Shot peening

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Shot peening is a process used to produce a decorative finish and to modify mechanical properties of metals. It entails impacting a surface with shot (round metallic particles) with force sufficient to create dimples and with enough shot that those dimples overlap. It is similar to sandblasting, except that it operates by the mechanism of plasticity (physics) rather than abrasion: each particle functions as a ball-peen hammer. In practice, this means that less material is removed by the process, and less dust created.

Peening a surface spreads it plastically in the manner of a rivet, causing changes in the mechanical properties of the surface. Shot peening is often called for in aircraft repairs to relieve stresses built up in the grinding process. See the article on peening for more on this effect.

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

Shot peening may be used for cosmetic effect. The surface roughness resulting from the overlapping dimples causes light to scatter upon reflection. Because peening typically produces larger surface features than sand-blasting, the resulting effect is more pronounced. Shot-peened aluminium sheet, for example, resembles white stone, and was used in the expansion of the Hearst Memorial Mining Building at the University of California, Berkeley.