Mechanisms of Reverted Austenite and Grain Refinement via Neutron Diffraction for Enhancing Mechanical Properties of 304 Stainless Steel

Austenitic stainless steels possess excellent properties; however, their low yield strengths limit their applications in structural settings. In this study, neutron diffraction was employed along with conventional microstructural characterization to investigate the microstructural and strain behaviors of reverted and deformed austenites in specimens rolled under cryogenic temperature (CRT) and room temperature (RT) and annealed at various temperatures, where different reversion mechanisms occur. After austenite reversion annealing, the yield strength was superior to that of the as-received specimens, regardless of the rolling conditions. The CRT specimens showed high strengths owing to grain refinement. They were composed of strain-induced martensite, leading to the formation of fine reverted austenite. The RT specimens consisted mostly of deformed austenite, resulting in coarse austenite. For the annealing at 590 °C, the reverted austenite grew under strain induced by the martensite. At 900 °C, recrystallization occurred during the heating process, with the fine grains growing preferentially, decreasing the yield strength. At 750 °C, reverted austenite formed through both the diffusional and diffusionless mechanisms. The diffusionally reverted austenite grew without strain, forming abnormally coarse grains, whereas the diffusionless reverted austenite produced recrystallized fine grains. Neutron diffraction analysis revealed that the annealed CRT specimens underwent full recrystallization and the RT specimens underwent incomplete recrystallization, indicating that the deformed austenite recrystallized more slowly than the diffusionless reverted austenite. Superior yield strength and elongation were achieved by annealing at the CRT for 1 min owing to the grain refinement combined with diffusional reverted austenite and diffusionless reverted austenite.

Graphical Abstract