Dr.-Ing. Jonathan Lentz

Materials Technology
Ruhr-Universität Bochum

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  • Effect of Deformation on the Magnetic Properties of C + N Austenitic Steel
    Torres-Mejía, Laura Gabriela and Paredes-Gil, Katherine and Parra Vargas, Carlos Arturo and Lentz, Jonathan and Weber, Sebastian and Mujica-Roncery, Lais
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 55 (2024)
    In this investigation, the effect of deformation on magnetic properties at low temperatures of FeCr18.2Mn18.9–0.96C + N high interstitial steel was studied. Tensile tests were carried out at room temperature and interrupted at 10, 20, and 30 pct deformation. Magnetic measurements were performed through the vibrating sample magnetometry (VSM) technique from 50 K to 370 K. Microstructural, morphological, and crystalline structural analyses by means of XRD and SEM showed that the material consisted of a homogenous and stable austenitic structure with no presence of α-martensite or ε-martensite. Twinning and dislocation cells are suggested as main deformation mechanisms. The material exhibits a paramagnetic–antiferromagnetic (T Néel) transition below 235 K. The Néel temperature of the material tends to increase due to the deformation. A decrease of the magnetization and magnetic susceptibility for the deformed material was measured. Ab initio calculations were performed and showed that the FCC phase is more stable when carbon and nitrogen are added as interstitial elements compared with the free C + N system, additionally, the critical transition temperature was calculated, with a value in agreement with the experimental data. An influence of the magnetic contribution on the SFE was established, being in the order of 5 mJ/m2. © 2023, The Author(s).
    view abstract10.1007/s11661-023-07237-z
  • Effect of Deformation on the Magnetic Properties of C + N Austenitic Steel
    Torres-Mejía, Laura Gabriela and Paredes-Gil, Katherine and Parra Vargas, Carlos Arturo and Lentz, Jonathan and Weber, Sebastian and Mujica-Roncery, Lais
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 55 (2024)
    In this investigation, the effect of deformation on magnetic properties at low temperatures of FeCr18.2Mn18.9–0.96C + N high interstitial steel was studied. Tensile tests were carried out at room temperature and interrupted at 10, 20, and 30 pct deformation. Magnetic measurements were performed through the vibrating sample magnetometry (VSM) technique from 50 K to 370 K. Microstructural, morphological, and crystalline structural analyses by means of XRD and SEM showed that the material consisted of a homogenous and stable austenitic structure with no presence of α-martensite or ε-martensite. Twinning and dislocation cells are suggested as main deformation mechanisms. The material exhibits a paramagnetic–antiferromagnetic (T Néel) transition below 235 K. The Néel temperature of the material tends to increase due to the deformation. A decrease of the magnetization and magnetic susceptibility for the deformed material was measured. Ab initio calculations were performed and showed that the FCC phase is more stable when carbon and nitrogen are added as interstitial elements compared with the free C + N system, additionally, the critical transition temperature was calculated, with a value in agreement with the experimental data. An influence of the magnetic contribution on the SFE was established, being in the order of 5 mJ/m2. © 2023, The Author(s).
    view abstract10.1007/s11661-023-07237-z
  • A comparative study of in-situ alloying in laser powder bed fusion for the stainless steel X2CrNiMoN20-10-3
    Becker, L. and Lentz, J. and Benito, S. and Cui, C. and Ellendt, N. and Fechte-Heinen, R. and Weber, S.
    Journal of Materials Processing Technology 318 (2023)
    This paper compares, for the first time, laser powder bed fusion (PBF-LB/M) processing of a powder mixture (PM), also known as in-situ alloying, with that of a pre-alloyed (PA) powder from gas atomization with the same chemical composition, using the example of X2CrNiMoN20–10–3 ferritic-austenitic stainless steel. The focus is on the differences in the microstructure formation mechanisms during PBF-LB/M between PM and PA using different energy inputs, in order to gain new insights into the process transferability of in-situ alloying to the processing of pre-alloyed powders. The microstructure investigations are carried out using electron backscattered diffraction (EBSD), energy dispersive (EDS) as well as wavelength dispersive X-ray spectrometry (WDS), X-ray diffraction (XRD) and magneto-inductive method (Feritscope®). The microstructures of samples produced from PM and PA differ significantly in terms of the resulting ferritic and austenitic phase fractions, so that a ferritic-austenitic microstructure forms for PM, while the PA is predominantly austenitic. The differences are mainly based on the increased chemical inhomogeneities for the PM in comparison to the PA state, which are discussed based on EDS map analysis through spatial statistics. With increasing energy input, the chemical homogeneity of the PM approaches that of the PA, but it cannot reach it even with maximum energy input. The formation of a ferritic-austenitic microstructure in the case of the PM leads to the formation of a finer microstructure compared to single-phase PA steel resulting in higher hardness of PBF-LB/M-built PM. © 2023 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2023.118038
  • A comparative study of in-situ alloying in laser powder bed fusion for the stainless steel X2CrNiMoN20-10-3
    Becker, L. and Lentz, J. and Benito, S. and Cui, C. and Ellendt, N. and Fechte-Heinen, R. and Weber, S.
