Study On Underwater Laser Welding Of Stainless Steel

Underwater welding is the important key repairing technology in nuclear power engineering. Governments proposed stricter requirements on repairing nuclear power equipment in service after Fukushima Daiichi nuclear disaster. Since laser welding has been characterized as easy system integration, high control accuracy, low heat input, small heat affected zone(HAZ), low residual stress and so on, underwater laser welding becomes more and more preferred method for repairing nuclear power equipment. However, the open published data of underwater laser welding technology is lacking because of technical blockade and confidentiality. There is little systematic research on its welding process, the microstructure, microhardness, mechanical property and failure behave, especially about the effect of laser beam quality.Aiming at AISI304 stainless steel, which has been widely used in nuclear power equipment, this study builds an experiment platform for underwater laser repair, investigates the influence of welding process parameters on laser beam quality and the weld depth and width, during underwater wet laser welding and local-dry underwater laser welding. For local-dry underwater laser welding 1.0/1.0 mm equal thickness plates and 1.5/2.0 mm unequal thickness plates, the effect of laser beam quality and welding process parameters on weld performance, microstructure, microhardness, mechanical property and failure behavior are systematic investigated, respectively.Scattering and refraction effect of bubbles and disturbed water surface accelerate the deterioration of laser beam quality during underwater wet welding. With the water depth increasing, the weld depth reduced gradually, however, the weld width increased first and then decreased. For local-dry underwater laser welding, there are three factors affect the laser repair quality: shielding gas pressure, scattering and refraction of aerosol particles, and air density distribution. The deciding factor for laser repair quality varies with the water depth and shielding gas rate.For 1.0/1.0 mm equal thickness plates butt welding, compared with traditional laser welding, the joint welded by local-dry underwater laser welding has finer crystal grain, higher microhardness, and the microstructure consists of austenite and a small amount of lath ferrite, while, the ferrite in traditional laser welding joint is ribbon-like. Selecting the appropriate shielding gas rate according to different water depth, joint with similar base metal mechanical property could be produced. The HAZ and fusion zone are sensitive regions of failure.For 1.5/2.0 mm unequal thickness plates butt welding, when the laser beam located at the center of weld, the produced joint has the largest tensile shear strength. However, when the laser beam located near thick plate side, the joint has the smallest tensile shear strength. The center of weld bead is equiaxed dendrite zone, which consists of austenite and little ferrite. The microstructure of both sides of weld bead center is asymmetry. The columnar dendrite zone of thick plate is larger than that of thin plate.