Peening is a cold working process where the surface of the metal is blasted by shot pellets . This induces compressive stress in the region just below the skin to compensate for the stretching of the skin. When a tensile load is applied on this residual compressed load, the net effect is a reduced amount of stress on the part. This makes the metal well suited for fatigue applications which involve cyclic loads-such as gears, crankshafts etc.
Most weldment surfaces are under tension due to shrinkage after being exposed to welding heat.
Peening can reduce tensile stress by inducing compressive stress on the part. In this type of application peening is done by using hammers.