Fatigue behavior of X70 microalloyed steel after severe shot peening

The so called “severe plastic deformation” (SPD) processes are object of increasing interest due to their ability to obtain a nanostructured surface layer of material with supposed superior properties. Among these processes, severe shot peening (SSP) is very attractive from an industrial point of view, due to its versatility and wide applicability. In this study the X70 microalloyed steel is considered and treated by severe shot peening. Fatigue tests were performed to assess how SSP treatment affects the fatigue behavior of this steel. Roughness and X-ray diffraction residual stress measurements as well as microscopy observations have been carried out on the treated specimens. Rotating bending fatigue tests at room temperature on smooth and notched specimens were performed to evaluate the effect of the treatment on fatigue strength. Fracture surfaces have been then observed by scanning electron microscopy. The results were compared with the ones of a series of not peened specimens and another series of specimens shot peened with conventional parameters and interpreted by considering the characteristics of the nanostructured layer, the induced residual stresses and the surface work hardening. The comparison shows a very significant fatigue strength improvement for severely shot peened specimens, especially for the notched specimens.