid solution strengthening and precipitation hardening. Due to their large atomic radii and high solubility in α-Fe, W and Mo were used for solid solution strengthening. Furthermore, the content of niobium, which is well known to form Laves phase precipitates, was raised. This led to a higher content of Laves phase precipitates compared to the reference material. The analyses concentrated on the effect of the Laves phase. Strength at elevated temperature was investigated in compression tests at 900°C with respect to the annealing time which was varied between 1h and 1440 h. © 2011 Published by Elsevier Ltd.
    view abstract10.1016/j.proeng.2011.04.276
  • Influence of machining-induced martensite on hydrogen-assisted fracture of AISI type 304 austenitic stainless steel
    Martin, M. and Weber, S. and Izawa, C. and Wagner, S. and Pundt, A. and Theisen, W.
    International Journal of Hydrogen Energy 36 (2011)
    Hydrogen-assisted fracture of AISI type 304 steel has been evaluated with a special focus on the strain-induced martensite that is produced below the specimen surface during standard turning operation. Two different surface conditions were investigated: one containing martensite, resulting from the machining process, and a martensite-free state which is obtained after a proper heat treatment. Additionally, chemical composition and thickness of oxide layers, occurring in both studied cases, were analyzed by secondary ion mass spectrometry. These two different conditions were tested at room temperature in air (ambient pressure) and in hydrogen gas (40 MPa) atmosphere, respectively. Experimental results reveal a detrimental effect of machining-induced martensite on AISI type 304 steel performance in hydrogen, leading to major differences in relative reduction of area (RRA) between the as-machined and the heat-treated state for the same material. In this context, an operating mechanism based on hydrogen diffusion is discussed. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijhydene.2011.05.133
  • Investigation of corrosion and wear properties of fe based metal matrix composites consolidated by sintering and hot isostatic pressing
    Hill, H. and Weber, S. and Siebert, S. and Theisen, W.
    Powder Metallurgy 54 (2011)
    Many industrial applications, e.g. processing of polymers, suffer from high costs caused by corrosion and wear. Particularly the combination of both increases the requirements for the materials used. Corrosion resistant cold work steels were developed to withstand the combined attack. Resistance is achieved by a sufficient content of chromium in the metal matrix and by carbides dispersed in a martensitic matrix. A further gain in wear resistance is possible by adding hard phases to the steel to produce a particulate reinforced metal matrix composite (MMC). The common consolidation process for such MMCs is hot isostatic pressing, but they can also be processed by solid state or liquid phase sintering. This work focuses on detailed investigations of the properties in dependence on the processing route. The results show that the resulting corrosion and wear resistance depend not only on the processing method, but also on the incorporated hard phases in combination with the manufacturing method. In addition, the unreinforced metal matrices were compared to the MMC. © 2011 Institute of Materials, Minerals and Mining.
    view abstract10.1179/003258910X12678035166656
  • Lean-alloyed austenitic stainless steel with high resistance against hydrogen environment embrittlement
    Weber, S. and Martin, M. and Theisen, W.
    Materials Science and Engineering A 528 (2011)
    To address the upcoming austenitic stainless steel market for automotive applications involving hydrogen technology, a novel lean - alloyed material was developed and characterized. It comprises lower contents of nickel and molybdenum compared to existing steels for high - pressure hydrogen uses, for instance 1.4435 (AISI 316L). Alloying with manganese and carbon ensures a sufficient stability of the austenite at 8. wt.% of nickel while silicon is added to improve resistance against embrittlement by dissolved hydrogen. Investigations were performed by tensile testing in air and 400. bar hydrogen at 25 °C, respectively. In comparison to a standard 1.4307 (AISI 304L) material, a significant improvement of ductility was found. The materials concept is presented in general and discussed with regard to austenite stability and microstructure. © 2011 Elsevier B.V.
    view abstract10.1016/j.msea.2011.06.049
  • Mechanical properties of thermally sprayed Fe based coatings
    Röttger, A. and Weber, S. and Theisen, W. and Rajasekeran, B. and Vassen, R.
