Severe Shot Peening to Get a Nano-Structured Surface Layer in Low-Alloy Steel

Shot peening is a cold work treatment that consists in impacting a metallic surface with a flux of spherical shots with the aim to introduce compressive residual stresses field and work hardening on the surface layer. Both aforementioned parameters are responsible for improved fatigue behaviour of treated components. Recent studies have shown that if applied using severe parameters (severe shot peening), this treatment can be categorized as a severe plastic deformation surface treatment. Severe shot peening is able to strongly modify the microstructure of the surface layer of material by increasing the dislocation density that define new grain boundaries and progressively form ultrafine structure. In this work, conventional shot peening and severe shot peening treatments were applied to 39NiCrMo3 steel samples with the aim to study the microstructural changes. The samples were also characterized in terms of surface roughness, micro hardness, residual stresses, and surface work-hardening. Particular attention was focused on the microstructural analysis as a function of the coverage level, with the aim to assess the evolution of grain size from the surface to the inner material and induced gradient microstructure. The results indicated that severe shot peening causes a more remarkable improvement of the general surface mechanical characteristics with respect to conventional shot peening, and revealed a notable microstructural alteration induced by the treatment.