    Journal of Materials Processing Technology 318 (2023)
    This paper compares, for the first time, laser powder bed fusion (PBF-LB/M) processing of a powder mixture (PM), also known as in-situ alloying, with that of a pre-alloyed (PA) powder from gas atomization with the same chemical composition, using the example of X2CrNiMoN20–10–3 ferritic-austenitic stainless steel. The focus is on the differences in the microstructure formation mechanisms during PBF-LB/M between PM and PA using different energy inputs, in order to gain new insights into the process transferability of in-situ alloying to the processing of pre-alloyed powders. The microstructure investigations are carried out using electron backscattered diffraction (EBSD), energy dispersive (EDS) as well as wavelength dispersive X-ray spectrometry (WDS), X-ray diffraction (XRD) and magneto-inductive method (Feritscope®). The microstructures of samples produced from PM and PA differ significantly in terms of the resulting ferritic and austenitic phase fractions, so that a ferritic-austenitic microstructure forms for PM, while the PA is predominantly austenitic. The differences are mainly based on the increased chemical inhomogeneities for the PM in comparison to the PA state, which are discussed based on EDS map analysis through spatial statistics. With increasing energy input, the chemical homogeneity of the PM approaches that of the PA, but it cannot reach it even with maximum energy input. The formation of a ferritic-austenitic microstructure in the case of the PM leads to the formation of a finer microstructure compared to single-phase PA steel resulting in higher hardness of PBF-LB/M-built PM. © 2023 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2023.118038
  • A new approach to the powder metallurgical production of high nitrogen steels by integrated diffusion alloying in a hot isostatic press
    Becker, L. and Radtke, F. and Lentz, J. and Herzog, S. and Broeckmann, C. and Weber, S.
    Materials Letters 352 (2023)
    Alloying with nitrogen in stainless steels, with nitrogen being dissolved interstitially, results in improved mechanical properties and higher resistance to local corrosion phenomena. However, the positive effect cannot be arbitrarily scaled to higher nitrogen contents, mainly due to the limited nitrogen solubility in the liquid phase. This paper presents a powder metallurgical approach to the production of a high nitrogen variant of the austenitic stainless steel X2CrNi18-9, in which a powder mixture of steel and Si3N4 powder is hot isostatically pressed and integrally cooled by uniform rapid quenching (URQ®). This procedure, known as diffusion alloying, allows nitrogen to dissociate from Si3N4 and dissolve into the solid austenitic phase, which provides a nitrogen solubility greater than 0.5 mass% under HIP conditions. With this approach, about 0.38 mass% N could be interstitially dissolved in the steel, which, however, can be further increased in the future by adjustments discussed here. © 2023 Elsevier B.V.
    view abstract10.1016/j.matlet.2023.135119
  • A new approach to the powder metallurgical production of high nitrogen steels by integrated diffusion alloying in a hot isostatic press
    Becker, L. and Radtke, F. and Lentz, J. and Herzog, S. and Broeckmann, C. and Weber, S.
    Materials Letters 352 (2023)
    Alloying with nitrogen in stainless steels, with nitrogen being dissolved interstitially, results in improved mechanical properties and higher resistance to local corrosion phenomena. However, the positive effect cannot be arbitrarily scaled to higher nitrogen contents, mainly due to the limited nitrogen solubility in the liquid phase. This paper presents a powder metallurgical approach to the production of a high nitrogen variant of the austenitic stainless steel X2CrNi18-9, in which a powder mixture of steel and Si3N4 powder is hot isostatically pressed and integrally cooled by uniform rapid quenching (URQ®). This procedure, known as diffusion alloying, allows nitrogen to dissociate from Si3N4 and dissolve into the solid austenitic phase, which provides a nitrogen solubility greater than 0.5 mass% under HIP conditions. With this approach, about 0.38 mass% N could be interstitially dissolved in the steel, which, however, can be further increased in the future by adjustments discussed here. © 2023 Elsevier B.V.
