Prof. Dr. Sebastian Weber

Materials Technology
Ruhr-Universität Bochum

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  • Additive manufacturing of a carbon-martensitic hot-work tool steel using a powder mixture – Microstructure, post-processing, mechanical properties
    Großwendt, F. and Röttger, A. and Strauch, A. and Chehreh, A. and Uhlenwinkel, V. and Fechte-Heinen, R. and Walther, F. and Weber, S. and Theisen, W.
    Materials Science and Engineering A 827 (2021)
    This work examines the processing of a hot-work tool steel using laser-based powder bed fusion of metals (PBF-LB/M). The hot-work tool steel was produced using a low-cost powder mixture consisting of pure iron and other elemental powders as well as ferroalloys. Furthermore, a prealloyed starting powder with the same nominal chemical composition as the powder mixture was produced by inert-gas atomization. Besides, a reference steel was produced by casting to compare the microstructures and mechanical properties resulting from the different processing routes. The first step examined the application of a chemically homogeneous and dense layer of the powder mixture prior to PBF-LB/M densification. In addition to evaluate suitable process parameters for PBF-LB/M processing of the starting materials, the microstructure formation was comprehensively examined using electron microscopy and the processes adapted to it. To eliminate defects (cracks, pores) and chemical inhomogeneities, thermal posttreatments, namely supersolidus liquid phase heat-treatment (SLPHT) and hot isostatic pressing (HIP) were performed. Suitable heat-treatment parameters were evaluated. Finally, the obtained microstructures and the associated properties of the post-processed PBF-LB/M samples were compared with those in the reference states. As a main result, it was possible to achieve full redensification and simultaneous chemical homogenization of the PBF-LB/M-processed powder mixture by SLPHT post-processing. The hardness of the additively manufactured and SLPHT-post-processed specimens exceeds that of the cast reference. © 2021 Elsevier B.V.
    view abstract10.1016/j.msea.2021.142038
  • CrMnFeCoNi high entropy alloys with carbon and nitrogen: mechanical properties, wear and corrosion resistance
    Chmielak, L. and Mujica Roncery, L. and Niederhofer, P. and Weber, S. and Theisen, W.
    SN Applied Sciences 3 (2021)
    The use of interstitial elements has been a key factor for the development of different kinds of steels. However, this aspect has been little explored in the field of high entropy alloys (HEAs). In this investigation, the effect of carbon and nitrogen in a near-equiatomic CrMnFeCoNi HEA is studied, analyzing their impact on the microstructure, and mechanical properties from 77K to 673K, as well as wear, and corrosion resistance. Carbon and nitrogen are part of the FCC solid solution and contribute to the formation of precipitates. An increase in the yield and ultimate tensile strength accompanied with a decrease in the ductility are the main effects of C and N. The impact toughness of the interstitial-free material is higher than that of C and C+N alloyed systems. Compared to CrNi and CrMn austenitic steels, the wear resistance of the alloys at room temperature is rather low. The surface corrosion resistance of HEAs is comparable to austenitic steels; nevertheless HEAs are more susceptible to pitting in chloride containing solutions. © 2021, The Author(s).
    view abstract10.1007/s42452-021-04814-y
  • Effect of Heat Treatment on the Microstructure of Cast Martensitic Stainless Steel Einfluss der Wärmebehandlung auf die Mikrostruktur von nichtrostendem martensitischen Stahlguss
    Van Gen Hassend, F. and Weber, S.
    Praktische Metallographie/Practical Metallography 58 (2021)
    The resistance of martensitic stainless steels to wear and corrosion is greatly influenced by the martensitic matrix and the presence of carbides. The precipitation of carbides along the grain boundaries will lead to a significant decrease in fracture toughness and furthermore, will increase the risk of intergranular corrosion. With tools made of corrosion-resistant steel castings, this fact is of particular relevance as coarse eutectic carbide precipitates are normally not sufficiently dissolved during conventional austenitization. In this context, the dissolution of carbides will be studied on the basis of systematic heat treatment experiments and observed using light optical microscopy and the resulting microstructure and its impact on the mechanical properties (hardness) will be discussed in the following sections. © 2021 Walter de Gruyter GmbH, Berlin/Boston 2021.
    view abstract10.1515/pm-2021-0013
  • 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
  • Hot wear of single phase fcc materials—influence of temperature, alloy composition and stacking fault energy
    Berger, A. and Walter, M. and Benito, S.M. and Weber, S.
    Metals 11 (2021)
    The severe sliding abrasion of single‐phase metallic materials is a complex issue with a gaining importance in industrial applications. Different materials with different lattice structures react distinctly to stresses, as the material reaction to wear of counter and base body is mainly de-termined by the deformation behavior of the base body. For this reason, fcc materials in particular are investigated in this work because, as shown in previous studies, they exhibit better hot wear behavior than bcc materials. In particular, three austenitic steels are investigated, with pure Ni as well as Ni20Cr also being studied as benchmark materials. This allows correlations to be worked out between the hot wear of the material and their microstructural parameters. For this reason, wear tests are carried out, which are analyzed on the basis of the wear characteristics and scratch marks using Electron Backscatter Diffraction. X‐Ray experiments at elevated temperatures were also carried out to determine the microstructural parameters. It was found that the stacking fault energy, which influences the strain hardening potential, governs the hot wear behavior at elevated temper-atures. These correlations can be underlined by analysis of the wear affected cross section, where the investigated materials have shown clear differences. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met11122062
  • Impact of the allowed compositional range of additively manufactured 316l stainless steel on processability and material properties
    Großwendt, F. and Becker, L. and Röttger, A. and Chehreh, A.B. and Strauch, A.L. and Uhlenwinkel, V. and Lentz, J. and Walther, F. and Fechte-Heinen, R. and Weber, S. and Theisen, W.
    Materials 14 (2021)
    This work aims to show the impact of the allowed chemical composition range of AISI 316L stainless steel on its processability in additive manufacturing and on the resulting part properties. ASTM A276 allows the chromium and nickel contents in 316L stainless steel to be set between 16 and 18 mass%, respectively, 10 and 14 mass%. Nevertheless, the allowed compositional range impacts the microstructure formation in additive manufacturing and thus the properties of the manufactured components. Therefore, this influence is analyzed using three different starting powders. Two starting powders are laboratory alloys, one containing the maximum allowed chromium content and the other one containing the maximum nickel content. The third material is a commercial powder with the chemical composition set in the middle ground of the allowed compositional range. The materials were processed by laser-based powder bed fusion (PBF-LB/M). The powder characteristics, the microstructure and defect formation, the corrosion resistance, and the mechanical properties were investigated as a function of the chemical composition of the powders used. As a main result, solid-state cracking could be observed in samples additively manufactured from the starting powder containing the maximum nickel content. This is related to a fully austenitic solidification, which occurs because of the low chromium to nickel equivalent ratio. These cracks reduce the corrosion resistance as well as the elongation at fracture of the additively manufactured material that possesses a low chromium to nickel equivalent ratio of 1.0. A limitation of the nickel equivalent of the 316L type steel is suggested for PBF-LB/M production. Based on the knowledge obtained, a more detailed specification of the chemical composition of the type 316L stainless steel is recommended so that this steel can be PBF-LB/M processed to defect-free components with the desired mechanical and chemical properties. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ma14154074
  • Influence of nitrogen uptake and heat treatment on the microstructural characteristics and corrosion performance of X190CrVMo20-4-1 steel produced by supersolidus liquid-phase sintering
    Farayibi, P.K. and van gen Hassend, F. and Blüm, M. and Weber, S.
