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Fig. 6. 11 Case hardening process scheme 20

6.6 Shot peening

Shot peening is a cold working process used to produce a compressive residual stress layer and modify
the mechanical properties of metals and composites. It entails striking a surface with a shot (round
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metallic, glass, or ceramic particles) with force sufficient to create plastic deformation. Popular
methods for propelling shot media include air blast systems and centrifugal blast wheels. In the air
blast systems, media are introduced by various methods into the path of high pressure air and
accelerated through a nozzle directed at the part to be peened. The centrifugal blast wheel consists of
a high speed paddle wheel. Shot media are introduced in the center of the spinning wheel and
propelled by the centrifugal force by the spinning paddles towards the part by adjusting the media
entrance location, effectively timing the release of the media. Other methods include ultrasonic
peening, wet peening, and laser peening (which does not use media).

Process description
Shot peening is similar mechanically to sandblasting, though its purpose is not to remove material, but
rather it employs the mechanism of plasticity to achieve its goal, with each particle functioning as a
ball-peen hammer (Fig. 6. 12).

Peening a surface spreads it plastically, causing changes in the mechanical properties of the surface.
Its main application is to avoid the propagation of microcracks in a surface. By putting a material under
compressive stress, shot peening prevents such cracks from propagating.
Shot peening is often called for in aircraft repairs to relieve tensile stresses built up in the grinding
process and replace them with beneficial compressive stresses. Depending on the part geometry, part
material, shot material, shot quality, shot intensity, and shot coverage, shot peening can increase
fatigue life up to 1000% (Fig. 6. 13). 22

Benefits
Plastic deformation induces a residual compressive stress in a peened surface, along with tensile stress
in the interior. Surface compressive stresses confer resistance to metal fatigue and to some forms of
stress corrosion. The tensile stresses deep in the part are not as troublesome as tensile stresses on the

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