Superplasticity of fine-grained austenitic stainless steels: A review

An appropriate fraction of a second phase for controlling the dynamic grain growth of the fine-grained microstructure during hot deformation can be easily achieved for the high and ultrahigh carbon steels as well as the duplex stainless steels (dual-phase ferritic-austenitic steels), which leads to good superplastic forming behaviors. However, the austenitic stainless steels are typically single-phase alloys at elevated temperatures, which might limit their tensile ductility, and hence, inducing superplastic ductility in these ferrous alloys needs special considerations. In the present review article, firstly, the methods for the grain refinement of austenitic stainless steels are summarized, which includes the formation of deformation-induced martensite during cold deformation and its reversion to austenite at elevated temperatures, severe plastic deformation (SPD) techniques, and thermomechanical processing routes that utilize the dynamic recrystallization (DRX). These methods are used to process fine-grained microstructures that are suitable for activating the grain boundary sliding (GBS) with strain rate sensitivity index (m) of ~0.5 at elevated temperatures. Afterward, the reported works on the superplasticity of austenitic stainless steels are critically discussed. It is revealed that the methods such as nitrogen addition, incorporating the carbonitride forming elements such as vanadium, increasing the carbon content of the material for the formation of carbides, and the incomplete reversion treatment for the retention of a small volume fraction of martensite can be used to increase the thermal stability of the ultrafine grained (UFG) microstructure against grain coarsening during superplastic deformation. Finally, some distinct suggestions for future works are introduced.