The purpose of shot peening is to strengthen, or lengthen, the fatigue life of metal. A cold working process used to change or modify the mechanical properties of the metal and produce a compressive residual stress layer.  Process uses steel shot thrown at the part with high velocity, with each particle acting as a ball peen hammer, operating by the mechanism of plasticity rather than abrasion.  The surface is stretched radially as the media hits, causing a plastic flow of the surface fibers, extending to a depth depending on the degree of impact from the shot and the physical properties of the material. There is a brief rise in surface temperature due to the energy created by the shot impact, possibly enough to affect the plastic flow of surface fibers. However, the effect of shot peening is known as cold working to distinguish it from metal flow at high temperatures. Fibers under the top layer are not stretched to yield point and retain elasticity.