Effect of Cavitation Number on the Improvement of Fatigue Strength of Carburized Steel Using Cavitation Shotless Peening

摘要

Peening can be used to produce a layer of compressive residual stress at the surface of components which are subject to fatigue or stress corrosion, thereby retarding crack initiation and/or impeding the development of new cracks and hence improving their fatigue life. We have developed a new peening method, Cavitation Shotless Peening (CSP), which makes use of cavitation impacts induced by the collapse of the cavitation bubbles to produce compressive residual stress and work hardening on the material surface. CSP is a surface enhancement technique which differs with shot peening in that shots are not used. CSP uses a submerged high-speed water jet with cavitation, herein referred to as a cavitating jet, whose intensity and occurring region can be controlled by parameters such as upstream pressure and nozzle size. Cavitation number σ, which is defined by the ratio of upstream pressure to downstream pressure, is the main parameter of the cavitating jet. In this paper, the pit distribution on the specimen was observed with cavitating numbers σ = 0.0057 and σ = 0.0142. The improvement of fatigue strength and introduction of residual stress were investigated for both conditions using carburized alloy steel (JIS SCM415). It was evident from a comparison between non-peened and cavitation shotless peened specimens that the cavitation number has influence on the fatigue strength of metallic materials. Comparison of shot peened and CSP specimens has also been discussed.