EFFECT OF STRAIN RATE ON STRAIN INDUCED α'--MARTENSITE TRANSFORMATION AND MECHANICAL RESPONSE OF AUSTENITIC STAINLESS STEELS

Tensile tests of cold rolled and annealed EN1.4318 (AISI301L) and EN1.4301 (AISI304) stainless steel sheet samples with 2 mm in thickness were performed at the strain rates of 5×10-4 s-1 (slow strain rate) and 2×10-2 s-1 (fast strain rate). The mechanism and volume fraction of strain induced α'--martensite transformation were investigated by using TEM, SEM and XRD. The amount of strain induced α'--martensite in EN1.4318 is much higher than that in EN1.4301 when both steels are deformed at the same strain rate. Adiabatic heating caused by the fast strain obviously decreases the α'--martensite transformation rate and work hardening rate in cold rolled EN1.4318 steel. The amount of α'--martensite and transformation rate for both steels are reduced during uniform deformation compared with those at slow strain rate, this behavior is more significant in cold rolled steels than that in annealed ones. For the more stable EN1.4301 with low saturated amount of α'--martensite (<0.3, volume fraction), rapidly plastic instability and tremendous reduction of uniform elongation are due to the small hardening effect at fast strain. In contrast, for EN1.4318 with low stacking fault energy and rather high saturated amount of α'--martensite, the tensile strength is significant decreased with increasing saturated amount of α'--martensite when deformation at fast strain. The strain rate sensitivity of EN1.4318 is much higher than that of EN1.4301.