Development and applications of laser peening without coating as a surface enhancement technology

Abstract

Laser peening without coating (LPwC) is an innovative surface enhancement technology to mitigate fatigue and stress corrosion of metallic materials by imparting a compressive residual stress. Toshiba has established a process without coating, whereas the coating is inevitably required in conventional process of laser peening to protect the surface from melting. Since the energy of laser pulses in LPwC is significantly small compared to that in the conventional process, a commercially available Nd:YAG laser can be used, and moreover, an optical fiber can be utilized to deliver the laser pulses. Compressive residual stress nearly equal to the yield strength of the materials was introduced on the surface after LPwC. The depth of the compressive residual stress reaches 1 mm or more from the surface. High-cycle fatigue tests proved that LPwC significantly prolonged the fatigue lives despite the increase in surface roughness due to ablative interaction of laser pulses with material surface. Accelerating stress corrosion cracking (SCC) tests showed that LPwC completely prevents SCC of sensitized austenitic stainless steels, nickel-base alloys and their weld metals. LPwC has been used since 1999 to prevent SCC of core shrouds or nozzle welds of ten nuclear power reactors of both boiling water reactor (BWR) and pressurized water reactor (PWR) types, already covering nearly one fifth of the existing nuclear power plants (NPPs) in Japan.