    Materials and Corrosion 72 (2021)
    Martensitic stainless steel powder exhibits a high nitrogen uptake when densified by supersolidus liquid-phase sintering in a nitrogen atmosphere, but the optimum uptake, which is beneficial to its resistance to corrosion, is unknown. In this study, the resistance of high-carbon martensitic stainless steel X190CrVMo20-4-1 densified in a nitrogen atmosphere against pitting corrosion was explored. This was to clarify the impact of nitrogen uptake in the steel matrix in the quenched and tempered condition on its corrosion resistance in an aqueous solution. Samples were subjected to potentiodynamic polarisation tests in a de-aerated, 1 wt% NaCl solution. Results revealed that the X190 steel densified in a nitrogen atmosphere at 40-kPa pressure, subjected to deep cryogenic treatment in liquid nitrogen at an austenitising temperature of 1150°C and tempered at 200°C, had the best pitting corrosion resistance with a breakdown potential of 142 ± 11 mV/SCE and a hardness of 738 ± 4 HV10. The matrix around the M7C3 carbides and MX carbonitrides suffered high pitting susceptibility. The implications of this study serve as a basis for the improvement of the functional properties of steels. © 2021 The Authors. Materials and Corrosion published by Wiley-VCH GmbH.
    view abstract10.1002/maco.202112432
  • Microstructural Changes During Short-Term Heat Treatment of Martensitic Stainless Steel—Simulation and Experimental Verification
    Schmidtseifer, N. and Weber, S.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 52 (2021)
    Short-term heat treatments of steels are used for tools and cutlery but also for the surface treatment of a variety of other workpieces. If corrosion resistance is required, martensitic stainless steels like AISI 420L or AISI 420MoV are typically used. The influence of short-term heat treatment on the different metastable states of the AISI 420L steel was examined and reported in this article. Starting from a defined microstructural state, the influence of a short-term heat treatment is investigated experimentally with the help of a quenching dilatometer and computer assisted simulations are carried out. With the results obtained, a simulation model is built up which allows to compute the microstructural changes during a short-term heat treatment to be evaluated without the need for an experiment. As an indicator, the value of the martensite start temperature is calculated as a function of different holding times at austenitizing temperature. The martensite start temperature is measured by dilatometry and compared to calculated values. Validation of simulated results reveals the potential of optimizing steel heat treatment processes and provides a reliable approach to save time, resources and energy. © 2021, The Author(s).
    view abstract10.1007/s11661-021-06280-y
  • Potential of the Recycling of Grinding Sludge by various Powder Metallurgical Processes
    Jäger, S. and Weber, S. and Röttger, A.
    Procedia CIRP 104 (2021)
    The metalworking industry produces a large amount of waste material by machining. In contrast to conventional machining, the waste material generated during a grinding process is not purely metallic, but consists of abrasives, water, lubricants and metal chips. This mixture is called grinding sludge. In terms of ecological and economical aspects, the recycling of these waste materials is very promising. By separating the individual components of the grinding sludge, the recycling potential of the individual components becomes visible. This study focuses on the recycling of the metallic part. For this purpose, various powder metallurgical processes were performed. The produced samples and their properties were then compared with samples made of conventional PM powder. © 2021 Elsevier B.V.. All rights reserved.
    view abstract10.1016/j.procir.2021.11.150
  • Processing of a newly developed nitrogen-alloyed ferritic-austenitic stainless steel by laser powder bed fusion – Microstructure and properties
    Becker, L. and Röttger, A. and Boes, J. and Weber, S. and Theisen, W.
    Additive Manufacturing 46 (2021)
    In this work, a novel alloy design of a stainless steel with a ferritic-austenitic microstructure is derived for PBF-LB/M (powder bed fusion-laser beam/metal). The alloy was developed based on X2CrNiMo17-12-2 steel, for which an austenite volume content of approx. 54 vol% in the PBF-LB/M state was achieved using a reduced Ni equivalent. Partial substitution of Ni by Mn increases the N solubility of the alloy. By melting and further gas-atomizing this melt in an N2 atmosphere, an N content of 0.27 mass% was set in the produced steel powder. This leads to both high strength and high corrosion resistance of the PBF-LB/M-processed steel. However, microstructural investigations in the PBF-LB/M state confirm a microstructure consisting of ferrite, austenite, and Mo- and Cr-rich nitrides of M2N type. The nitrides were not completely eliminated by a subsequent heat treatment of the PBF-LB/M samples. As a result of the solution annealing, the microstructure approaches the thermodynamic equilibrium so that the austenite volume content increases from 54.2 vol% to 92.7 vol%. The higher Cr and N contents result in a higher corrosion resistance of the investigated steel compared to PBF-LB/M-processed X2CrNiMo17-12-2, regarded as the reference material. In addition, the measured strengths are significantly higher due to the larger amounts of austenite/ferrite interfaces and the N-induced solid-solution strengthening effect compared to X2CrNiMo17-12-2. © 2021 Elsevier B.V.
    view abstract10.1016/j.addma.2021.102185
  • Stacking fault energy in relation to hydrogen environment embrittlement of metastable austenitic stainless crni‐steels
    Fussik, R. and Egels, G. and Theisen, W. and Weber, S.
