Peening using cavitation impact is called "cavitation shotless peening CSP", since there is no requirement for shot in the process. Micro- and macro-strain of polycrystalline metal peened by CSP were evaluated using X-ray diffraction methods, as the full width at half maximum (FWHM) of the X-ray diffraction profile from the peened surface was decreased, although compressive residual stress was introduced. It was found that CSP reduced the micro-strain in the surface, but simultaneously introduced compressive residual stress, i.e., a macro-strain. The results demonstrate that the micro-strain is relieved by CSP without the need for heat treatment, and is, therefore, a sort of annealing. Thus, CSP can renew the metallic material while the shape itself is maintained.
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