Abstract

Laser shock peening(LSP) is an innovative surface treatment technique, which is successfully applied to improve fatigue performance of metallic components. After the treatment, the fatigue strength and fatigue life of a metallic material can be increased remarkably owing to the presence of compressive residual stresses in the material. Recently, the incidences of cracking in Alloy 600 small-caliber penetration nozzles (CRDM (control rod drive mechanism) and BMI(bottom mounted instrument)) have increased significantly. The cracking mechanism has been attributed to primary water stress corrosion cracking (PWSCC) and has been shown to be driven by welding residual stresses and operational stresses in the weld region. For this reason, to mitigating weld residual stress, preventive maintenance of BMI nozzles was considered application of laser shock peening process. Effects of parameters related to finite element simulation of laser shock peening process to determine residual stresses are discussed, in particular parameters associated with the LSP process, such as the maximum pressure, pressure pulse duration, laser spot size and number of shots. It is found that certain ranges of the maximum pressure and pulse duration can produce maximum compressive residual stresses near the surface, and thus proper choices of these parameters are important. For the laser spot size, residual stresses are not affected, provided it is larger than a certain size. Magnitudes of compressive residual stresses are found to increase with increasing number of shots, but the effect is less pronounced for more shots.