    view abstract10.1016/j.matlet.2023.135119
  • High nitrogen steels produced by PBF-LB/M - Process strategy and properties for ceramic additivated metal powders
    Radtke, Felix and Becker, Louis and Herzog, Simone and Lentz, Jonathan and Weber, Sebastian and Broeckmann, Christoph
    Euro Powder Metallurgy 2023 Congress and Exhibition, PM 2023 (2023)
    Additivated powders for powder bed fusion - laser beam (PBF-LB/M) process allow for innovative materials, which cannot be produced by conventional manufacturing techniques. In this study, Si3N4 powder is added to an austenitic steel powder for the manufacturing of high nitrogen steels (HNS). Therefore, two different types of additivation are investigated using tumbling mixers with and without grinding balls. Variations in the oxygen content and particle size distribution of the ceramic powder are analyzed to gain knowledge regarding influences on the rheological and reflective properties of the additivated powder. The flowability and packing density are determined as a function of humidity and drying parameters of the powder. The first samples of the investigated powders were processed by PBF-LB/M. Microscopic investigation reveals new insights into the melting mechanisms of metal/ceramic mixtures. An analysis of the residual powder gives first indications regarding recyclability. © European Powder Metallurgy Association (EPMA)
    view abstract10.59499/EP235764243
  • High nitrogen steels produced by PBF-LB/M - Process strategy and properties for ceramic additivated metal powders
    Radtke, Felix and Becker, Louis and Herzog, Simone and Lentz, Jonathan and Weber, Sebastian and Broeckmann, Christoph
    Euro Powder Metallurgy 2023 Congress and Exhibition, PM 2023 (2023)
    Additivated powders for powder bed fusion - laser beam (PBF-LB/M) process allow for innovative materials, which cannot be produced by conventional manufacturing techniques. In this study, Si3N4 powder is added to an austenitic steel powder for the manufacturing of high nitrogen steels (HNS). Therefore, two different types of additivation are investigated using tumbling mixers with and without grinding balls. Variations in the oxygen content and particle size distribution of the ceramic powder are analyzed to gain knowledge regarding influences on the rheological and reflective properties of the additivated powder. The flowability and packing density are determined as a function of humidity and drying parameters of the powder. The first samples of the investigated powders were processed by PBF-LB/M. Microscopic investigation reveals new insights into the melting mechanisms of metal/ceramic mixtures. An analysis of the residual powder gives first indications regarding recyclability. © European Powder Metallurgy Association (EPMA)
    view abstract10.59499/EP235764243
  • Impact of in Situ Heat Treatment Effects during Laser-Based Powder Bed Fusion of 1.3343 High-Speed Steel with Preheating Temperatures up to 700 °C
    Moritz, S. and Schwanekamp, T. and Reuber, M. and Lentz, J. and Boes, J. and Weber, S.
    Steel Research International (2023)
    view abstract10.1002/srin.202200775
  • Impact of in Situ Heat Treatment Effects during Laser-Based Powder Bed Fusion of 1.3343 High-Speed Steel with Preheating Temperatures up to 700 °C
    Moritz, S. and Schwanekamp, T. and Reuber, M. and Lentz, J. and Boes, J. and Weber, S.
    Steel Research International (2023)
    view abstract10.1002/srin.202200775
  • Improving the Defect Tolerance of PBF-LB/M Processed 316L Steel by Increasing the Nitrogen Content
    Stern, Felix and Becker, Louis and Cui, Chengsong and Tenkamp, Jochen and Uhlenwinkel, Volker and Steinbacher, Matthias and Boes, Johannes and Lentz, Jonathan and Fechte-Heinen, Rainer and Weber, Sebastian and Walther, Frank
    Advanced Engineering Materials 25 (2023)
    Nitrogen (N) in steels can improve their mechanical strength by solid solution strengthening. Processing N-alloyed steels with additive manufacturing, here laser powder bed fusion (PBF-LB), is challenging as the N-solubility in the melt can be exceeded. This degassing of N counteracts its intended positive effects. Herein, the PBF-LB processed 316L stainless steel with increased N-content is investigated and compared to PBF-LB 316L with conventional N-content. The N is introduced into the steel by nitriding the powder and mixing it with the starting powder to achieve an N-content of approximately 0.16 mass%. Thermodynamic calculations for maximum solubility to avoid N outgassing and pore formation under PBF-LB conditions are performed beforehand. Based on the results, a higher defect tolerance under fatigue characterized by Murakami model can be achieved without negatively influencing the PBF-LB processability of the 316L steel. The increased N-content leads to higher hardness (+14%), yield strength (+16%), tensile strength (+9%), and higher failure stress in short time fatigue test (+16%). © 2022 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH.
