Peening

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.

Plastic deformation from peening induces a residual compressive stress in a peened surface, along with tensile stress in the interior. This stress state resembles the one seen in toughened glass, and is useful for similar reasons.

Surface compressive stresses confer resistance to metal fatigue and to some forms of corrosion, since cracks will not grow in a compressive environment. The benefit comes at the expense of higher tensile stresses deeper in the part. However, the fatigue properties of the part will be improved, since the stresses are normally significantly higher at the surface in part due to surface imperfections and damage.

Cold work also serves to harden the material's surface. This makes cracks less likely to form at the surface and provides resistance to abrasion. When a metal undergoes strain hardening its yield strength increases but its ductility decreases. Strain hardening actually increases the number of dislocations in the crystal lattice of the material. When a material has a great number of dislocations, plastic deformation is hindered, and the material will continue to behave in an elastic way well beyond the elastic yield stress of the non-strain hardened material.

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