Low temperature mechanical behavior of fine- and ultrafine-grained 304 austenitic stainless steel fabricated by cryogenic-rolling and annealing

Fine-grained (FG) austenite structure, and ultrafine-grained (UFG) structure consisting of austenite matrix and residual strain-induced α'-martensite (SIMα′) were fabricated by combining cryogenic-rolling with annealing for 304 austenitic stainless steel (ASS). Tensile and Charpy V-notch impact tests at 20–298 K were carried out to investigate the low temperature mechanical behavior of FG- and UFG-ASS. We found that FG-ASS showed a better match among strength, plasticity and toughness at 20–298 K. The prevailing cleavage was observed in the fracture surface of UFG-ASS at 77 K and 20 K, resulting from the inhibited SIMα′ transformation and residual SIMα′. Single serration of both FG- and UFG-ASS can be divided into Stage I–IV at 20 K, in which Stage II exhibited two mini work-hardening types of three-stage and continuous-reduction. Stress cliff fall, abrupt increase of strain and instantaneous strain rate were found simultaneously during Stage III. The critical temperature to occur serrated flow increased and the serrated flow intensified with refining grain size, accompanying with that more SIMα′ and more stacking faults were detected in FG- and UFG-ASS, respectively. This paper indicated that there was a strong grain size-dependent interaction between serrated flow and SIMα′ transformation for metastable ASS, which was also in favor of understanding the grain size-dependent serrated flow of other metastable alloys.