Peening

Hearst Mining building (stone, left) with expansion (shot peened aluminum alloy, right).

Peening is the mechanical working of metals by means of hammer blows or by blasting with shot (shot peening). Peening is a cold work process. It tends to expand the surface of the cold metal, thereby relieving tensile stresses and/or inducing compressive stresses. Peening also encourages strain hardening of the surface metal.

Residual stress

Plastic deformation 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. The tensile stresses deep in the part are not as problematic as tensile stresses on the surface because cracks are less likely to start in the interior.

Work hardening

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.

Copper and other malleable metals respond well to strain hardening. Some forms of copper, such as ductile wire, are easily deformed, yet beaten copper articles are quite stiff. Strain hardening may be reversed by annealing.

Hand peening may be performed using a peening hammer. It is still used today in the hand manufacture of high quality cutting blades.

Use with welding

Hand peening may also be performed after welding to help relieve the tensile stresses that develop in the weld metal and surrounding base metal on cooling. The level of reduction in tensile stress is minimal and only occurs on or near to the weld surface. Peening will induce a higher hardness in to the weld and this is something that should be avoided. For this reason peening is not normally accepted by the majority of codes, standards or specifications. Any peening that is carried out on a weld should have been carried out on the weld procedure qualification test piece.

The welding procedure qualification test piece replicates all of the essential variables that will be used in production welding. If the weld is peened during the qualification of a welding procedure the subsequent mechanical testing of the procedure qualification test piece will demonstrate the mechanical properties of the weld. These mechanical properties must as a minimum match the mechanical properties of the materials that have been welded together. If they do not the procedure has failed and the welding procedure is not acceptable for use in production welding. Peening should only be carried out to a production weld were the procedure test piece has been peened.

Residual stress

Work hardening

Use with welding

See also