    view abstract10.1002/adem.202200751
  • Improving the Defect Tolerance of PBF-LB/M Processed 316L Steel by Increasing the Nitrogen Content
    Stern, Felix and Becker, Louis and Cui, Chengsong and Tenkamp, Jochen and Uhlenwinkel, Volker and Steinbacher, Matthias and Boes, Johannes and Lentz, Jonathan and Fechte-Heinen, Rainer and Weber, Sebastian and Walther, Frank
    Advanced Engineering Materials 25 (2023)
    Nitrogen (N) in steels can improve their mechanical strength by solid solution strengthening. Processing N-alloyed steels with additive manufacturing, here laser powder bed fusion (PBF-LB), is challenging as the N-solubility in the melt can be exceeded. This degassing of N counteracts its intended positive effects. Herein, the PBF-LB processed 316L stainless steel with increased N-content is investigated and compared to PBF-LB 316L with conventional N-content. The N is introduced into the steel by nitriding the powder and mixing it with the starting powder to achieve an N-content of approximately 0.16 mass%. Thermodynamic calculations for maximum solubility to avoid N outgassing and pore formation under PBF-LB conditions are performed beforehand. Based on the results, a higher defect tolerance under fatigue characterized by Murakami model can be achieved without negatively influencing the PBF-LB processability of the 316L steel. The increased N-content leads to higher hardness (+14%), yield strength (+16%), tensile strength (+9%), and higher failure stress in short time fatigue test (+16%). © 2022 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH.
    view abstract10.1002/adem.202200751
  • Influence of annealing time on the microstructure and properties of additively manufactured X2CrNiMoN25–7–4 duplex stainless steel: Experiment and simulation
    Becker, L. and Boes, J. and Lentz, J. and Cui, C. and Steinbacher, M. and Li, Y. and Fechte-Heinen, R. and Theisen, W. and Weber, S.
    Materialia 28 (2023)
    view abstract10.1016/j.mtla.2023.101720
  • Influence of annealing time on the microstructure and properties of additively manufactured X2CrNiMoN25–7–4 duplex stainless steel: Experiment and simulation
    Becker, L. and Boes, J. and Lentz, J. and Cui, C. and Steinbacher, M. and Li, Y. and Fechte-Heinen, R. and Theisen, W. and Weber, S.
    Materialia 28 (2023)
    view abstract10.1016/j.mtla.2023.101720
  • Influence of nitrogen content on the corrosion fatigue behavior of additively manufactured AISI 316L stainless steel in chloride solution
    Stern, F. and Becker, L. and Tenkamp, J. and Boes, J. and Lentz, J. and Weber, S. and Walther, F.
    International Journal of Fatigue 172 (2023)
    view abstract10.1016/j.ijfatigue.2023.107666
  • Influence of nitrogen content on the corrosion fatigue behavior of additively manufactured AISI 316L stainless steel in chloride solution
    Stern, F. and Becker, L. and Tenkamp, J. and Boes, J. and Lentz, J. and Weber, S. and Walther, F.
    International Journal of Fatigue 172 (2023)
    view abstract10.1016/j.ijfatigue.2023.107666
  • Improving the Defect Tolerance of PBF-LB/M Processed 316L Steel by Increasing the Nitrogen Content
    Stern, F. and Becker, L. and Cui, C. and Tenkamp, J. and Uhlenwinkel, V. and Steinbacher, M. and Boes, J. and Lentz, J. and Fechte-Heinen, R. and Weber, S. and Walther, F.
    Advanced Engineering Materials (2022)
    Nitrogen (N) in steels can improve their mechanical strength by solid solution strengthening. Processing N-alloyed steels with additive manufacturing, here laser powder bed fusion (PBF-LB), is challenging as the N-solubility in the melt can be exceeded. This degassing of N counteracts its intended positive effects. Herein, the PBF-LB processed 316L stainless steel with increased N-content is investigated and compared to PBF-LB 316L with conventional N-content. The N is introduced into the steel by nitriding the powder and mixing it with the starting powder to achieve an N-content of approximately 0.16 mass%. Thermodynamic calculations for maximum solubility to avoid N outgassing and pore formation under PBF-LB conditions are performed beforehand. Based on the results, a higher defect tolerance under fatigue characterized by Murakami model can be achieved without negatively influencing the PBF-LB processability of the 316L steel. The increased N-content leads to higher hardness (+14%), yield strength (+16%), tensile strength (+9%), and higher failure stress in short time fatigue test (+16%). © 2022 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH.
    view abstract10.1002/adem.202200751
  • Influence of preheating temperatures on material properties of PBF-LB manufactured hot-work tool steel X37CrMoV5-1
    Moritz, S. and Ziesing, U. and Boes, J. and Lentz, J. and Weber, S. and Reuber, M.
    Procedia CIRP 111 (2022)
    view abstract10.1016/j.procir.2022.08.143
  • Laser Additive Manufacturing of Duplex Stainless Steel via Powder Mixture
    Cui, C. and Becker, L. and Gärtner, E. and Boes, J. and Lentz, J. and Uhlenwinkel, V. and Steinbacher, M. and Weber, S. and Fechte-Heinen, R.
