Peening

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

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Peening is the process of working a metal's surface to improve its material properties, usually by mechanical means such as hammer blows or by blasting with shot (shot 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.

Residual stress

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.

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. Other methods like heat spots if applicable help reduce residual tensile stresses. 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 (ex. ASME B31.3 para 328.5.1 (d)). 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 on a production weld where the procedure test piece has been peened.

History

The first published article about peening was written in Germany in 1929 and was specifically about shot peening. The first patent for shot peening was also taken out in Germany in 1934, however it was never commercially implemented. In 1930, a few engineers at Buick, independent of the Germans, noticed that "shot blasted" (as it was originally termed) springs had a much better fatigue life than the springs that were not. The process was then commercial used in the automotive industry to overcome fatigue issues. Zimmerli was the first to do research on it, publishing a report in 1940, but John Almen was the one who did the most extensive research on the topic. Almen helped expand shot peening to the aircraft industry during World War 2.^[1]

By 1950, peening became an accepted process and was being included in engineering literature. In the same year peen forming was invented to form the wing skin of the Super Constellation aircraft.^[1]

See also

• Almen strip
• Ultrasonic Impact Treatment
• Laser peening
• Peen plating
• Heat treatment
• Case hardening
• Fatigue (material)

References