Shot Peening is a technique used to increase the surface hardness and service life of metal machine parts that are subject to repetitive strains such as tension and compression.

In this process, the metal part is subject to a hail or blast of very small, round metal shot. This takes place in a Shot Peening machine that slings the shot into the part using either a wheel sling or compressed air mechanism. The thousands of small, repeated blows act to work harden, or cold hammer the outer metal layers of the object. As a result, the part exhibits greatly increased metal fatigue limits and resitance to stress fractures and other failure.

One might wish to shot peen connecting rods in an internal combustion engine, as these are constantly being put into compression by the explosive force of the engine. Gearbox internals such as shafts and gears are generally desired to be very hard and can be subject to shot peening. Aircraft turbine blades are excellent examples of parts that are subject to repetitive stress. These are often removed and shot peened to reclaim surface hardness.

Compare also case hardened objects.

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