    Journal of Manufacturing and Materials Processing 6 (2022)
    Laser additively manufactured duplex stainless steels contain mostly ferrite in the as-built parts due to rapid solidification of the printed layers. To achieve duplex microstructures (ferrite and austenite in roughly equal proportions) and, thus, a good combination of mechanical properties and corrosion resistance, an austenitic stainless steel powder (X2CrNiMo17-12-2) and a super duplex stainless steel powder (X2CrNiMoN25-7-4) were mixed in different proportions and the powder mixtures were processed via PBF-LB/M (Laser Powder Bed Fusion) under various processing conditions by varying the laser power and the laser scanning speed. The optimal process parameters for dense as-built parts were determined by means of light optical microscopy and density measurements. The austenitic and ferritic phase formation of the mixed alloys was significantly influenced by the chemical composition adjusted by powder mixing and the laser energy input during PBF-LB/M. The austenite content increases, on the one hand, with an increasing proportion of X2CrNiMo17-12-2 in the powder mixtures and on the other hand with increasing laser energy input. The latter phenomenon could be attributed to a slower solidification and a higher melt pool homogeneity with increasing energy input influencing the phase formation during solidification and cooling. The desired duplex microstructures could be achieved by mixing the X2CrNiMo17-12-2 powder and the X2CrNiMoN25-7-4 powder at a specific mixing ratio and building with the optimal PBF-LB/M parameters. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/jmmp6040072
  • Martensite Transformation in Tool Steels under Isostatic Pressure–Implementation of In-Situ Electrical Resistivity Measurements into a Hot Isostatic Press with Rapid Quenching Technology
    Kramer, B. and Deng, Y. and Lentz, J. and Broeckmann, C. and Theisen, W. and Weber, S.
    Metals 12 (2022)
    Powder metallurgical (PM) parts usually benefit from more homogenous and finer mi-crostructures as opposed to conventionally processed material. In particular, hot isostatic pressing (HIP) combined with near-net-shape technologies can produce almost defect free PM tools with com-plex geometries. Recent advances in the plant technology of smaller HIP units allow the integration of hardening heat treatments in HIP processes. Thus, additional processing steps, transportation, energy consumption and cost are reduced. However, it is known that high pressure influences phase stability and transformation temperatures. Still, knowledge of the martensite start temperature (MS) is crucial for the design of hardening heat treatment. Since the influence of pressure on MS in HIP heat treatment is insufficiently investigated, it is the aim of this study to deploy a measurement method that allows to record MS as a function of pressure, temperature and cooling rate. Taking the hot working tool steel AISI H11 (X37CrMoV5-1, 1.2343) as the reference material, in this study for the first time the method of an in-situ electrical resistivity measurement was used to measure MS within a HIP. To investigate the influence of HIP pressure on Ms, resulting microstructures and hardness, specimens were austenitized at a temperature of TAUS = 1050◦ C for tAUS = 30 min at pAUS = 25, 50, 100 or 150 MPa. Additionally, the MS temperature of the same material was determined by quenching dilatometry at ambient pressure for comparison purposes. Characterization of microstructures was conducted by scanning electron microscopy while hardness as an important technological property of tool steels was measured according to the Vickers method. Furthermore, the CALPHAD method was used to compute the thermodynamic influence of pressure on phase stabilities. The experimental results indicate that the method of in-situ resistivity measurement can be used to measure MS during an integrated HIP heat-treatment process. Besides, a stabilizing effect of pressure on the close packed crystal structure of the austenitic fcc phase is clearly detected, resulting in a reducing influence on the MS temperature of AISI H11 by up to 90 K. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met12050708
  • Processing of a Martensitic Tool Steel by Wire-Arc Additive Manufacturing
    Ziesing, U. and Lentz, J. and Röttger, A. and Theisen, W. and Weber, S.
    Materials 15 (2022)
    view abstract10.3390/ma15217408
  • Quantification of extremely small-structured ferritic-austenitic phase fractions in stainless steels manufactured by laser powder bed fusion
    Becker, L. and Boes, J. and Lentz, J. and Cui, C. and Uhlenwinkel, V. and Steinbacher, M. and Fechte-Heinen, R. and Theisen, W. and Weber, S.
