Residual stress improvement mechanism on metal material by underwater laser irradiation

Residual stress improvement technology for component surface by underwater pulsed laser irradiation has been developed as a method of preventing stres s corrosion cracking (SCC) of core components in nuclear reactors. In order to optimize the laser irradiation conditions based on a complete u nderstanding of the mechanism, the propagation of a shock wave induced by t he impulse of laser irradiation and the dynamic response of the irradiated material were analyzed through time-dependent elasto-plastic calculations w ith a finite element program. The calculated results are compared with the measured results obtained by experiments in which laser pulses with an ener gy of 200mJ are focused to a diameter of 0.8mm on a water-immersed test pie ce of 20% cold-worked Type 304 austenitic stainless steel to simulate neutr on irradiation hardening. A residual compressive stress, which is nearly equivalent to the yield stre ss of the processed material, remains on the material surface after passage of the shock wave with enough amplitude to induce a permanent strain. Mult iple irradiation of laser pulses extends the stress-improved depth to about 1 mm, which would be the limit corresponding to the three-dimensional disp ersion effect of the shock wave.