Severe shot peening of AISI 321 with 1000% and 1300% coverages: A comparative study on the surface nanocrystallization, phase transformation, sub-surface microcracks and microhardness

In this study, AISI 321 austenitic stainless steel samples were surface treated using severe shot peening (SSP) with 1000 % and 1300 % coverages. Microstructural features including the grain size, phase transformation, and formation of sub-surface microcracks were investigated at the rough top surface and about 40 mu m depth (top surface after grinding and removal of initial rough surface layer created by SSP). In addition, microhardness variations were thoroughly analyzed in-depth. Experimental results demonstrated that for both 1000 % and 1300% coverages, the microstructures of top surface and 40 mu m depth are respectively composed of equiaxed nano-grains and lamell-ashaped cells; however, enhanced imparted strain in the case of 1300 % coverage leads to the formation of considerable amounts of strain-induced martensite (alpha') phase in the surface layers and consequently, due to strength increase and lack of deformability, some microcracks are created in the sub-surface layers (up to 20 mu m depth).