    Materialia 22 (2022)
    This work investigated processing of stainless steel powders and powder mixtures using powder bed fusion - laser beam/metal (PBF-LB/M), which produced different ferritic and austenitic phase fractions in the as-built state. The rapid cooling and solidification rates in the PBF-LB/M process led to the formation of an extremely small-structured microstructure in which the austenitic phase was found on the grain boundaries and as acicular Widmanstätten austenite (width < 1 µm) within the primary δ-ferritic solidified matrix. This work shows that the time-saving quantification of the ferritic and austenitic phase fractions of these particular microstructures is nontrivial. Common time-efficient phase quantification methods such as image analysis of etched cross-sections or magneto-inductive methods (Feritscope®) have proven to be inaccurate. On the other hand, electron backscattered diffraction (EBSD) investigations proved to be extremely time-consuming in order to resolve the small microstructural constituents sufficiently well and to obtain a reliably large sample section. The highest accuracy was achieved with X-ray diffraction. Two different methods were considered: the Debye-Scherrer method, which was characterized by short measuring times, and the Bragg-Brentano method (quantification using Rietveld refinement), which showed the highest accuracy for the entire range of ferritic-austenitic phase fractions. © 2022 Acta Materialia Inc.
    view abstract10.1016/j.mtla.2022.101393
  • Validation of the Powder Metallurgical Processing of Duplex Stainless Steels through Hot Isostatic Pressing with Integrated Heat Treatment
    Becker, L. and Lentz, J. and Kramer, B. and Rottstegge, A. and Broeckmann, C. and Theisen, W. and Weber, S.
    Materials 15 (2022)
    Duplex stainless steels exhibit an excellent combination of corrosion resistance and strength and are increasingly being manufactured through powder metallurgy (PM) to produce large, near-net-shaped components, such as those used for offshore applications. Hot isostatic pressing (HIP) is often used for PM production, in which pre-alloyed powders are compacted under high pressures and temperatures. Recent developments in HIP technology enable fast cooling as part of the process cycle, reaching cooling rates comparable to oil quenching or even faster. This enables the integrated solution annealing of duplex stainless steels directly after compaction. In contrast to the conventional HIP route, which requires another separate solution annealing step after compaction, the integrated heat treatment within the HIP process saves both energy and time. Due to this potential gain, HIP compaction at a high pressure of 170 MPa and 1150 °C with integrated solution annealing for the production of duplex stainless steels was investigated in this work. Firstly, the focus was to investigate the influence of pressure on the phase stability during the integrated solution annealing of the steel X2CrNiMoN22-5-3. Secondly, the steel X2CrNiMoCuWN25-7-4, which is highly susceptible to sigma phase embrittlement, was used to investigate whether the cooling rates used in the HIP are sufficient for preventing the formation of this brittle microstructural constituent. This work shows that the high pressure used during the solution heat treatment stabilizes the austenite. In addition, it was verified that the cooling rates during quenching stage in HIP are sufficient for preventing the formation of the sigma phase in the X2CrNiMoCuWN25-7-4 duplex stainless steel. © 2022 by the authors.
    view abstract10.3390/ma15186224
  • 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
  • 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
  • Microstructures, Heat Treatment, and Properties of Boron-Alloyed Tool Steels
    Lentz, J. and Röttger, A. and Theisen, W.
    Steel Research International 91 (2020)
    To enable the development of novel Fe–C–B–Cr and Fe–C–B–Cr–Mo cold work tool steels, the microstructures and hardness-tempering behaviors of hypoeutectic laboratory melts are investigated. The results show that increasing Cr content enhances the thermodynamic stability of the ultrahard M2B borides. The formation of carboborides is suppressed by adjusting the B/(C + B) ratio, Cr content, and austenitization temperature. A secondary hardenability at 500 °C is achieved by Mo addition. In addition, Mo stabilizes the M23(C,B)6 phase and at higher contents the M3B2 boride. Based on these investigations, Fe0.4C1B–Cr alloys are designed which, inspired by the microstructure of the steel X153CrMoV12-1, feature a α′-Fe hardenable matrix but 15 vol% of eutectic M2B borides instead of M7C3 for wear protection. The Fe0.4C1B–Cr steels are produced by casting and hot rolling as well as powder metallurgy and hot isostatic pressing. The (tribo-) mechanical properties are investigated and compared with X153CrMoV12-1. Fracture toughness, bending strength, wear resistance, and hardness of the novel Fe0.4C1B–Cr alloys are found to be similar or superior to the steel X153CrMoV12-1, at decreased material cost. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/srin.201900416
  • Time-Dependent Evolution of Microstructure and Mechanical Properties of Mortar
    Röttger, A. and Youn-Čale, B.-Y. and Küpferle, J. and Lentz, J. and Breitenbücher, R. and Theisen, W.
