Shot peening

Shot peening is a process used as a decorative finish and/or 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 sand blasting, except that it operates by the mechanism of plasticity 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.

Mechanical effects are caused by peening the surface; that is, spreading it plastically in the manner of a rivet. This induces a residual compressive stress at the surface along with tensile stress in the interior, as seen in toughened glass. Such a stress distribution confers resistance to metal fatigue, as well as some forms of corrosion. Cold work is another major result, and serves to harden the material, making cracks less likely to form at the surface and providing resistance to abrasion.

Cosmetic affects are due to surface roughness from the overlapping dimples, which cause light to scatter upon reflection. Because it produces larger surface features than most sand-blasting processes, 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.