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

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 (ε=0, 0.2 and 0.4) and pre-straining temperature (203 K∼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 ε-martensite increased yield stress and fatigue resistance by fostering deflected-and-branched propagation of fatigue cracks, whereas a high fraction of ε-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 ~20% ε-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.