    International Journal of Civil Engineering 17 (2019)
    This study investigates the evolution of the microstructure and mechanical properties of mortar. Mortar samples consisting of Portland cement CEM I42.5 R (~ 60 vol% of quartz sand 0/2 mm, w/c-ratio of 0.5) were prepared and stored according to EN 1015. After 1, 2, 7, 14 and 28 days, the samples were oven-dried until constant weight as well as vacuum-dried. The microstructure of the mortar samples was investigated using scanning electron microscopy. Phase analysis was performed using X-ray diffraction, allowing the description of the crystalline phase evolution during hardening. Mechanical properties were evaluated using nanoindentation. Based on the nanoindentation results, the effective Young’s modulus was calculated using the model by Hashin and Shtrikman. The moduli calculated based on the values of the nanoindentation experiments were compared to the Young’s modulus determined in compression experiments. The results show that the Young’s modulus determined by the nanoindentation and compression test describes a degressive curve progression. The studies show a correlation between the results from nanoindentation tests and the mechanical properties obtained from the compression tests. Therefore, the microstructural evolution of mortar, including the influence of pores on Young’s modulus, must be taken into account to estimate the macroproperties from the nanoindentation tests. © 2018, Iran University of Science and Technology.
    view abstract10.1007/s40999-018-0305-0
  • Enhancement of hardness, modulus and fracture toughness of the tetragonal (Fe,Cr)2B and orthorhombic (Cr,Fe)2B phases with addition of Cr
    Lentz, J. and Röttger, A. and Großwendt, F. and Theisen, W.
    Materials and Design 156 (2018)
    This study analyzes the influence of Cr content on hardness H, elastic modulus E and fracture toughness KIC of the M2B boride by means of nanoindentation experiments. Additionally, properties of the Fe3(C,B) phase are determined. Samples of the M2B phase are casted and microstructurally characterized by means of scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. At a Cr content higher than 14.7 atom% the M2B phase transforms from tetragonal into orthorhombic structure. The tetragonal M2B type possesses an optimum of H (21 ± 1 GPa), E (373 ± 6) GPa and KIC (3.5 ± 0.7 MPam) at 4–5 atom% Cr. The hardness, modulus and toughness of the orthorhombic M2B phase increase with Cr content and reach values of H = 27 ± 0.7 GPa, E = 473 ± 9 of and KIC = 3.26 ± 0.8 MPam at maximal investigated Cr content of 55 atom%. The hardness of the M2B phases decreases around 2.3–3.2 GPa as a function of indentation depth, which is known as the indentation size effect. Hardness and fracture toughness of M2B phase outperform conventionally used M7C3 carbides and are similar to MC-carbides. Findings can be used in novel alloying approaches in order to optimize the performance and reduce cost of tool steels. © 2018 Elsevier Ltd
    view abstract10.1016/j.matdes.2018.06.040
  • Hardness and modulus of Fe2B, Fe3(C,B), and Fe23(C,B)6 borides and carboborides in the Fe-C-B system
    Lentz, J. and Röttger, A. and Theisen, W.
    Materials Characterization 135 (2018)
    This work provides a comparative and comprehensive study of the indentation hardness and indentation modulus of iron-rich borides and carboborides of types Fe2B, Fe3(C,B), and Fe23(C,B)6. In addition, the hardness and elastic modulus of Cr-rich M7C are investigated for comparative purposes. We investigated the impact of increasing B content and indentation size effect (ISE). The phases of interest were stabilized in cast Fe-C-B alloys that varied with respect to the B / (B + C) ratio and heat treatment. The resulting microstructures were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and wavelength X-ray spectroscopy (WDS). Dynamic in-situ nanoindentation experiments based on the method of continuous stiffness measurement (CSM) were coupled to SEM and EBSD investigations to determine the mechanical properties of the individual borides and carboborides as a function of the indentation depth. The results were compared to values obtained for the Cr-rich M7C3 carbide. It was found that the hardness of the B-rich Fe3(C,B) phase is considerably higher than pure Fe3C and increases with increasing B content. The ISE was present in all investigated phases, and the hardness decreased as a function of indentation depth. The hardness at infinite indentation depth H0 was estimated according to the model of Nix and Gao. The Fe2B phase was found to be the hardest phase (H0 = 19.04 GPa), followed by M7C3 (H0 = 16.43 GPa), Fe3(C,B) (H0 = 11.18 to 12.24 GPa), and Fe23(C,B)6 (H0 = 10.39 GPa). © 2017 Elsevier Inc.
    view abstract10.1016/j.matchar.2017.11.012
  • Mechanism of the Fe3(B,C) and Fe23(C,B)6 solid-state transformation in the hypoeutectic region of the Fe-C-B system
    Lentz, J. and Röttger, A. and Theisen, W.