    Metals 11 (2021)
    Metastable austenitic steels react to plastic deformation with a thermally and/or mechan-ically induced martensitic phase transformation. The martensitic transformation to α’‐martensite can take place directly or indirectly via the intermediate stage of ε‐martensite from the single‐phase austenite. This effect is influenced by the stacking fault energy (SFE) of austenitic steels. An SFE < 20 mJ/m2 is known to promote indirect conversion, while an SFE > 20 mJ/m2 promotes the direct conversion of austenite into α’‐martensite. This relationship has thus far not been considered in relation to the hydrogen environment embrittlement (HEE) of metastable austenitic CrNi steels. To gain new insights into HEE under consideration of the SFE and martensite formation of metastable CrNi steels, tensile tests were carried out in this study at room temperature in an air environment and in a hydrogen gas atmosphere with a pressure of p = 10 MPa. These tests were conducted on a conventionally produced alloy AISI 304L and a laboratory‐scale modification of this alloy. In terms of metal physics, the steels under consideration differed in the value of the experimentally deter-mined SFE. The SFE of the AISI 304L was 22.7 ± 0.8 mJ/m2 and the SFE of the 304 mod alloy was 18.7 ± 0.4 mJ/m2. The tensile specimens tested in air revealed a direct γ→α’ conversion for AISI 304L and an indirect γ→ε→α’ conversion for 304mod. From the results it could be deduced that the indirect phase transformation is responsible for a significant increase in the content of deformation‐induced α’‐martensite due to a reduction of the SFE value below 20 mJ/m2 in hydrogen gas atmosphere. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met11081170
  • Study on the Austemperability of Thin-wall Ductile Cast Iron Produced by High-Pressure Die-casting
    van gen Hassend, F. and Ninnemann, L. and Töberich, F. and Breuckmann, M. and Röttger, A. and Weber, S.
    Journal of Materials Engineering and Performance (2021)
    The production of thin-wall ductile iron (TWDI) by high-pressure die-casting (HPDC) is complex because of several metallurgical and microstructural challenges. The present work aims to evaluate the austemperability of components (4 mm thickness) produced by HPDC process. The graphitization kinetics, the pearlite formation during continuous cooling, and the effect of austempering on the evolution of the ausferritic microstructure were investigated using dilatometric tests, microstructural analysis as well as Vickers hardness tests and tensile tests. Results show that components exhibit a brittle behavior because of white structures, small shrinkage cavities, and microporosity in the as-cast condition. Graphitization at 1100 °C allows rapid formation of small graphite particles within a short time (40 s). The critical cooling time (t8/5) to avoid the formation of pearlite upon cooling was found to be 5 s at a martensite start temperature of 193 ± 14 °C. Austempering at 360 °C for 40 min results in an ausferritic microstructure with stable carbon-enriched austenite which provides a high hardness (355 ± 4 HV10) and tensile strength (Rm = 709 ± 65 MPa). The results represent main criteria regarding the producibility of die-casted TWDI, which are helpful for future alloy and heat treatment design. © 2021, The Author(s).
    view abstract10.1007/s11665-021-06252-8
  • 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
  • Uncovering process-structure relationships associated to the hot isostatic pressing of the high-speed steel PMHS 3-3-4 through novel microstructural characterization methods
    Benito, S. and Boes, J. and Matsuo, M. and Weber, S. and Theisen, W.
    Materials and Design 208 (2021)
    Gaining insight into the many agents that determine the underlying material microstructure is essential to engineer new and efficient solutions. Tool steels resistant to abrasive wear are particularly interesting because it is possible to tailor their macroscopic properties by adjusting some of the primary carbide phase features: micromechanical properties, volume fraction, size, and shape. For many popular ledeburitic cold-work and high-speed tool steel alloys, there is a generally good understanding of the effects of the HIP temperature, pressure, and holding time on these traits. Nevertheless, there is still no thorough investigation on the influence of powder size distribution on the primary carbide phase. To that end, we employ in this work novel microstructural characterization methods that shed light on the nature and extent of its influence. We show that powder size has an enduring effect on primary carbide geometric features associated with the solidification process during powder atomization. This work contributes significant process-structure links, which uncover new opportunities for microstructural design. © 2021 The Author(s)
    view abstract10.1016/j.matdes.2021.109925
  • A comparative study on the tribological properties of a cobalt-free superaustenitic stainless steel at elevated temperature
    van gen Hassend, F. and Weber, S.
    Metals 10 (2020)
    The properties of a cobalt-free cast superaustenitic stainless steel (SASS) is investigated comparatively to the commercial high-cobalt alloyed GX15CrNiCo21-20-20 (1.4957, N-155) steel regarding its global hardness and wear resistance at elevated temperature by means of in situ hot hardness tests and cyclic abrasive sliding wear tests against an Al2O3 (corundum) counter-body at 600◦C. In the aged condition, results show that the 1.4957 steel suffers a higher material loss due to brittle failure initiated by coarse eutectic Cr-rich carbides which are incorporated into a mechanically mixed layer during abrasive loading. In contrast, within the Co-free steel eutectic M6(C,N) carbonitrides are distributed more homogeneously showing less tendency to form network structures. Due to the combination of primary Nb-rich globular-blocky MX-type carbonitrides and eutectic M6(C,N) carbonitrides dispersed within an Laves phase strengthened austenitic matrix, this steel provides comparable hardness and significantly improved wear resistance at elevated temperature. Thus, it may be an adequate alternative material to commercial SASS and offers the possibility to save cobalt for future applications. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met10091123
  • 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
  • Densification of a high chromium cold work tool steel powder in different atmospheres by SLPS: Microstructure, heat treatment and micromechanical properties
    Farayibi, P.K. and Blüm, M. and Weber, S.
    Materials Science and Engineering A 777 (2020)
    The degradation of moulds, dies and tools employed in plastic, food and chemical processing industries has necessitated the development of suitable wear and corrosion-resistant materials. As improving the wear and corrosion resistance of iron base alloys tend to have opposing demands regarding chemical composition and heat treatment, optimisation of both parameters has to be kept in mind. One alloying element that is known to improve both corrosion and wear resistance of steels is nitrogen. Hence, an investigation into the densification of high chromium X190CrVMo20-4-1 cold work tool steel in a vacuum and under a nitrogen atmosphere at different pressures via supersolidus liquid-phase sintering (SLPS) process is reported in this paper. The investigation aimed to elucidate the influence of different atmospheres and nitrogen partial pressures employed during densification on the microstructure, optimal heat treatment parameters and micromechanical properties of the steel. Experimental findings were supplemented by computational thermodynamics calculations. The results revealed that increasing nitrogen pressure promoted the diffusion of vanadium from Cr-rich carbides (M7C3) to form V-rich carbonitrides, M(C,N). Optimum quench-hardening temperature was strongly influenced by the matrix chemistry. Upon tempering, the nitrogen-sintered samples had higher secondary hardening potential than the vacuum-sintered at a higher temperature, but a low-temperature tempering is beneficial to the corrosion resistance of the steel. The mechanical properties of the carbides in the densified steels in different atmospheres were influenced by their chemical composition. Experimental observations are in good agreement with computational thermodynamic evaluations. © 2020 Elsevier B.V.
    view abstract10.1016/j.msea.2020.139053
  • Development of a recycling strategy for grinding sludge using supersolidus liquid phase sintering
    Hankel, J. and Jäger, S. and Weber, S.
