Principle of shot peening, work hardening and cold working process.

J.Almen discovered shot peening in the 1930s while working for General Motors.

Shot peening is a process used to induce superficial compressive stress, and to modify the mechanical properties of metals. The method consists in striking a surface with projectiles (metal, glass or ceramic beads) at high speed, with sufficient force to create a plastic deformation: each ball acts like a mini-hammer.

Objective of shot peening

Shot peening, due to the successive impacts, causes changes in the mechanical properties of the surface. Shot peening is often required for repairs to fragile areas, to compensate for tensile stresses due to machining and to replace them with compressive stresses.

Shot peening can improve fatigue resistance from 0 to 1000%. But this depends of:

• the part geometry and material
• the projectile material,
• the projectile quality,
• the projectile intensity
• it field of action

The compressive surface stresses create fatigue resistance and resistance to certain forms of stress corrosion cracking., mains advantages or this work hardening and cold working process.