Homogenization of microstructure and hardness of surface layer of SS400 steel welded joint

The surface layer of the welded joint of SS400 steel was treated by high-energy shot peening (HESP) technique, and characterized by means of the XRD, TEM and SEM. The hardness variation along the depth of the treated sample was examined after treatment. Experimental results show that the coarse-grained microstructure of top-surface layer of welded joint can be refined into nano-scale after the HESP treatment, and these nanocrystalline grains are about 10 nm in diameter and possess nearly random crystallographic orientations. The same hardness can be found for the nanostructured surface layers of the base metal, HAZ and weld of SS400 steel respectively, which is much higher than that of the sample before the HESP treatment. Homogenization of microstructure and hardness of the surface layer of SS400 steel welded joint are achieved. The strengthening of the surface layer can be primarily attributed to the surface nanocrystallization.