Shot peening is a very popular method of imparting compressive residual stress on the surface of components to increase their fatigue life. It is a process whereby small balls of iron or steel shot are bombarded at the component surface at high velocity. Peening can help improve the fatigue life and strength of a welded joint. This happens by increasing the surface area and compressive stress on the weld. Peening can also help eliminate surface defects, such as slag inclusions. Slag inclusions are small areas of molten metal trapped within the weld.