Tensile residual stress at the component surfaces after manufacturing processes such as milling, grinding or heat treatment process such as carburizing, carbonitriding and quenching, increase the stress during fatigue loading. This accelerates the fatigue crack initiation and increases the fatigue crack propagation rate. On contrary compressive residual stress induced over the surface of components by shot peening and leads to improving the fatigue resistance which results in delayed crack initiation and retarded crack propagation from the surface. Shot peening is a cold-working process intended to produce surface deformations with the purpose of developing residual compressive stress.

When impact loads on the material surface would cause the surface layer to expand laterally, the layer underneath prevents surface layer expansion, creating the compressive residual stress at the surface. Shot size is an important parameter which will effect on compressive residual stress and surface finish of a component. As shot diameter increase which will increase the magnitude of maximum compressive residual stress as well as push its location inside away from surface. As hardness of shot increases there will be increase in the value of surface compressive residual stress.