    Materials Science and Technology 27 (2011)
    An additional coating against wear or corrosion on component parts is required for many applications. These coatings protect the substrate material against external influences, thus increasing the economic lifetime of the component. Coating processes such as build-up welding and thermal spraying are well established and commonly used. The thermal spray process, in particular, permits deposition of metals, ceramics, or cermets materials to produce near net shape coatings on complex surface geometries. However, commonly used coating materials suffer from high raw material costs, thus decreasing the cost effectiveness of the coating process. Fe based materials are low priced and possess noteworthy mechanical properties; they thus provide the possibility of substituting the expensive Ni and Co based materials commonly used for thermal spray processes. In this work, 2 mm thick high velocity oxyfuel sprayed Fe based coatings in the as sprayed and thermally sprayed and hot isostatic pressed condition were investigated with respect to their mechanical and wear properties. Additionally, the fracture surface was investigated by scanning electron microscopy to characterise the fracture behaviour. It could be demonstrated that the substrate and the heat treatment have the greatest impact on the shear strength of thermally sprayed cold work tool steel. It is shown that the substrate materials as well as the heat treatment are promoting diffusion processes across the interface between the coating and the substrate. Hence, a material integrated bond is formed. The microstructures of the thermally sprayed coatings become more important regarding the mechanisms of failure of the four point bending tests. The material strength is influenced by quenching and tempering and the specimen deflection is influenced by diffusion reactions induced by hot isostatic pressing treatment. The thermally sprayed coatings in the as sprayed condition feature the highest wear resistance due to their hardness. © 2011 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute.
    view abstract10.1179/1743284710Y.0000000002
  • Microstructures and properties of laser cladding NiTi alloy with W for biomedical applications
    Yan, X.J. and Gugel, H. and Huth, S. and Theisen, W.
    Materials Letters 65 (2011)
    The feasibility of improving the radiopacity of NiTi by means of Nd:YAG laser cladding is investigated in the present study. Fine elementary tungsten powder was pasted on NiTi sheets and then melted using laser. The resulting microstructure was analyzed by a scanning electron microscope (SEM) equipped with energy-dispersive X-ray (EDX) spectrometry. The results show that the solubility of tungsten in the NiTi-matrix is low and the excessive tungsten forms fine precipitates and W-rich compounds, which result in the average W content in the fusion zone up to 8 at.%. The laser-clad sample failed near the end of the plateau with a tensile strength of about 410 MPa. © 2011 Elsevier B.V. All Rights Reserved.
    view abstract10.1016/j.matlet.2011.06.040
  • New cast iron alloys with high wear resistance at elevated temperatures
    Theisen, W. and Gevelmann, G.
    Journal of ASTM International 8 (2011)
    New production processes demand a higher wear resistance of tools, especially at elevated temperatures, in order to make the business more profitable and effective. Based on equilibrium calculations, new cast iron alloys have been developed. The influence of different alloying elements was investigated before first bulk materials were casted. The materials were tested with respect to their microstructure, hardness, (micro)red hardness, fracture toughness, and wear properties (at room and elevated temperatures) and compared to the standard high speed steel HS6-5-3 (≈ M3 class 2). It turns out that one of the new alloys has comparable mechanical properties but superior wear resistance, especially at elevated temperatures, thanks to a tripled carbide content and increased temperature strength of the matrix material. Copyright © 1996-2011 ASTM.
    view abstract10.1520/JAI103236
  • Nucleation and precipitation kinetics of M23C6 and M2N in an Fe-Mn-Cr-C-N austenitic matrix and their relationship with the sensitization phenomenon
    Mújica Roncery, L. and Weber, S. and Theisen, W.
