Shot peening

 Shot peening is a cold work process, in which the metal part is struck by a stream of small hard spheres (shot) creating numerous overlapped dimples on the part surface.
The surface material resists to stretching induced by the shots impacts resulting in a formation of a compression stressed skin of about 0.01” (0.25 mm) thickness.

Glass, steel or ceramic balls of a diameter from the range 0.007-0.14” (0.18-0.36 mm) are used as shot media.

The residual compression stresses inhibits both crack initiation and propagation. Therefore shot peening is used mainly for increasing fatigue strength.

Dimples formed on a part surface as a result of shot peening may serve as lubricant “pockets”, which provide continuous lubrication of the part preventing galling. Steels, nodular (ductile) cast irons, Aluminum alloys, Nickel alloys and Titanium alloys may be treated by shot peening.

The properties, which may be increased by shot peening:

• fatigue strength
• Stress corrosion cracking
• Corrosion fatigue
• Resistance to Intergranular corrosion
• Galling resistance
• Wear resistance.

The components, which are normally shot peened: springs, wheels, turbine parts (blades, shafts), marine propellers, aircraft wings, internal combustion engine parts (valves, gears, crankshafts, connecting rods, pistons, cylinder heads).
Shot peening may increase the components life by 500-1000%.