Interpretation for the fast sigma phase precipitation in the high intensity shot peened nanocrystallined Super304H stainless steel

Surface shot peening (SP) with an intensity of 0.5 MPa/12 min plus 650 degrees C aging was performed on the Super304H austenitic stainless steel with an attempt to decrease its intergranular corrosion susceptibility more effectively. It was found that the local stress/strain concentration with higher energy at some typical sites caused by over saturated cold deformation induced the early nucleation of sigma phase and the enhanced chromium diffusion in nanocrystallized microstructure promoted its fast growth in the material. The critical shot peening time to trigger fast precipitation of sigma phase is between 8 and 12 min under 0.5 MPa for the Super304H steel. The occurrence of massive sigma phase during aging brought a positive move of the reactivation potential of chromium depleted zones around sigma phase compared to M23C6.