Engineered surface nanocrystalline structures in SS304 via severe shot peening: Insights into microstructural evolution and mechanical property modulation

Shot peening is a powerful surface engineering technique to improve the mechanical properties of metal alloys by introducing compressive residual stresses. During severe shot peening, a nanocrystalline layer tends to form on the surface, and is accompanied by the variation in surface integrity. In this work, shot peening with high coverage percentage was used to create severe plastic deformation on stainless steel 304 (SUS304) and the formation of the nanocrystalline layer was discussed with the investigation into microstructure, phase distribution, micro-hardness, and young’s moduli. Phase transformation of γ austenite to α´martensite and grain refinement occurred during the mechanical peening process. Four layers with distinctly different microstructures were found after severe peening, which include i) martensitic phased nanocrystals with preferred orientation. ii) martensitic phased nanocrystals with random orientations. iii) transition layer with martensite phase partially transformed from austenite phase. iv) matrix phase. The underlying mechanism of the formation of nanocrystal layer is interpreted as “high impact and repeated peening process triggered phase transformation followed by nano-crystallisation” . Significant increase in hardness by around 100% was found after severe shot peening, and the same can be attributed to both phase transformation and grain refinement induced by severe plastic deformation. A decrease in young’s modulus by ∼5% was observed in the near surface region (<100 μm) which can be ascribed to the severe grain refinement. The findings 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.