Thermal stability of structure and hardness of the surface layer of 316l stainless steel after surface mechanical attrition treatment

After surface mechanical attrition treatment (SMAT) for 316L stainless steel, a gradient structure in its surface layer with grain size from nano-scale to micro-scale was obtained. The samples before and after the SMAT were annealed in vacuum at different temperatures for different durations, and the structural evolution as well as the hardness and the residual stress variations along the depth were analyzed. Experimental results show that when the annealing temperature is lower than 0.5 T_m (T_m is the melt point), no obvious change can be found for the grain size in the gradient structure, except the martensite transformation in the affected layer due to the release of residual stress, and the hardness distribution along the depth remains unchanged. When the annealing temperature is higher than 0.5 T_m, recovery, recrystallization and sharp drop of the residual stress occur in the gradient structure, which induce significant reduction of the hardness along the depth. The effect of annealing duration on the structure and property of the SMAT sample is less important comparing with the annealing temperature.