Shot peening or shot peeling is the mechanical process that consists in hammering the surface "cold", by means of a jet of metal shot thrown, at high speed and high intensity, against the surface.

Shot peening is carried out to improve the degree of finish and the resistance capacity of mechanical products, when they are subjected to fatigue stress.
Shot peening is frequently used for:

increase the fatigue resistance capacity and the degree of surface hardness of the machines and structural elements. In this case, shot-peening is a much preferable treatment compared to traditional thermochemical treatments, both because it guarantees, unlike these, greater versatility, and because it allows to reduce the impact of the treatment on the environment and to limit production costs;
improve the distribution of stresses on surfaces, altered by mechanical processing or heat treatments
considerably reduce the concentration of stresses caused by surface notches, threads and decarburization
guarantee greater resistance to stress-corrosion
decrease the porosity of the surface
ensure that the surface is able to more effectively retain any traces of lubricating oils and greases
delay or prevent the formation of cracks on alloy components subjected to tension and stress