a method of hardening metal parts , such as a rotor or drive link , and parts thus produced . preferably the method includes imparting residual compressive stresses into the metal parts and uses various small ball type structures to create small compressions in the surfaces of the metal parts . the compressions apply residual stresses to the parts , which strengthen the metal . the substantial uniform ball type structures are pressed into the metal part to control the application of stress into the part and maintain substantial uniform properties within the metal part , which resists future stresses as the part is used in its desired machinery and / or processes .