Microstructures in Friction Stir Welded 304 Austenitic Stainless Steel

Friction stir welding was applied to 304 austenitic stainless steel. The microstructural evolution and hardness distribution in the weld were investigated. The stir zone (SZ) and thermomechanically affected zone (TMAZ) showed dynamically recrystallised and recovered microstructures, respectively. The hardness of the SZ was higher than that of the base material and the maximum hardness was located in the TMAZ. The higher hardness in TMAZ was attributed to high density of dislocations and sub-grains. Electron microscopic observations revealed that ferrite and sigma phase were formed in austenite matrix in the SZ depending on the cooling rate during FSW.