Research on the Structure and Performance of Local Dry Underwater Laser Welding of 304 Stainless Steel

The underwater welding process is influenced by various factors, including water intrusion, cooling effects, and their impact on the microstructure and properties of the weld joint. To investigate the effects of the underwater environment on the performance of 304 stainless steel partial dry laser welding joints, experiments were conducted using a self-designed double-layer gas drainage device for local dry underwater laser welding of 304 stainless steel at a water depth of 50 mm. The underwater and terrestrial welds were studied under laser power of 3.0 kW, welding speed of 0.8 m/min, and inner gas flow rate of 25 L/min. The results indicated that water intrusion during the welding process caused partial oxidation of the weld seam surface. The cooling effect of water reduced the peak temperature and cooling rate during the welding process, leading to an increase in nucleation rate and undercooling, promoting grain refinement, and altering the solidification mode, resulting in a microstructure consisting of austenite and partial ferrite after solidification. The average hardness of the underwater joint was higher than that of the terrestrial joint, and the tensile strength could reach 97% of the terrestrial joint. Fracture morphology and mechanism analysis revealed that both joints exhibited ductile fracture with micro-pore aggregation, while the underwater joint exhibited some brittle characteristics. Moreover, the cooling effect of water exacerbated the segregation of Cr and Ni elements, reducing the corrosion resistance of the underwater joint.