, Volume 36, Issue 2, pp 421-432

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

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Abstract

The effect of strain rate on strain-induced γα′-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 3×10−4 and 200 s−1. The most important effect of the strain rate was found to be the adiabatic heating that suppresses the strain-induced γα′ transformation. A correlation between the work-hardening rate and the rate of γα′ transformation was found. Therefore, the changes in the extent of the α′-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 α′-martensite fraction is a linear function of flow stress. This indicates that the formation of α′-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 β parameter decreased with increasing strain rate, which indicates a decrease in the chemical driving force of the αα′ transformation.