Shot peening

A method of work hardening the surface of metals by shooting small steel balls (called shot) at it using a special machine similar to a shot blaster.

Steel shot is used since it is ductile and less-likely to break up than cast iron shot. The shot must be spherical and of a size selected for the application. It is therefore carefully filtered to remove any small or broken shot which could damage the surface.

The objective is to strengthen the surface by developing compressive stresses (residual stress) in the surface layers and thereby improve the fatigue properties.

See also shot blasting, for comparison.