ABSTRACT: Shot peening is a surface treatment regularly used to enhance the fatigue life of industrial and mechanical components. Plastic deformation layers are generated by the effects of the bombarded surface. In this regard, recent studies have demonstrated that the Severe Shot Peening (SSP) treatment has an efficient effect on microstructure layers. This event is due to the change of surface grains to nano-scale. Although in some cases, the SSP treatment had an inverse effect, which is called Over Shot Peeing (OSP). Because the OSP treatment results in the formation of nano-cracks below and near the surface. In the present research, different types of SP treatments (conventional and severe) were applied to molybdenum alloy steel to assess the differences induced by microstructures and their effects on fatigue behavior. The microstructures and surface roughness were characterized for each group of specimens. Also, four-point rotating bending fatigue tests were performed on shot-peened specimens. The obtained results showed the notable influence of SP treatment on surface layer microstructures, while at the same time in some coverages, an over peeing was observed.
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