    Acta Materialia 59 (2011)
    This work addresses the austenite decomposition in Fe-20Mn-12Cr-0.24C-0.32N steel. Decomposition of austenite in C + N steels leads to the formation of M23C6 and M2N precipitates. Owing to the fact that decomposition is a temperature- and time-dependent phenomenon, thermodynamic and diffusion simulations of nucleation and growth of precipitates were performed based on the CALPHAD method. A well-known consequence of decomposition is sensitization of the areas surrounding the carbides and nitrides, which is normally associated with Cr depletion. In this study, the influence of element redistribution on the Gibbs free energy of the austenitic phase was analyzed. From the electrochemical equivalence of the difference in the Gibbs energy and the difference in the potential, it was found that the sensitized areas exhibit a less noble potential. A mechanism based on a sensitized anode and a matrix cathode is proposed and is correlated with experimental measures of intergranular corrosion. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2011.06.038
  • Simulation of the external pressure influence on the micro-Structural evolution of a Single Crystal Ni-Based superalloy
    Lopez-Galilea, I. and Huth, S. and Fries, S.G. and Steinbach, I. and Theisen, W.
    Advanced Materials Research 278 (2011)
    The phase field method has been applied to simulate the microstructural evolution of a commercial single crystal Ni-based superalloy during both, HIP and annealing treatments. The effects of applying high isostatic pressure on the microstructural evolution, which mainly retards the diffusion of the alloying elements causing the loss of the orientational coherency between the phases is demonstrated by the simulation and experimental results. © (2011) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/AMR.278.247
  • The impact of processing on microstructure, single-phase properties and wear resistance of MMCs
    Hill, H. and Weber, S. and Huth, S. and Niederhofer, P. and Theisen, W.
    Wear 271 (2011)
    High mechanical loads and abrasive wear are boundary conditions for many tooling materials used in modern economy. One of the standard materials used in high abrasive environments is tool steel, or more specifically cold work tool steel. Wear resistance can be increased by adding hard phases like titanium carbides (TiC) to obtain a particle-reinforced metal-matrix composite (MMC) that can be produced either by hot isostatic pressing (HIP) or super solidus liquid-phase sintering (SLPS). Starting from the same raw materials, these two processes lead to different microstructures. The amount and dispersion of the hard phases as well as their hardness and fracture toughness control the wear resistance. In this study the hardness, Young's modulus and the fracture toughness of titanium carbides in Fe-base MMCs have been investigated by nanoindentation measurements. Owing to the small size of the TiC particles, the matrix properties have a pronounced effect on the results. This effect was studied in dependence on the indentation depth to enable the discussion of both the apparent and the real changes in the properties resulting from the production process and the heat treatment. This contribution also discusses the link between the microstructural properties and the macroscopic wear behavior. © 2011 Elsevier B.V.
    view abstract10.1016/j.wear.2010.11.031
  • Coating of high-alloyed, ledeburitic cold work tool steel applied by HVOF spraying
    Rajasekaran, B. and Mauer, G. and Vassen, R. and Röttger, A. and Weber, S. and Theisen, W.
    Journal of Thermal Spray Technology 19 (2010)
    This study demonstrates the processing of a cold work tool steel (X220CrVMo13-4) coating using HVOF spraying. The coating formation was analyzed based on microstructure, phase, hardness, porosity, oxidation, and adhesion characteristics. An online diagnostic tool was utilized to find out the in-flight characteristics of powder such as temperature and velocity during the coating process to identify the influencing parameters to achieve dense cold work tool steel coatings with low oxidation. The influence of powder size, process parameters, and in-flight characteristics on the formation of cold work tool steel coatings was demonstrated. The results indicated that thick and dense cold work tool steel coatings with low oxidation can be obtained by the selection of appropriate powder size and process parameters. © ASM International.