    Acta Materialia 119 (2016)
    This study investigates the microstructural mechanisms involved in the solid-state transformation of the Fe3(B,C) → Fe23(C,B)6 phases in the hypoeutectic region of the iron-carbon-boron (Fe-C-B) system. We analyzed the influence of different initial microstructural characteristics on the Fe3(B,C) → Fe23(C,B)6 transformation with regards to the matrix phase, matrix C content, B/(C + B) ratio, and agglomeration of the parental Fe3(B,C) phase. We performed thermodynamic calculations using the CALPHAD method, validated by laboratory melts with varying B/(B + C) ratios. These laboratory melts were then microstructurally characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and wavelength-dispersive X-ray spectroscopy (WDS). We particularly focused on solid-state transformation of borides and carboborides of type M3(C,B) and M23(C,B)6 in the hypoeutectic region of the ternary system Fe-C-B, investigated via both in situ and ex situ XRD measurements. It was found that the solid-state transformations are influenced by enriched B inside the eutectic structure, a result of solidification. This increased B content is not reduced in solid state due to the kinetic limitations of B and C inside the hard-phase structure. Thus phase stability is subject to local equilibria depending on the local C and B concentration of the hard-phase structure. In this process the Fe23(C,B)6 phase also forms a shell-like structure surrounding the Fe3(B,C) and Fe2B phases. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.08.009
  • Boron-alloyed Fe-Cr-C-B tool steels - Thermodynamic calculations and experimental validation
    Röttger, A. and Lentz, J. and Theisen, W.
    Materials and Design 88 (2015)
    This study focuses on the development of boron-alloyed tool steels. The influence of Cr additions from 0 to 10mass% on microstructural changes were investigated for a constant metalloid content (C+B=2.4mass%). In the first step, thermodynamic calculations were performed to map the quaternary Fe-Cr-C-B system. In the second step, thermodynamic calculations were validated with laboratory melts that were investigated with respect to the microstructure and phase composition. The results of thermodynamic calculations correspond to real material behavior of Fe-Cr-C-B alloys. Furthermore, the influence of chromium on hard phase formation was investigated by means of phase analysis methods, X-ray diffraction (XRD), and energy dispersive spectrometry (EDS). Nanoindentation was used to determine hard phase properties (hardness, Young's modulus). It was shown that chromium promotes the formation of M2B-type borides. An increase in the Cr content within the M2B phase led to a transformation from the tetragonal structure into an orthorhombic structure. This transformation is accompanied by an increase in hardness and in the Young's modulus. In contrast, Cr also promotes the formation of Cr-rich carboborides of type M23(C,B)6. However, an increased Cr content within the M23(C,B)6 phase is not associated with an increase in hardness or elastic modulus. © 2015 Published by Elsevier Ltd.
    view abstract10.1016/j.matdes.2015.08.157
  • Solidification and phase formation of alloys in the hypoeutectic region of the Fe-C-B system
    Lentz, J. and Röttger, A. and Theisen, W.
    Acta Materialia 99 (2015)
    In this work, alloys from the hypoeutectic iron-rich region of the iron-carbon-boron (Fe-C-B) system were investigated with respect to the solidification and the phase formation. Laboratory melts with a constant carbon content of 0.6 mass% and boron contents of 0.2 mass%, 0.6 mass%, and 1.8 mass% were fabricated and metallographically examined. In addition the microstructures were investigated by CALPHAD method in the state of equilibrium and by multiphase-field (MPF) method to reproduce the non-equilibrium process of the technical solidification. The results were analyzed with respect to the effect of boron on the solidification paths, microstructural crystallization processes as well as the morphological and chemical characteristics of the solidified phases. The investigated alloys undergo primary crystallization of austenite (γ-Fe). Due to the low solubility of B in the primary phase γ-Fe, B is strongly segregated in the melt and the solidification paths are deviated to high B contents. Therefore, as the B content increases, the eutectic solidification sequence starts with the B-rich Fe<inf>2</inf>B phase and continues with the formation of the B-rich Fe<inf>3</inf>(B,C) phase in the latter process. The B content of the melt thus decreases during the eutectic reaction, and the eutectic Fe<inf>3</inf>(B,C) phase exhibits a decreasing B gradient in the direction of growth. Consequently, the low-melting phase of the Fe-C-B system is the Fe<inf>3</inf>(B,C) phase with a low B content and a composition closest to its low-melting B content of 14.10 at.% B. Increasing B/(C + B) ratios of the alloy composition raise the average B content of the Fe<inf>3</inf>(B,C) phase (up to >20 at.% B) and hence at the same time increase the solidus temperature of the alloy. These findings revealed consistency with experimental results for chemical composition (WDX), phase analysis (diffraction with synchrotron radiation, EBSD), and thermal analysis (DTA). © 2015, Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.07.037

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