Thermal stability of nanocrystalline surface layer of AISI 321 stainless steel

A nanocrystalline layer with an average grain size of 75 nm was formed on the top surface layer of 321 stainless steel (321SS) alloy via severe shot peening (SSP). Grazing incidence X-ray diffraction (GI-XRD) and transmission electron microscopy (TEM) were applied to characterize the grain size changes, phase compositions and microstructure evolution after vacuum annealing at temperatures ranging from 200 °C to 1000 °C. The results revealed that nanograins slowly grow to ≈411 nm as the temperature increases to 600 °C. Dislocation annihilation is shown to be responsible for the changes in grain size up to 600 °C. However, an abnormal grain growth is observed at annealing temperatures of 800 °C and 1000 °C in which grains grow to ≈1267 nm and ≈2012 nm, respectively. This abnormal grain growth is attributed to the synergistic effect of dislocation annihilation in grain boundaries and triple points, re-arrangement of dislocations, and formation of middle angle (13°-17°) grain boundaries. Transformation of stress induced martensite (α′) to austenite (γ) and microhardness evolution in the nanocrystalline top surface layer during annealing are also discussed.