What is double shot peening?

Double shot peening is a variant of the shot peening process that applies two successive blasting operations on the same component, with abrasives of different sizes and different parameters. The objective is to maximize residual compressive stresses in the surface layer of the material — the most critical zone for fatigue crack initiation.

In the single process, a single spherical abrasive is projected at the Almen intensity defined by the specification. In the double process, a second pass is added with a smaller diameter abrasive, which impacts at lower intensity than the first process. This second impact generates additional compressive stresses of the order of 40 to 50% in the layer closest to the surface — exactly where fatigue cracks initiate.

As an additional effect, the finer abrasive of the second process rounds the peaks generated by the first process, improving the surface finish. In gears and ring gears, this better surface finish reduces localized contact stresses on tooth faces, increasing service life.

Double shot peening processes typically use metallic abrasives of different granulometries. In some applications — especially in case-hardened or carburized gears — the second process may use glass beads or ceramic shot, which being harder than the treated surface generate an additional compression effect in the narrower surface layer.

Double shot peening is the standard specification for high-demand components: competition and premium transmission gears, high-fatigue springs and critical aeronautical components.