Phase reversion induced nanograined austenitic stainless steels: microstructure, reversion and deformation mechanisms

We describe here the attributes of a promising 'phase reversion' approach that results in nanograined/ultrafine grained (NG/UFG) structure in austenitic stainless steels with high strength-high ductility combination. The approach involves severe cold deformation (about 45-75%) of metastable austenite to martensite, which, on annealing for short durations, reverts to austenite via diffusional or shear mechanism, depending on the chemical composition of steel. There was, however, a need to optimise the severity of cold deformation and temperature-time annealing sequence to obtain NG/UFG structure. The fundamental criteria to obtain NG/UFG structure via 'phase reversion' approach were to obtain dislocation cell type martensite. In high strength NG/UFG steel, mechanical twinning contributed to the excellent ductility, while in low strength coarse grained (CG) steel, ductility was also good, but due to nucleation of strain induced martensite at shear bands. The difference in deformation mechanism between NG/UFG and CG steels was attributed to increase in the stability of austenite with decrease in austenite grain size.