Surface nanocrystallization of 316L stainless steel induced by ultrasonic shot peening

A new technique was introduced to realize surface nanocrystallization (i.e. generation of a surface layer of nanostructures) on a 316L stainless steel by means of ultrasonic shot peening treatment. The microstructural evolution of the 316L stainless steel was characterized by using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. After the ultrasonic shot peening treatment, obvious grain refinement was observed and a nanocrystalline surface layer was found. The thickness of the nanocrystalline surface layer varies from a few to about 30 μm depending upon the treatment duration and the grain size increases from about 10 nm at the top surface layer, gradually to more than 100 nm at a depth of ≈30 μm. The nanocrystallization mechanism was analyzed in terms of the deformation behavior and TEM observations of the microstructural evolution of the treated samples.