    Journal of Cleaner Production 263 (2020)
    Recycling strategies for waste products from grinding processes have become an essential concern for the industrial sector, as up to 250,000 tons of grinding sludge is generated annually in Germany alone. In this paper, a suitable recycling strategy for the recovery of the metallic component of industrial sludge generated from cold work tool steel grinding is investigated and reported. Possible reuse of the recovered metallic material as a precursor for supersolidus liquid phase sintering (SLPS), a powder metallurgy process, is assessed for the first time. Using a novel technique, including washing, dry screening and magnetic separation, 50 wt% metallic swarf and 50 wt% abrasives can be recovered from dried, industrial grinding sludge. These metallic swarf are a mixture of discontinuous and continuous microchips with a particle size of up to 250 μm. The metallic swarf tend to agglomerate, resulting in larger accumulations and a correspondingly larger overall size with a particle size of over 1000 μm. Using SLPS, the densification of the metallic swarf was considered promising, as density increases with increasing sintering temperature. A maximum density of 76 vol% was achieved, owing to the morphology of the swarf. Occasionally, a few abrasives were observed in the microstructure. Hence, a new sustainable recycling strategy for the recovery of the metallic swarf in industrial grinding sludge has been proposed and the reuse of swarf as precursor for SLPS has demonstrated promising results. Further work is being undertaken to improve the densification of the metallic swarf by SLPS. © 2020 Elsevier Ltd
    view abstract10.1016/j.jclepro.2020.121501
  • Hard Cladding by Supersolidus Liquid Phase Sintering: An Experimental and Simulation Study on Martensitic Stainless Steels
    Farayibi, P.K. and Blüm, M. and Weber, S.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 51 (2020)
    Martensitic stainless steels are suitable for diverse structural applications but degrade when subjected to wear-prone activities in service. To enhance their service life, the densification of high Cr, martensitic, X190CrVMo20-4-1 tool steel powder on two different martensitic stainless steel substrates via supersolidus liquid-phase sinter (SLPS) cladding was investigated. The objective was to assess the influence of the difference in compositions of the martensitic stainless steels employed as substrates on the interfacial diffusion, microstructure, hardness and bonding strength of the steel-to-steel claddings. Computational thermodynamics and diffusion simulations were employed to supplement experimental findings. Owing to interdiffusion, a M7C3 carbide-free, banded region exists in the X190 adjacent to the interface with the width dictated by chemical potential gradient of carbon. The hardness of the substrate was lower near the interface region because of carbon enrichment, which promoted the presence of retained austenite. An interfacial strength of 798 MPa was achieved with fairly ductile X190 matrix near the cladding interface as the fracture surface was characterized by mixed fracture modes of dimple rupture and cleavage with localized quasi-cleavage features. Experimental observations and computational simulations are in agreement. The implications of the SLPS cladding technique are discussed in the context of tool development. © 2020, The Author(s).
    view abstract10.1007/s11661-020-05953-4
  • Heat treatment optimisation of supersolidus sintered steel compounds [Optimierung der Wärmebehandlung von supersolidus gesinterten Schichtverbunden]
    Farayibi, P.K. and Blüm, M. and Weber, S.
    HTM - Journal of Heat Treatment and Materials 75 (2020)
    The high demands on wear resistant tools have led to the development of wear resistant claddings on a substrate, which can be a low alloyed steel with higher ductility than the cladding to improve the resistance of the tool against fracture. In this study, the post heat treatment of sinter-cladded X245VCrMo9-4 steel coating on X120Mn12 steel substrate was investigated, as it is expected that the substrate remained austenitic while the coating possessed a tough martensitic matrix with uniform dispersion of carbide precipitates. Samples were prepared by sintering at 1250 °C in a vacuum furnace under a nitrogen atmosphere at 80 kPa and a heating rate of 10 K/min, and was allowed to cool in the furnace after a dwell of 30 min at sintering temperature. These samples were subjected to heat treatment by austenitisation, oil quenching and tempering. The effect of heat treatment procedures deployed on the samples was examined using optical microscopy, scanning electron microscopy, X-ray diffraction and hardness. Experimental results were supported by computational thermodynamic calculations. The results indicated that the optimised heat treatment, through which the hardness of the steel coating is significantly enhanced while the substrate microstructure remained austenitic, is by austenitising at 950 °C, quenching and low temperature tempering at 150 °C. Quenching temperature was significant to the hardness of the steel coating, as quenching from higher temperature led to a lower hardness of the matrix when compared to quenching at lower austenitisation temperature owing to a high fraction of retained austenite. © 2020 Carl Hanser Verlag. All rights reserved.
    view abstract10.3139/105.110400
  • Impermeable Charge Transport Layers Enable Aqueous Processing on Top of Perovskite Solar Cells
    Gahlmann, T. and Brinkmann, K.O. and Becker, T. and Tückmantel, C. and Kreusel, C. and van gen Hassend, F. and Weber, S. and Riedl, T.
    Advanced Energy Materials 10 (2020)
    Several applications of perovskite solar cells (PSCs) demand a semitransparent top electrode to afford top-illumination or see-through devices. Transparent conductive oxides, such as indium tin oxide (ITO), typically require postdeposition annealing at elevated temperatures, which would thermally decompose the perovskite. In contrast, silver nanowires (AgNWs) in dispersions of water would be a very attractive alternative that can be deposited at ambient conditions. Water is environmentally friendly without safety concerns associated with alcohols, such as flammability. Due to the notorious moisture sensitivity of lead-halide perovskites, aqueous processing of functional layers, such as electrodes, on top of a perovskite device stack is elusive. Here, impermeable electron transport layers (ETLs) are shown to enable the deposition of semitransparent AgNW electrodes from green aqueous dispersions on top of the perovskite cell without damage. The polyvinylpyrrolidone (PVP) capping agent of the AgNWs is found to cause a work–function shift and an energy barrier between the AgNWs and the adjacent ETL. Thus, a high carrier density (≈1018 cm−3) in the ETL is required to achieve well-behaved J/V characteristics free of s-shapes. Ultimately, semitransparent PSCs are demonstrated that provide an efficiency of 17.4%, which is the highest efficiency of semitransparent p-i-n perovskite solar cells with an AgNW top electrode. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/aenm.201903897
  • Influence of Chemical Inhomogeneities on Local Phase Stabilities and Material Properties in Cast Martensitic Stainless Steel
    van gen Hassend, F. and Weber, S.
