Cyclic deformation and phase transformation of 6Mo superaustenitic stainless steel

A fatigue behavior analysis was performed on superaustenitic stainless steel UNS S31254 (Avesta Sheffield 254 SMO), which contains about 6 wt.% molybdenum, to examine the cyclic hardening/softening trend, hysteresis loops, the degree of hardening, and fatigue life during cyclic straining in the total strain amplitude range from 0.2 to 1.5 %. Independent of strain rate, hardening occurs first, followed by softening. The degree of hardening is dependent on the magnitude of strain amplitude. The cyclic stress-strain curve shows material softening. The lower slope of the degree of hardening versus the strain amplitude curve at a high strain rate is attributed to the fast development of dislocation structures and quick saturation. The s martensite formation, either in band or sheath form, depending on the strain rate, leads to secondary hardening at the high strain amplitude of 1.5 %.