Microstructure evolution in dual-phase stainless steel during severe deformation

Deformation microstructures of an Fe–27% Cr–9% Ni dual-phase stainless steel, which was bar rolled/swaged to a total strain of 6.9 at ambient temperature, were studied. After a rapid increase in the hardness during early deformation, the rate of strain hardening slowed and produced a steady-state-like deformation behaviour at strains above 4. The severe deformation resulted in the evolution of similar microstructures in both austenite and ferrite consisting of elongated (sub)grains with a final transverse size of about 0.1 μm and about 70% of high-angle (sub)boundaries. However, the different phases were characterised by different structural change kinetics. The ferrite transverse (sub)grain size decreased continuously, approaching its minimum at large strains above 5.0, while the distinct grain subdivision in the austenite reduced the transverse (sub)grain size to its final value quickly at an early processing stage. The main mechanism of microstructure evolution during the large strain processing was considered to be micro-shearing with dynamic recovery.