    Steel Research International 91 (2020)
    Cr–Mo-alloyed cast martensitic stainless steels are suitable tool materials for a wide field of applications. Local inhomogeneities in the chemical composition, however, affect their local and global properties such as the hardenability and the corrosion resistance. Herein, the influence of microsegregations on phase stabilities and properties is investigated by means of property distribution maps (PDM) which are determined via thermodynamic and empirical calculations based on measured local chemical composition data. The results show that the enrichment of Cr and Mo in interdendritic regions benefits the local corrosion resistance but increases the solvus temperature of M23C6 carbides from 1040 to 1150 °C and depresses the martensite start temperature (Ms) to temperatures below 50 °C locally. As predicted from the PDM, high-temperature austenitization at 1150 °C combined with a cryogenic treatment at −80 °C ensures a martensitic microstructure with relatively high hardness (592 ± 12 HV10) and significantly higher critical pitting potential compared with specimens austenitizized at 1050 °C, which proves PDM to be a powerful tool for the optimization of heat treatment parameters. However, local transformation of austenite into δ-ferrite during austenitization at 1150 °C must be considered. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/srin.201900481
  • Influence of particle reinforcement and heat treatment on the wear resistance of inductively melted hardpaint coatings
    Schwarz, P. and Weber, S. and Deuerler, F.
    Metals 10 (2020)
    Wear-resistant coatings can reduce the high economic damage caused by wear processes. In this study, various protective layers based on the alloy X400CrVMo17-15-2 were investigated. Commonly, the prealloyed metal powder is used for plasma transferred arc powder surfacing. However, in this work, the cost-efficient hardpaint technology was used to produce particle-reinforced (fused tungsten carbides) and non-reinforced coatings. To analyze the wear behavior, the coatings were subjected to abrasion wear and scratch tests. For the abrasion wear test, a grinding pin (Al2O3) is pressed with a defined force against the surface of the rotating sample for 6 h. For the scratch test, a loaded diamond pyramid indenter was employed to create a circular groove on the coatings at a predefined speed. The wear grooves were analyzed with the aid of laser scanning microscopy. In comparison to the coatings in the as-processed condition, the non-reinforced protective layers were investigated after quenching, with and without deep cryogenic treatment, and tempering. The determination of proper heat treatment parameters was supported by computational thermodynamics. It has been confirmed that it is possible to improve the wear resistance of the unreinforced coatings by heat treatment. However, the reinforced layers showed the highest resistance against abrasion. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met10070968
  • Laser scanning microscopy for the evaluation of the solid particle erosion resistance of different hardpaint wear protection layers [Laserscanningmikroskopie zur Beurteilung der Strahlverschleißbeständigkeit unterschiedlicher Hardpaint-Verschleißschutzschichten]
    Schwarz, P. and Weber, S. and Deuerler, F.
    Praktische Metallographie/Practical Metallography 57 (2020)
    view abstract10.3139/147.110643
  • Long-term heat treatment of collector bars for aluminium electrolysis: impact on microstructure and electrical properties
    Hankel, J. and Kernebeck, S. and Deuerler, F. and Weber, S.
    SN Applied Sciences 2 (2020)
    In order to identify possible optimizations regarding the electrical energy efficiency of an aluminium electrolysis cell, the impact of service temperature on microstructure and electrical properties of the cell cathode was investigated. The investigations include experiments regarding the chemical composition, especially the content of carbon, the electrical conductivity and the microstructure at selected positions. Thermodynamic calculations were used to estimate local service temperatures and explain phase transformations and formations. It was found that due to the increased service temperature diffusion processes of carbon took place to a particular extent between cast iron and collector bar. As a result, the carbon content in the collector bar changed from 0.06 to 1.05–1.4 wt%, while in the cast iron a reduction from 3.47 to < 1.50 wt% took place. These processes led to isothermal phase transformations and formations, that changed the matrix of the collector bar from austenitic with low content of ferrite to an austenitic matrix accompanied by precipitation of secondary, predominantly allotriomorphic cementite at service temperature. It was then shown that this has a negative effect on collector bar and decreases the electrical conductivity by up to 26 %. It was also discovered that graphite spheroidization within the grey cast iron has a positive effect on its electrical conductivity, which has increased by 52 %. The results provide the basis to gain an understanding of the carbon diffusion related processes within the cathode of an electrolysis cell and reveal further potential to increase the energy efficiency of primary aluminium production. © 2020, The Author(s).
    view abstract10.1007/s42452-020-03391-w
  • Upcycling strategy of grinding swarf by supersolidus liquid phase sintering
    Jäger, S. and Weber, S.
    Procedia CIRP 90 (2020)
    Machining is an important process in the metalworking industry. All machining operations, like milling, turning or grinding, generate large amounts of waste products. Due to the limited resources, it is necessary to find a working recycling strategy and to close the loop in the circular economy. However, there is no recycling strategy for grinding waste. This could be explained by the composition of the grinding waste, called grinding sludge. Therefore, this research deals with the upcycling of the grinding swarf in a PM process. The grinding swarf are characterized by SEM and XRD and processed by a ball mill treatment and a Supersolidus liquid phase sintering process. © 2020 The Author(s). Published by Elsevier B.V.
    view abstract10.1016/j.procir.2020.01.079
  • 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
  • Critical evaluation of the material characteristics and environmental potential of laser beam melting processes for the additive manufacturing of metallic components
    Teubler, J. and Weber, S. and Suski, P. and Peschke, I. and Liedtke, C.
    Journal of Cleaner Production 237 (2019)
    Nowadays, the main impetus to apply additive manufacturing (AM) of metals is the high geometric flexibility of the processes and its ability to produce pilot or small batch series. In contrast, resource and energy intensities are often not considered as constraints, even though the turnout of additive manufacturing is high, at least compared to chip removing processes. The study at hand analyses the material characteristics and environmental impacts of a hose nozzle as an example of a commercial product of simple geometry. The production routes turning (conventional manufacturing) and laser beam melting (additive manufacturing) are compared to each other in terms of natural resource use, climate change potential and primary energy demand. It is found, that the product shows a lower demand for natural resources when produced via AM, but higher carbon emissions and energy demand when using a steel, that is mainly (80%) produced from high-alloyed steel scrap. However, different case studies during the sensitivity analyses showed that a number of factors highly influence the results: the steel source as well as the source of electricity play a major role in determining the environmental performance of the production routes. The authors also found that other production processes (here cold forging of tubes) might be an eco-friendly alternative to both routes, if feasible from an economic point of view. In regard to the material characteristics, experimental testing revealed that the material advantages of AM produced hose nozzles (in particular higher yield strength) are reduced after a solution heat treatment is applied to the as-produced material, in order to increase corrosion resistance. However, products that do not require this production step might benefit from the higher yield strength, as a lower wall thickness could be realised. © 2019
    view abstract10.1016/j.jclepro.2019.117775
  • 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
  • Extremely Robust Gas-Quenching Deposition of Halide Perovskites on Top of Hydrophobic Hole Transport Materials for Inverted (p-i-n) Solar Cells by Targeting the Precursor Wetting Issue
    Brinkmann, K.O. and He, J. and Schubert, F. and Malerczyk, J. and Kreusel, C. and Van Gen Hassend, F. and Weber, S. and Song, J. and Qu, J. and Riedl, T.