    view abstract10.1007/s11666-009-9456-5
  • Comprehensive investigations of the supersolidus liquid-phase sintering of two plastic mold steels
    Hill, H. and Weber, S. and Siebert, S. and Huth, S. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 41 (2010)
    The processing of plastics, particularly reinforced composites, necessitates the use of corrosion- and wear-resistant materials for tools that come into contact with the polymer. For such applications, plastic mold steels were developed that offer not only a good wear resistance due to the presence of carbides in a martensitic matrix, but also good corrosion resistance provided primarily by a sufficient amount of dissolved chromium. The common processing route for these high-alloyed materials is the hot isostatic pressing (HIP) of gas-atomized powders (PM-HIP). In this context, sintering plays an insignificant role, except for the processing of metal-matrix composites (MMCs). The development of novel wear- and corrosion-resistant MMCs based on plastic mold steels requires knowledge of the sintering behavior of prealloyed powders of such tool steels. It is well known that alloyed powders can be processed by supersolidus liquid-phase sintering (SLPS), a method leading to almost full densification and to microstructures without significant coarsening effects. In this work, two different gas-atomized powders of plastic mold steels were investigated by computational thermodynamics, thermal analysis, sintering experiments, and microstructural characterization. The results show that both powders can be sintered to almost full density (1 to 3 pct porosity) by SLPS in a vacuum or a nitrogen atmosphere. Experimental findings on the densification behavior, nitrogen uptake, and carbide volume fractions are in good agreement with calculations performed by computational thermodynamics. © 2010 The Minerals, Metals & Materials Society and ASM International.
    view abstract10.1007/s11661-009-0148-z
  • Development of cold work tool steel based-MMC coating using HVOF spraying and its HIP densification behaviour
    Rajasekaran, B. and Mauer, G. and Vaßen, R. and Röttger, A. and Weber, S. and Theisen, W.
    Surface and Coatings Technology 204 (2010)
    The aim of the present study is to develop a Fe-based metal matrix composite (MMC) coating using high velocity oxy-fuel spraying (HVOF) process. A ledeburitic high alloyed cold work tool steel (X220CrVMo13-4) and NbC with an average size of 2μm at different volume fractions have been considered as metal matrix and hard particles respectively. MMC coatings were deposited on austenitic stainless substrates and the coatings were subsequently densified by hot isostatic pressing (HIP) with and without encapsulation. Microstructural analysis of the as-sprayed and HIPed coatings were characterized by SEM and XRD methods. Results showed that the feedstock preparation involving fine NbC was an influencing factor on the coating deposition. A relatively homogeneous dispersion of fine NbC up to 30. vol.% in cold work tool steel matrix was possible using optimized HVOF spraying. Besides, HVOF spraying and its subsequent HIP treatment induced significant microstructural and phase changes in the MMC coatings. The study showed the potential of HVOF spraying for the development of steel based MMC coatings and its subsequent densification can be achieved by HIP process with and without encapsulation. © 2010 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2010.05.001
  • Development of Mn-Cr-(C-N) corrosion resistant twinning induced plasticity steels: Thermodynamic and diffusion calculations, production, and characterization
    Roncery, L.M. and Weber, S. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 41 (2010)
    In this work, the development of corrosion-resistant twinning induced plasticity steels is presented, supported by thermodynamic and diffusion calculations within the (Fe-Mn-Cr)-(C-N) alloy system. For the calculations, ambient pressure and primary austenitic solidification were considered as necessary to avoid nitrogen degassing in all processing steps. Manganese is used as an austenite stabilizer, chromium is used to increase nitrogen solubility and provide corrosion resistance, and carbon and nitrogen are used as interstitial elements to provide mechanical strength. Isopleths of the different elements vs temperature as well as isothermal sections were calculated to determine the proper amount of Mn, Cr, total interstitial content, and the C/N ratio. Scheil and diffusion calculations were used to predict the extent of microsegregations and additionally to evaluate the effect of diffusion annealing treatments. The materials were produced in laboratory scale, being followed by thermomechanical processing and the characterization of the microstructure. Tensile tests were performed with three different alloys, exhibiting yield strengths of 460 Mpa to 480 MPa and elongations to fracture between 85 pct and 100 pct. © The Minerals, Metals & Materials Society and ASM International 2010.
    view abstract10.1007/s11661-010-0334-z
  • Evaluation of factors influencing deep cryogenic treatment that affect the properties of tool steels
    Oppenkowski, A. and Weber, S. and Theisen, W.
