Excellent strength-toughness synergy in metastable austenitic stainless steel due to gradient structure formation

The effect of cold swaging and subsequent annealing on the structure and mechanical properties of a 321-type metastable austenitic stainless steel was studied. Cold swaging to a strain of 90% produced a gradient structure with a gradual decrease in the alpha-phase content from the edge to the center of the bar. Globular and lamellar regions of both strain-induced martensite and retained austenite were found in the center, meanwhile, a completely globular austenite-martensite dual-phase structure was formed at the edge. After annealing at 500 degrees C, (similar to 30 degrees below the reverse martensite-to-austenite phase transformation temperature) microstructure refinement due to simultaneous development of recrystallization and reverse martensite-to-austenite transformation was observed. Both ultimate tensile strength and yield strength increased noticeably after the annealing. Besides, notch toughness of the annealed specimen doubled in comparison to that of the cold-swaged condition and almost reached the level of a coarse-grained condition.