    ACS Applied Materials and Interfaces 11 (2019)
    Lead halide perovskite solar cells afford high power conversion efficiencies, even though the photoactive layer is formed in a solution process. At the same time, solution processing may impose some severe dewetting issues, especially if organic, hydrophobic charge transport layers are considered. Ultimately, very narrow processing windows with a relatively large spread in device performance and a considerable lab-to-lab variation result. Here, we unambiguously identify dimethylsulfoxide (DMSO), which is commonly used as a co-solvent and complexing agent, to be the main reason for dewetting of the precursor solution on hydrophobic hole transport layers, such as polytriarylamine, in a gas-quenching-assisted deposition process. In striking contrast, we will show that N-methyl-2-pyrrolidon (NMP), which has a lower hydrophilic-lipophilic-balance, can be favorably used instead of DMSO to strongly mitigate these dewetting issues. The resulting high-quality perovskite layers are extremely tolerant with respect to the mixing ratio (NMP/dimethylformamide) and other process parameters. Thus, our findings afford an outstandingly robust, easy to use, and fail-safe deposition technique, yielding single (MAPbI3) and double (FA0.94Cs0.06PbI3) cation perovskite solar cells with high efficiencies (∼18.5%). Most notably, the statistical variation of the devices is significantly reduced, even if the deposition process is performed by different persons. We foresee that our results will further the reliable preparation of perovskite thin films and mitigate process-to-process variations that still hinder the prospects of upscaling perovskite solar technology. Copyright © 2019 American Chemical Society.
    view abstract10.1021/acsami.9b15867
  • Handling of blackplate in metallographic preparation and heat treatment optimization via quenching dilatometry [Handhabung von Feinstblechen in der metallographischen Präparation und Wärmebehand-lungsoptimierung mittels Abschreckdilatometrie]
    Schmidtseifer, N. and Weber, S.
    Praktische Metallographie/Practical Metallography 56 (2019)
    A prerequisite for the successful optimization of a heat treatment for high-alloyed steels is the knowledge about the present initial microstructure. The metallographic preparation and characterization of the initial microstructure poses a challenge, especially for blackplate, due to its low thickness. In this article, a method is presented which facilitates the handling of blackplate during metallographic preparation. With a precise knowledge of the initial microstructure, tests for the optimization of a heat treatment for such blackplate can also be performed. An optimization of the heat treatment process is particularly important for energy and resource efficient manufacturing processes. In this work, a preparation method was developed which allows winding blackplate to hollow cylinders to determine the optimized heat treatment parameters with the help of a quenching and deformation dilatometer. It will be shown that the two methods presented are an efficient but yet reliable way to optimize heat treatment processes of blackplate via laboratory tests. © Carl Hanser Verlag GmbH & Co. KG
    view abstract10.3139/147.110554
  • 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
  • Production and investigation of the wear behavior of inductively melted hardpaint coatings [Herstellung und Untersuchung des Verschleißverhaltens von induktiv eingeschmolzenen Hardpaint-Schutzschichten]
    Schwarz, P. and Deuerler, F. and Weber, S.
    Materialwissenschaft und Werkstofftechnik 50 (2019)
    Friction and wear processes generate significant economic damage annually in industrial production due to maintenance and repair costs as well as loss associated with production downtime. Wear-resistant coatings are a measure for reducing wear. In this context, the novel hardpaint technology for coating of parts with a fusible metal powder composition is described. Components with complex geometries as well holes or undercuts can be coated easily and inexpensively. Two protective layers are discussed and characterized in terms of their microstructure. Density measurements, hardness tests and scanning electron microscopic investigations were carried out. Both powder layers were inductively melted after application and are based on a hard alloy (iron-based) commonly used for plasma transferred arc powder surfacing. Abrasion wear resistance is evaluated based on the results of the wear tests and microstructural investigations. Furthermore the results are discussed in comparison to a martensitic fine-grain steel (Hardox 450), which is commonly used in abrasively stressed areas. Compared to Hardox 450 both hardpaint coatings have a much more wear-resistant behavior due to their hard phases. In addition, it was confirmed that the hardpaint technology is able to embed thermally sensitive fused tungsten carbides which are significantly increasing the wear resistance against abrasion. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/mawe.201800143
  • Relationship between hydrogen embrittlement and Md30 temperature: Prediction of low-nickel austenitic stainless steel's resistance
    Izawa, C. and Wagner, S. and Deutges, M. and Martín, M. and Weber, S. and Pargeter, R. and Michler, T. and Uchida, H.-H. and Gemma, R. and Pundt, A.
    International Journal of Hydrogen Energy 44 (2019)
    Hydrogen embrittlement (HE) of several low-nickel austenitic stainless steels (AISI 300 series) was studied with special attention to the impact of strain induced α′-martensite. The susceptibility of the steels to HE is judged with respect to the relative reduction of area (RRA): The HE susceptibility is lower for larger RRA-values. Strain-induced martensite formation was evaluated within in the framework of the Olson-Cohen model, revealing a linear relationship between RRA and the probability β of martensite nucleus formation in the steels. In order to widen the scope of data evaluation to literature data, the consideration of a parameter alternative to β is required. It is demonstrated that among other parameters the Md30 temperature (Nohara), which assesses the stability against martensitic transformation, can serve as an indicator to predict HE of AISI 300 series steels. Regarding the Md30 temperature (Nohara), a trend-line with respect to the RRA-values is found. Thereby, the RRA-values of low-nickel austenitic stainless steels group into three distinct regimes; (1) for Md30 &gt; −80 °C, where RRA-values decrease with increasing Md30 temperature, (2) at Md30 ≈ −80 °C, where RRA-values show a large variation (‘threshold band’), and (3) for Md30 &lt; −80 °C, showing constant RRA-values of nearly 100%. Some RRA data points that deviate from the trend line can be explained by the special microstructure of the investigated samples. © 2019 Hydrogen Energy Publications LLC
    view abstract10.1016/j.ijhydene.2019.07.179
  • Role of surface oxide layers in the hydrogen embrittlement of austenitic stainless steels: A TOF-SIMS study
    Izawa, C. and Wagner, S. and Deutges, M. and Martín, M. and Weber, S. and Pargeter, R. and Michler, T. and Uchida, H.-H. and Gemma, R. and Pundt, A.