    Journal of Materials Processing Technology 210 (2010)
    Deep cryogenic treatment (DCT) of tool steels is used as an additive process to conventional heat treatment and usually involves cooling the material to liquid nitrogen temperature (-196 °C). This kind of treatment has been reported to improve the wear resistance of tools. In this study, the Taguchi method was used to identify the main factors of DCT that influence the mechanical properties and the wear resistance of the powder metallurgically produced cold-work tool steel X153CrVMo12 (AISI D2). Factors investigated were the austenitizing temperature, cooling rate, holding time, heating rate, and tempering temperature. In order to study the significance of these factors and the effect of possible two-factor interactions L27(313), an orthogonal array (OA) was applied to conduct several heat treatments, including a single DCT cycle directly after quenching prior to tempering. The results show that the most significant factors influencing the properties of tool steels are the austenitizing and tempering temperatures. In contrast, the parameters of deep cryogenic treatment exhibit a lower level of significance. Further investigations identified a nearly constant wear rate for holding times of up to 24 h. The wear rate reaches a minimum for a longer holding time of 36 h and increases again with further holding. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2010.07.007
  • Low-temperature martensitic transformation and deep cryogenic treatment of a tool steel
    Tyshchenko, A.I. and Theisen, W. and Oppenkowski, A. and Siebert, S. and Razumov, O.N. and Skoblik, A.P. and Sirosh, V.A. and Petrov, Y. and Gavriljuk, V.G.
    Materials Science and Engineering A 527 (2010)
    The tool steel X220CrVMo 13-4 (DIN 1.2380) containing (mass%) 2.2C, 13Cr, 4V, 1Mo and the binary alloy Fe-2.03. mass% C were studied using transmission electron microscopy, Mössbauer spectroscopy, X-ray diffraction and internal friction with the aim of shedding light on processes occurring during deep cryogenic treatment. It is shown that the carbon atoms are essentially immobile at temperatures below -50 °C, whereas carbon clustering in the virgin martensite occurs during heating above this temperature. An increase in the density of dislocations, the capture of immobile carbon atoms by moving dislocations, the strain-induced partial dissolution of the carbide phase, and the abnormally low tetragonality of the virgin martensite are found and interpreted in terms of plastic deformation that occurs during martensitic transformation at low temperatures where the virgin martensite is sufficiently ductile. © 2010 Elsevier B.V.
    view abstract10.1016/j.msea.2010.07.056
  • Mechanical properties of blood clots - A new test method
    Krasokha, N. and Theisen, W. and Reese, S. and Mordasini, P. and Brekenfeld, C. and Gralla, J. and Slotboom, J. and Schrott, G. and Monstadt, H.
