The effect of Ni depletion on athermal martensitic transformation in 304 austenitic stainless steel

Chemical and microstructural analyses were performed on Ni depletion zones in 304 austenitic stainless steel sheets after annealing and cooling with three different rates (0.2, 18.6, 160.0 ?C/s). The analysis was focused on the characteristics and the origin of the retained 6-ferrite and ??-martensite phases in the austenite matrix. Simultaneous Ni depletion and Cr enrichment belong to the retained 6-ferrite due to incomplete decomposition after the solidification process. Athermal ??-martensite was observed to grow within Ni-depleted zones with no Cr enrichment. The fraction of ??-martensite increases as the cooling rate increases. EPMA analysis reveals that the 1 wt% reduction of Ni contents to the bulk chemical composition promotes the thermally-activated martensitic transformation. Such athermal martensite tended to grow within twinning boundaries holding the Nishiyama-Wassermann orientation relationship with the austenite matrix confirmed with the electron diffraction patterns in TEM and the misorientation analysis in EBSD.