Surface Integrity Variations of Stainless Steel 304 upon Severe Shot Peening

Abstract

Shot peening is one of the most powerful mechanical peening techniques applied in surface engineering to improve the mechanical properties of metal alloys by introducing compressive residual stresses. During shot peening process, surface integrity, such as microstructures, surface roughness, hardness and residual stress can be varied by adjusting the operating parameters of shot peening. To facilitate the understanding of the microstructure and properties variation, in our latest work, shot peening with high coverage percentage has been employed to create severe plastic deformation on stainless steel 304 (SUS304). Morphological and topological study on the shot peened coupons were carried out using microscopes and 3D optical profiler. Residual stress depth profiling was evaluated using X-ray diffraction technique. Microstructures, phase distribution and crystal orientation were analyzed by electron backscatter diffraction technique. The results show that phase transition γ - α’ and grain refinement occur during this mechanical peening process. A nanocrystalline layer with preferred orientation formed near the surface, and dislocations accumulated within sub-surface area, which can be attributed to the high energy input from the peening process. The maximum compressive residual stress, which is around 1000-1200 MPa, occurred beneath the top surface. All these findings will provide guideline for surface engineering at various scales and designing of the surface enhancement process via mechanical peening for achieving optimum surface integrity of metallic alloys.