Effects of Strain Rate on Deformation-Induced Martensite in 304 Stainless Steel

Abstract

Sheet specimens of 304 stainless steel were tested in uniaxial tension over a temperature range of −173 to +100°C at low strain rate (10−3/s) and high strain rate (103/s). Two shook loading experiments were conducted; one at −105°C and one at ambient temperature. The amount of strain-induced α′ martensite was measured magnetically, the temperature increase during deformation was measured with a thermocouple, and the substructure was examined by transmission electron microscopy. At room temperature and small strain levels, more α′ martensite was produced at high rate than at low rate. At strains >0.25, low-rate deformation produced significantly more α′ martensite. Tests over a range of temperatures showed that the decrease in α′ at large strains and high strain rates resulted from an increase in temperature due to adiabatic heating. Micro structurally, we found that α′ nucleates at shear band intersections. At high rate, there are more shear bands, but the growth of the α′ embryos is restricted by the temperature increase. During shock loading, α′ martensite formed at both test temperatures, thereby demonstrating that transformation is possible at such high rates, and under the predominantly compressive stress state.