Microstructural evolution of a nanostructure of shot peened 304 stainless steel upon heat treatment

In this work, the austenitic (γ) 304 stainless steel was shot peened, and heat treated at 500~800 °C for one hour. The microstructural evolutions during the whole processes were studied by using the electron back scattered diffraction (EBSD) and transmission electron microscope (TEM), of which the specimens were prepared by modified samples preparation procedures, like the depth-specific plan-view specimens, utilizing a TEM specimen for EBSD-TEM analyses (3 mm thin foil, and carbon extraction replica). After shot peening, the gradient microstructures from the top treated surface to deep matrix were characterized as, (1) nanostructured (NS) layer containing the phase of γ (fcc), α' (bcc)-, ε (hcp)-martensite, (2) ultra-fine grains (phases of γ, ε, and with deformation twins, shear band, etc.), (3) coarse grains with planar dislocation arrays, indicating the strain induced phase transformation of γ to α'- and ε'-martensite after shot peening. Analyses of the heat treated specimens indicated that reverse phase transformation of α'-, ε'-martensite to γ occurred with the precipitation of M₂₃C₆ in the reversed transformed grains.