INTRODUCTION OF MACRO-STRAIN WITH RELEASING MICRO-STRAIN BY CAVITATION SHOTLESS PEENING

When cavitation bubble is collapsed, shock wave which can deform metallic materials is produced. Cavitation impacts can be used for surface modification to enhance fatigue life of metallic materials in the same way as shot peening. As a peening method using cavitation impact does not require shots in shot peening, it is called “cavitation shotless peening CSP”. Although CSP can introduce compressive residual stress, i.e., macro-strain into metallic materials, full width at half maximum of diffractive X-ray profile was decreased by CSP. In the present paper, tool alloy steel for forging die was chosen as tested material to investigate mechanism of improvement of fatigue life, as CSP improved the life time of the forging die. The introduction of macro-strain was revealed by measuring residual stress, which was evaluated by X-ray diffraction method. The fatigue life was investigated by using a plate bending fatigue test changing with processing time of CSP. The micro-strain was evaluated by a fundamental parameter approach, which is one of X-ray diffraction method. It was concluded that the fatigue life of the steel was improved about 90 times by CSP and CSP can introduce macro-strain, i.e., compressive residual stress and releasing micro-strain. The micro-strain becomes about 1/20 of heat treated specimen by CSP.