    Materialwissenschaft und Werkstofftechnik 41 (2010)
    A blood clot needs to have the right degree of mechanical, chemical and biological properties to stem the flow of blood and yet to be suitable for lytic enzymes or mechanical thrombectomy so as not to form a thrombotic event. The origin and understanding of these mechanical properties are still not known in detail. Clots are made of a three-dimensional network of fibrin fibers stabilized through ligation with a transglutaminase, factor XIIIa. New methods to achieve information about mechanical properties were established in this work. We performed compressive strength experiments of aged human blood clots. Furthermore after the set up of a new test environment, it was possible to perform tensile strength measurements of aged animal blood clots. Stress strain curves of aged clots were measured and discussed. The viscoelastic properties of the clot material were quantitatively described. This work should make a contribution to a better understanding of the behaviour of aged blood thrombus bulk material and induced mechanical stress. In der Behandlung ischämischer Schlaganfälle sind mechanische, chemische und biologische Eigenschaften von Blutgerinnseln von essentieller Bedeutung und entscheidend über den Erfolg einer Lyse-Therapie bzw. mechanischer Thrombektomie. Die Einflussfaktoren der Gerinnungsbedingungen auf die mechanischen Eigenschaften sind jedoch bisher noch nicht eingehend untersucht. Aus Schrifttum ist bekannt, dass auf mikrostruktureller Ebene die Blutgerinnsel aus Fibrinfasern, die untereinander in Knotenpunkten verbunden sind und in der Gesamtheit ein dreidimensionales Netzwerk bilden, bestehen. Dabei haben besonders die Faserdicke und die Knotenpunktendichte einen direkten Einfluss auf die mechanischen Eigenschaften eines Thrombus. Im Rahmen dieser Arbeit wurden Druck- und Zugversuche an Thrombus-Proben (hergestellt aus humanem bzw. tierischem Blut) durchgeführt und die Messergebnisse hinsichtlich der Gerinnungsbedingungen diskutiert. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201000703
  • Microstructure and wear properties of novel sintered cold work steel and related particle reinforced composite materials
    Weber, S. and Li, J.R. and Theisen, W.
    Materials Science and Technology 26 (2010)
    In the field of wear resistant materials it is known, that for certain applications steel based composites produced by powder metallurgy are beneficial due to a higher wear resistance compared to conventional cast materials. Early developments of these high wear resistant MMC were dependent on hot isostatic pressing but latest experimental findings nowadays also allow for a production by liquid phase sintering. Several materials systems have already been investigated which, however, lack of a sufficient hardness in the as sintered state. Especially for wear resistant coatings it would be beneficial to avoid a separate hardening of a coated component. The low hardness in the as sintered state is related to the transformation kinetics of the metallic matrix of the coating materials, typically leading to a comparatively soft pearlitic microstructure, if the cooling rate is too low. The use of a nickel alloyed PM cold work steel as matrix material avoids this restriction, as the formation of pearlite and bainite is delayed significantly, improving the hardenability of the steel. Adding coarse hard particles of chromium carbide (Cr 3C2), aluminium zirconium oxide (AlZrO) or titanium carbide (TiC) to the steel powder, composite materials with a high abrasive wear resistance can be obtained by liquid phase sintering. The development of these materials, supported by thermodynamic calculations, is presented here together with results of the microstructural investigation and wear tests. © 2010 Institute of Materials, Minerals and Mining.
    view abstract10.1179/026708309X12506933872982
  • Thick tool steel coatings using HVOF spraying for wear resistance applications
    Rajasekaran, B. and Mauer, G. and Vaßen, R. and Röttger, A. and Weber, S. and Theisen, W.
    Surface and Coatings Technology 205 (2010)
    This paper deals with the processing of thick cold work tool steel coating using high velocity oxy-fuel (HVOF) spraying process. A full factorial experimental design was established to identify the influencing process parameters on the formation of dense coating with low oxidation. Microstructural analysis of the coating was carried out using optical, SEM and XRD techniques. Cold work tool steel coatings with a thickness up to 2 mm were developed on bond coated low carbon steel substrates for wear resistance evaluation. A pin on disc test was performed to examine the wear resistance of thick cold work tool steel coatings on different types and sizes of abrasive papers. The wear results were compared with the wear resistance of a standard high speed steel pin. The abrasive wear resistance of cold work tool steel coated pins was found to be superior against soft and fine abrasive papers than the standard high speed steel. Besides, the performance of the coated pins against hard and coarser abrasive papers was found to be similar to standard high speed steel. The study showed the potential of HVOF spraying on the development of thick cold work tool steel coatings for wear resistance applications. © 2010 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2010.09.041
  • alloys

  • calphad

  • corrosion

  • heat treatment

  • mechanical properties

  • microstructure

  • sintering

  • steel

  • superalloys

  • thermodynamics

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