    Acta Materialia 180 (2019)
    Hydrogen environment embrittlement (HEE) of low-nickel austenitic stainless steels (AISI 300 series) with different chemical compositions was studied focusing on the impact of the steels surface oxides, grain sizes and dislocation arrangements. The susceptibility of the steels to HEE is judged with respect to the relative reduction of area (RRA), where the HEE susceptibility is lower for larger RRA values. For many AISI 300 steels a linear trend is observed correlating RRA and the probability of strain induced martensite formation in tensile tests. Some steels, however, depart from the general trend, revealing greater HEE resistances. A careful examination of possible factors influencing HEE of the investigated steels reveals that high RRA values are linked to a specific type of oxide layer, namely the “high constant level oxide”, as categorized by TOF-SIMS evaluation. Thus, this type of oxide layer may be able to lower the steels HEE susceptibility. Other types of surface oxides, grain sizes and dislocation arrangements in the matrix of the particular AISI 300 steels appear to be of secondary importance. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.09.019
  • Analysis of the micro-processes occurring on different wear protection layers under abrasive and impact wear using laser scanning microscopy [Analyse der Mikroprozesse beim Abrasiv-und Prallverschleiß an unterschiedlichen Verschleißschutzschichten mittels Laser-Scanning-Mikroskopie]
    Schwarz, P. and Deuerler, F. and Weber, S. and Peterseim, J.
    Praktische Metallographie/Practical Metallography 55 (2018)
    Laser scanning microscopy is a particularly suitable technique for the non-destructive analysis of surfaces as well as for the identification of micro-processes under abrasive wear. It is furthermore very well suited to reveal material damage under impact load and to simultaneously determine the gravimetric loss of mass and quantitatively determine the advancing depth of the depression. The benefit of the separately added hard materials under abrasive load and the considerably less favorable behavior of layer A under impact load (due to the more brittle network microstructure) could be evidenced. © Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/147.110529
  • Derivation of property distribution images from microstructural analyses of X2CrNi18-9 with regard to hydrogen embrittlement [Ableitung von Eigenschaftsverteilungsbildern aus mikrostrukturellen Analysen des X2CrNi18-9 im Zusammenhang mit der Wasserstoffversprödung]
    Fussik, R. and Weber, S.
    Praktische Metallographie/Practical Metallography 55 (2018)
    The scope of this research project covers the examination of the microstructure of different austenitic steels with regard to H embrittlement. It is known from the literature that the alloy X2CrNi18-9 is susceptible to H embrittlement, whereas steel X2CrNiMo18-14-3 is highly resistant to the embrittlement process. Austenitic steels with a low γ stability are susceptible to form α' martensite during plastic deformation. The γ stability can be estimated using empirical formulas, while one drawback of this approach is that only a limited amount of alloying elements is considered by those equations. A thermodynamic approach has proven effective for the estimation of the γ stability of multicomponent alloys. Hence, results from the microsegregation analysis for alloy X2CrNi18 - 9 using energy dispersive X-ray spectroscopy are presented and their influence on the local γ stability is discussed based on property distribution images. © 2018 Carl Hanser Verlag, München.
    view abstract10.3139/147.110522
  • 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
  • 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 short-term heat treatment on the mechanical properties of Al–Mg–Si profiles
    Kernebeck, S. and Weber, S.
    Metals 8 (2018)
    Aluminum profiles—for instance, profiles made of precipitation-hardenable alloys—are increasingly used for decorative details in the automotive industry. Typically, after hot extrusion and at least two to three days of natural aging (NA), the aluminum profiles are artificially aged. A commercial EN AW-6060 alloy of high purity was used for this investigation. Tensile tests were used as the main measurement method. This article focuses on the effect of short-term heat treatment on the point in time at which a significant increase of the ultimate tensile strength (UTS) during NA can be measured. Short-term heat treatment is shown to delay this point in time by almost four days, but it increases the variation of UTS. A heterogeneous temperature profile during short-term heat treatment was identified as one reason for this result. Finally, a strategy for minimizing variations in mechanical properties of artificially-aged aluminum alloys was developed, based on the experimental results of this study. © 2018 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met8090664
  • Investigation of austenitic FeCrNi steels with regard to stacking-fault energy and thermal austenite stability
    Fussik, R. and Walter, M. and Theisen, W. and Weber, S.
    Materialia 3 (2018)
    The mechanical properties of face-centered cubic (fcc) metals are influenced by physical parameters of the material, such as the stacking fault energy (SFE). It is known that a low SFE improves the strain hardening, thus increasing the abrasive wear resistance over a wide temperature range. Therefore, investigating the SFE is highly important for the characterization of the physical properties of materials at elevated temperatures. In the present study, the SFE of several austenitic stainless steels was determined by using a calculation model based on Calphad data for investigating the SFE depending on temperature. It can be shown that the lowest SFE value was calculated for the system Fe-27Cr-22Ni including interstitial elements (C+N < 0.1 mass%). This constitution was found by increasing the Cr content to a maximum considering the thermal austenite stability. In this context, the influence on the SFE and austenitic stability of the main alloying elements (Cr, Ni) were examined in detail. To determine the SFE values experimentally, alloys were produced on a laboratory scale and analyzed using X-ray diffraction line-profile analysis (XRD-LP). The results show good match between the calculated and measured SFE values. The calculations show that an increase of the Cr/Ni ratio decreases the SFE in FeCrNi alloys. Moreover, the represented calculation model is suitable for estimating the SFE over a wide temperature range, avoiding costly and time-consuming experiments. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.mtla.2018.08.020
  • 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
  • Investigation of the local austenite stability related to hydrogen environment embrittlement of austenitic stainless steels
    Fussik, R. and Egels, G. and Theisen, W. and Weber, S.
    Materials Science Forum 941 MSF (2018)
    Hydrogen is increasingly considered as fuel for future mobility or for stationary applications. However, the safe distribution and storage of pure hydrogen is only possible with suitable materials. Interstitially dissolved hydrogen atoms in the lattice of numerous metals are responsible for hydrogen embrittlement (HE). If hydrogen is introduced by an external source, it is called hydrogen environment embrittlement (HEE). Commonly, steels like AISI 316L with a high resistance to HEE include a large number of alloying elements and in high amount. High alloying levels result in a decrease of cost-efficiency. Therefore, the systematic investigation of lean-alloyed austenitic stainless steels is necessary in order to understand the mechanism of HEE. For that purpose, the steel grades AISI 304L and AISI 316L are selected in this work. Tensile tests in air and 400 bar hydrogen gas atmospheres are performed. After tensile testing in H, AISI 304L revealed secondary cracks at the specimen surface, which are related to the local austenite stability, which in turn is affected by the level of micro-segregation. The microstructural investigations of the crack environment directly contribute to the understanding of the micro-mechanisms of HEE. Property-maps generated from experimentally measured distributions of alloying elements allow to correlate the impact of micro-segregations on the local austenite stability. It is shown, that local segregation-bands affect the initiation and propagation of secondary cracks. In this context, the local austenite stability which is significantly affected by the Ni distribution will be discussed in detail by comparison of the metastable austenitic steel grades AISI 304L and AISI 316L. © 2018 Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/MSF.941.263
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • S-N fatigue properties of a stable high-aluminum austenitic stainless steel for hydrogen applications
    Michler, T. and Naumann, J. and Weber, S. and Martin, M. and Pargeter, R.
