peening

Peening is a metalworking surface-treatment process in which the surface is plastically deformed by repeated impacts or pressure. The goal is to introduce a compressive residual stress near the surface, which improves fatigue strength, reduces susceptibility to stress corrosion cracking, and helps stabilize dimensions after machining, forming, or welding. The resulting surface may also be work-hardened and slightly roughened.

The most common forms are shot peening and hammer or roller peening. In shot peening, a stream of small spherical media (steel, ceramic, or glass) bombards the surface, producing distributed impacts. Hammer peening uses a peening hammer to strike the surface directly, while roller peening employs rotating rollers to impart compressive stress along a surface band. Laser peening uses high-energy laser pulses to generate a shock wave that deepens the compressive zone with minimal surface roughening. Ultrasonic peening uses high-frequency vibrations to drive a peening tool into contact with the surface. Each method offers different depths of influence, coverage, and surface finish.

Process control relies on coverage, intensity, and consistency. Coverage describes the treated fraction of the surface, with full or near-full coverage typical for critical parts. Intensity is often specified in Almen units, measured with Almen strips to correlate with residual-stress depth and performance. Parameters such as media type, flow rate, nozzle distance, angle, and exposure time are selected to achieve the desired effect.

Applications span aerospace, automotive, and power-generation components, including gears, springs, shafts, turbine blades, and welded assemblies. Peening is used to extend service life, improve reliability, and mitigate damage from prior manufacturing or service loads, while balancing potential dimensional changes and surface roughening.