What is "peening"

n. The hardening of a metal surface by hammering, or by blasting with shot

Peening is the process of working a metal's surface to improve its material properties, usually by mechanical means, such as hammer blows, by blasting with shot ( shot peening), or blasts of light beams with laser peening. Peening is normally a cold work process ( laser peening being a notable exception). It tends to expand the surface of the cold metal, thereby inducing compressive stresses or relieving tensile stresses already present. Peening can also encourage strain hardening of the surface metal.