US Patent 7677070 Shot-peening process

A method of shot peening in which with respect to a carburized and quenched metal part, only its surface abnormal layer detrimental to the fatigue strength thereof is scraped without scraping of the martensitic structure underlying the surface abnormal layer, namely, in which the fatigue strength can be rendered stable and enhanced without surface cracking. As bombardment shot, use is made of a shot with hardness higher than that (first hardness) of the surface abnormal layer occurring at a surface layer portion of metal part prior to shot peening but lower than that (second hardness) of the martensitic structure.