    International Journal of Hydrogen Energy 38 (2013)
    The fatigue properties of a novel high aluminum austenitic stainless steel with a high resistance against hydrogen embrittlement were investigated. S-N tests in 40 MPa H2 at -50 C resulted in a reduction in fatigue life by a factor of about 2 compared to air. Striation analysis revealed no acceleration of crack growth rate but accelerated crack initiation or accelerated short crack growth in H2. No apparent difference in fatigue fracture characteristics and striation morphology between the air and H2 tested specimens could be identified. Copyright © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijhydene.2013.05.145
  • SIMS analysis on austenitic stainless steel: The influence of type of oxide surface layer on hydrogen embrittlement
    Izawa, C. and Wagner, S. and Martin, M. and Weber, S. and Pargeter, R. and Michler, T. and Uchida, H.H. and Pundt, A.
    Journal of Alloys and Compounds 580 (2013)
    The surface oxide layer on nine different types of solution-annealed austenitic stainless steel (ASS) was evaluated by Secondary Ion Mass Spectrometry (SIMS). Tensile tests were conducted at 25 C and -50 C in hydrogen at 40 MPa and in air at atmospheric pressure. Relative reduction of area (RRA) was calculated accordingly. Additionally the martensite transformation curves were measured in air at -50 C to evaluate the probability of the formation of α′ martensite nuclei. A linear relationship between RRA and the calculated probability of the nucleation of α′ martensite nuclei was found on samples represented by the following three oxides types; (a) thick oxide, (b) decaying sub-surface oxide and (c) thin surface oxides. However, type (d) with high constant oxide level down to 60 nm depth does not follow the trend. The observed variation of the oxide layer is linked to the variability of tensile test results. © 2013 Published by Elsevier B.V.
    view abstract10.1016/j.jallcom.2013.02.153
  • 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
  • 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
  • Hydrogen environment embrittlement of stable austenitic steels
    Michler, T. and San Marchi, C. and Naumann, J. and Weber, S. and Martin, M.
    International Journal of Hydrogen Energy 37 (2012)
    Seven stable austenitic steels (stable with respect to γ → α′ transformation at room temperature) of different alloy compositions (18Cr-12.5Ni, 18Cr-35Ni, 18Cr-8Ni-6Mn-0.25N, 0.6C-23Mn, 1.3C-12Mn, 1C-31Mn-9Al, 18Cr-19Mn-0.8N) were tensile tested in high-pressure hydrogen atmosphere to assess the role of austenite stability on hydrogen environment embrittlement (HEE). The influence of hydrogen on tensile ductility was small in steels that are believed to have a high initial portion of dislocation cross slip (18Cr-12.5Ni, 18Cr-35Ni, 18Cr-8Ni-6Mn-0.25N), while the effects of hydrogen were significantly greater in steels with other primary deformation modes (planar slip in 18Cr-19Mn-0.8N and 1C-31Mn-9Al or mechanical twinning in 0.6C-23Mn and 1.3C-12Mn) despite comparable austenite stability at the given test conditions. It appears that initial deformation mode is one important parameter controlling susceptibility to HEE and that martensitic transformation is not a sufficient explanation for HEE of austenitic steels. Copyright © 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijhydene.2012.08.071
  • 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
  • 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
  • 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
  • Investigations of Wear Mechanisms in Diamond Tools with Fe-Based Matrices Reinforced with WC-Co Particles
    Garcia, J. and Weber, S. and Kostka, A. and Pyzalla, A. and Garcia, L.F. and Lammer, A.
    Friction, Wear and Wear Protection: International Symposium on Friction, Wear and Wear Protection 2008 Aachen, Germany (2011)
    view abstract10.1002/9783527628513.ch92
  • 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
  • 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
  • SIMS study on the surface chemistry of stainless steel AISI 304 cylindrical tensile test samples showing hydrogen embrittlement
    Izawa, C. and Wagner, S. and Martin, M. and Weber, S. and Bourgeon, A. and Pargeter, R. and Michler, T. and Pundt, A.
    Journal of Alloys and Compounds 509 (2011)
    The local surface chemistry of a low-Ni austenitic stainless steel AISI type 304 used for tensile testing in hydrogen atmosphere is characterized by secondary ion mass spectrometry (SIMS). A chemical map on cylindrical austenitic stainless steel samples can be obtained even after a tensile test. In an effort to obtain the proper chemical element distribution on the samples, the influence of contamination and sample orientation is discussed. An iron oxide on top of a chromium oxide layer is present and Si segregation at grain boundaries is detected. Oxides are judged to reduce the initial hydrogen attack but to be of minor importance for crack propagation during the embrittlement process. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jallcom.2010.12.143
  • 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
  • Microstructure and mechanical properties of laser-welded joints of TWIP and TRIP steels
    Mujica, L. and Weber, S. and Pinto, H. and Thomy, C. and Vollertsen, F.
    Materials Science and Engineering A 527 (2010)
    With the aim of investigating a laser-welded dissimilar joint of TWIP and TRIP steel sheets, the microstructure was characterized by means of OM, SEM, and EBSD to differentiate the fusion zone, heat-affected zone, and the base material. OIM was used to differentiate between ferritic, bainitic, and martensitic structures. Compositions were measured by means of optical emission spectrometry and EDX to evaluate the effect of manganese segregation. Microhardness measurements and tensile tests were performed to evaluate the mechanical properties of the joint. Residual stresses and XRD phase quantification were used to characterize the weld. Grain coarsening and martensitic areas were found in the fusion zone, and they had significant effects on the mechanical properties of the weld. The heat-affected zone of the TRIP steel and the corresponding base material showed considerable differences in the microstructure and properties. © 2009 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.msea.2009.11.050
  • 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

  • corrosion

  • damage

  • mechanical properties

  • microstructure

  • powder metallurgy

  • sintering

  • steel

  • thermal properties

  • wear resistance

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