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Effect of the amount and temperature of prestrain on tensile and low-cycle fatigue properties of Fe-17Mn-0.5C TRIP/TWIP steel

Title
Effect of the amount and temperature of prestrain on tensile and low-cycle fatigue properties of Fe-17Mn-0.5C TRIP/TWIP steel
Authors
Song, Seok WeonLee, Jeong HunLee, TaekyungLee, Chong Soo
POSTECH Authors
Lee, Chong Soo
Date Issued
Jun-2017
Publisher
ELSEVIER SCIENCE SA
Abstract
A high-Mn austenitic steel represents excellent combination of tensile strength and ductility, but shows very low yield strength. To increase the yield strength, pre-deformation is applied before use. However, the other properties such as fatigue resistance should also remain high. In this study the influence of pre-strain on tensile and low cycle fatigue (LCF) resistance were investigated. The amount of pre-strain (epsilon = 0, 0.2 and 0.4) and pre-straining temperature (203 K similar to 490 K) were varied, which resulted in the variation of deformation-induced twin boundaries and fraction of martensite. Tensile tests and fully-reversed strain-controlled fatigue tests were conducted on plate-type samples. As the amount of pre-strain increased, yield and tensile strength were increased, but ductility and LCF life were decreased. The LCF properties of samples at a fixed amount of pre-strain were analyzed in the context of the fractions of martensite and mechanical twin boundaries. The presence of a small amount of epsilon-martensite increased yield stress and fatigue resistance by fostering deflected-and-branched propagation of fatigue cracks, whereas a high fraction of e-martensite degraded the mechanical properties. In contrast, the presence of a high fraction of mechanical twin boundaries enhanced both tensile and fatigue properties under the conditions used in this work. The sizes of dislocation cells decreased and spacing of fatigue striations narrowed as the fraction of mechanical twin boundaries increased. Results suggest that a microstructure composed of similar to 20% epsilon-martensite and numerous mechanical twin boundaries can represent superior tensile and fatigue properties; this microstructure was obtained by step pre-straining (SP) process, where pre-strain was imposed twice at different temperatures.
A high-Mn austenitic steel represents excellent combination of tensile strength and ductility, but shows very low yield strength. To increase the yield strength, pre-deformation is applied before use. However, the other properties such as fatigue resistance should also remain high. In this study the influence of pre-strain on tensile and low cycle fatigue (LCF) resistance were investigated. The amount of pre-strain (epsilon = 0, 0.2 and 0.4) and pre-straining temperature (203 K similar to 490 K) were varied, which resulted in the variation of deformation-induced twin boundaries and fraction of martensite. Tensile tests and fully-reversed strain-controlled fatigue tests were conducted on plate-type samples. As the amount of pre-strain increased, yield and tensile strength were increased, but ductility and LCF life were decreased. The LCF properties of samples at a fixed amount of pre-strain were analyzed in the context of the fractions of martensite and mechanical twin boundaries. The presence of a small amount of epsilon-martensite increased yield stress and fatigue resistance by fostering deflected-and-branched propagation of fatigue cracks, whereas a high fraction of e-martensite degraded the mechanical properties. In contrast, the presence of a high fraction of mechanical twin boundaries enhanced both tensile and fatigue properties under the conditions used in this work. The sizes of dislocation cells decreased and spacing of fatigue striations narrowed as the fraction of mechanical twin boundaries increased. Results suggest that a microstructure composed of similar to 20% epsilon-martensite and numerous mechanical twin boundaries can represent superior tensile and fatigue properties; this microstructure was obtained by step pre-straining (SP) process, where pre-strain was imposed twice at different temperatures.
Keywords
INDUCED PLASTICITY STEEL; STACKING-FAULT ENERGY; EPSILON-MARTENSITIC TRANSFORMATION; MEDIUM MN STEEL; TWIP STEELS; DEFORMATION-BEHAVIOR; AUSTENITIC STAINLESS; CRACK PROPAGATION; ALLOY; MICROSTRUCTURE
URI
http://oasis.postech.ac.kr/handle/2014.oak/50406
DOI
10.1016/j.msea.2017.04.099
ISSN
0921-5093
Article Type
Article
Citation
MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, vol. 696, page. 493 - 502, 2017-06
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 LEE, CHONG SOO
Graduate Institute of Ferrous Technology
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