Stress Improvement by Laser Peening in the Air

Abstract

Underwater laser peening (LP) system has been developed for the purpose of preventing occurrence of Stress Corrosion Cracking. Toshiba has already applied LP for actual nuclear reactors, such as Control-Rot Drive housings in BWR and bottom mounted instrumentations in PWR and so on. At the same time, LP system in the air is expected since Reactor Vessel Head (RVH) is set on an operating floor during outage. In this study, LP system in the air was proposed which employs laser beam irradiation with concentric water jet. LP uses shock waves made by underwater confinement of plasma generated during laser abrasion which enables to induce compressive residual stress at 1mm in depth by optimizing laser energy, laser spot diameter and pulse density. A water jet has the functions of transmitting a laser beam without attenuating power and creating partial under water environment in the air. Hence potential core, which has no disturbance area in a water jet, should be increased in diameter and longer in distance from nozzle exit. LP tests in the air were performed by using the developed nozzles and measurements of residual stress were conducted. Test conditions are below; 70 mJ in laser energy, 1.0 mm in spot diameter, more than 30 mm in distance from nozzle exit to irradiation point, 36 shot/mm2 in pulse density, 90 degree in irradiation angle between laser axis and surface direction of irradiated area. Material of specimens was Alloy 600 nickel based alloy. Residual stress was measured by X-ray diffraction. It was confirmed that stress state at 1.0 mm in depth from the surface indicates compressive stress. These results concluded that LP in the air was conducted successfully.