Effect of strain rate on the strain-induced γ→α′-martensite transformation and mechanical properties of austenitic stainless steels

The effect of strain rate on strain-induced gamma --> alpha'-martensite transformation and mechanical behavior of austenitic stainless steel grades EN 1.4318 (AISI 301LN) and EN 1.4301 (AISI 304) was studied at strain rates ranging between 3 x 10(-4) and 200 s(-1). The most important effect of the strain rate was found to be the adiabatic heating that suppresses the strain-induced gamma --> alpha' transformation. A correlation between the work-hardening rate and the rate of gamma --> alpha' transformation was found. Therefore, the changes in the extent of the alpha'-martensite formation strongly affected the work-hardening rate and the ultimate tensile strength of the materials. Changes in the martensite formation and work-hardening rate affected also the ductility of the studied steels. Furthermore, it was shown that the square root of the alpha'-martensite fraction is a linear function of flow stress. This indicates that the formation of alpha'-martensite affects the stress by influencing the dislocation density of the austenite phase. Olson-Cohen analysis of the martensite measurement results did not indicate any effect of strain rate on shear band formation, which was contrary to the transmission electron microscopy (TEM) examinations. The beta parameter decreased with increasing strain rate, which indicates a decrease in the chemical driving force of the gamma --> alpha' transformation.