Details
Sometimes components that are subject to fatigue failure can be strengthened by shot peening. Bombarding the surface with steel shot propelled at a high velocity introduces compressive residual stresses at the surface that increase the resistance of the metal to fatigue failure.
Shot Peening is a cold working method that reduces stress. Small, round metal balls, or shot, are projected onto the surface of the weldment. The shot imparts small indentations into the surface, which induces compressive stress. The tensile residual stresses at the surface of the weldment must "overcome" the compressive stress for a fatigue crack to initiate. If properly applied, the compression works to counteract the tensile stresses. Fatigue cracks have a low probability of developing in the shot-peened area.
Caution must be taken to ensure the shot-peening operation is performed with knowledge of its variables. There are three important variables to control in its application:
surface compressive stress,
maximum compressive stress, and
depth of compressive stress.
The velocity of the shot is another controlling factor. If the impingement of the shot is too deep, detrimental stresses may be induced, negating the desired results.