Mechanische Oberflächenbehandlung von hochmanganhaltigen TWIP/TRIP Stählen – Mikrostrukturelle Stabilität and Mechanische Eigenschaften
Mechanische Eigenschaften von metallischen Werkstoffen und ihre mikrostrukturellen Ursachen
Zusammenfassung der Projektergebnisse
The main objective of the project reported here was to analyse and rationalize the characteristic properties of high-manganese steels (HMnS) with respect to their near surface properties directly established by mechanical surface treatment processes. Focus was on characteristic gradients as well as their stability under mechanical and thermal loading. The thermo-mechanically processed high-manganese TWIP steel X40MnCrAl19-2 was the material of choice. Besides shot peening, deep rolling treatments in different temperature regimes (-196 °C, RT, 200 °C and 550 °C) were the main surface treatment process used. Through application of high-resolution electron microscopic analysis as well as X-Ray diffraction, profound insights into microstructural evolution as well as most relevant elementary mechanisms contributing to the final properties of the surface-treated layers were obtained. A detailed analysis of the cyclic stress-strain (CSS) behavior in direct relation to the nearsurface properties allowed to evaluate the most important contributions to the fatigue properties of the surfacetreated conditions in the HCF-regime. The results obtained clearly show that the near-surface layers are characterized by activation of different mechanisms depending on the process temperatures considered. While deformation induced phase transformation is promoted at low temperatures (TRIP-effect), the near-surface layers after treatments at RT as well as elevated temperatures are characterized by the formation of deformation twins (TWIP-effect). Depending on the resistance of the material against plastic deformation, the different conditions are also characterized by different effective depths as well as residual stress profiles. The CSS behavior derived from the fatigue tests in the HCF-regime allows a classification of the investigated conditions into four different groups depending on the plastic strains seen. These four groups can be rationalized by the effective depth of the surface treatment processes. The influence of the effective depth can be linked to a surface-core-model. The simplistic model elaborated allows, in consideration of the near-surface properties, for final evaluation of the fatigue behavior as a function of the different surface treatment processes, even for novel materials such as high entropy alloys (HEA). To proof that the basic findings can be transferred to other alloy systems, in addition to the results summarized above, comprehensive investigations were carried out on the CoCrFeMnNi HEA. Besides investigations on the evolution of residual stress, microstructure and cyclic performance after deep rolling at RT and cryogenic temperature, the effect of severe plastic deformation (SPD) on the cyclic properties in the LCF-regime was studied. In case of the TWIP steel in focus, final investigations focusing on the influence of the number of passes during deep rolling revealed a significant influence regardless of the process temperature considered. At this point further tests need to be conducted in order to expand the database. Future work should, thus, focus on consecutive process control applying different deep rolling temperatures for the purpose of exploiting possible synergies of different conditions in the near-surface area.
Projektbezogene Publikationen (Auswahl)
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On the Evolution of Residual Stresses, Microstructure and Cyclic Performance of High-Manganese Austenitic TWIP-Steel after Deep Rolling. Metals, 9(8), 825.
Oevermann, Torben; Wegener, Thomas & Niendorf, Thomas
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Consequences of Deep Rolling at Elevated Temperature on Near-Surface and Fatigue Properties of High-Manganese TWIP Steel X40MnCrAl19-2. Applied Sciences, 11(21), 10406.
Wegener, Thomas; Krochmal, Marcel; Oevermann, Torben & Niendorf, Thomas
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Evolution of residual stress, microstructure and cyclic performance of the equiatomic high-entropy alloy CoCrFeMnNi after deep rolling. International Journal of Fatigue, 153, 106513.
Oevermann, T.; Wegener, T.; Liehr, A.; Hübner, L. & Niendorf, T.
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On the low-cycle fatigue response of CoCrNiFeMn high entropy alloy with ultra-fine grain structure. Acta Materialia, 205, 116540.
Picak, S.; Wegener, T.; Sajadifar, S.V.; Sobrero, C.; Richter, J.; Kim, H.; Niendorf, T. & Karaman, I.
