shotpeening

Shot peening is a mechanical surface treatment in which small, hard spheres, typically steel, stainless steel, glass, or ceramic media, are blasted at high velocity onto a metal surface. The repeated impacts plastically deform the surface, producing a compressive residual stress field that extends beneath the surface. This compressive layer improves fatigue strength, resistance to cracking, and sometimes wear resistance.

Methods and equipment: The media are delivered by air blast, wheel blast, or centrifugal machines. Key process parameters include media size and hardness, shot velocity, exposure time, coverage, and standoff distance. The peening intensity is commonly controlled using Almen strips, reporting intensity in Almen units; coverage is expressed as percent of surface treated.

Outcomes and limitations: Shot peening can slightly roughen the surface, which may be desirable for adhesion in some cases. If misapplied, it can cause excessive deformation, dimensional changes, or embedding of media in porous surfaces. Surfaces must be free of contamination to avoid embedding of residue.

Materials and uses: It is widely used on metallic components such as gears, springs, shafts, fasteners, turbine blades, and aerospace structural parts. While steel shot is common, other media such as stainless steel, glass, and ceramic are used for non-ferrous or temperature-sensitive materials. Shot peening is often used in combination with other finishing treatments and is distinct from laser peening, which induces deeper compressive stresses with a different mechanism.

Standards and history: Shot peening originated in the early 20th century and became prominent in aerospace and automotive industries in the mid- to late 20th century. Industry standards and specifications govern media quality, machine calibration, coverage, and intensity testing (Almen).