Residual Stress Improvement on Inside Wall of Small-Diameter Pipe by Underwater Laser Ablation

Abstract

The authors showed that the residual stress on the inside wall of a pipe with inner diameter of 15 mm could be improved by an underwater laser ablation process, in which laser pulses with energy of 130 mJ from a Q-switched and frequency-doubled Nd: YAG laser were irradiated with diameter of 0.7 mm and with 18 or 27 impacts for every 1 mm² on material surface. A compact total-reflection mirror was invented, which reflected laser pulses in water with broad angle of incidence, and a device of 14 mm in outer diameter was assembled using the mirror in order to irradiate laser pulses to the inner surface of the pipe. The laser irradiation condition was optimized through experiments with Alloy 600 plates, and was applied to the cylindrical test piece of 15 mm in inner diameter. The results showed that a compressive residual stress was built on the inner surface of the test piece and the effect penetrated up to 1 mm from the surface. The compressive residual stress on surface is expected to prevent stress corrosion cracking (SCC) of structural materials in corrosive environments and to enhance fatigue strength of components under cyclic load.