Abstract: To improve the underwater welding environment and weld quality, a small portable underwater TIG welding torch with movable gas hood is developed. With the double gas shielded welding torch, the influence of welding parameters on weld quality of 2507 duplex stainless steel is studied. The results show that when the internal gas flow rate is 10 L/min and the external gas flow rate is 5 L/min, the weld formation is the best and the depth-width ratio is the largest. When the internal and external gas flow are kept constant, the stability of the molten pool is reduced with increasing welding current. But when the welding current is too high, the welding arc may be restrained by the inner nozzle and the protective gas, which increases the penetration depth and depth-width ratio. It is also found that pulsed welding can increase the arc stability under the water environment. The comparative analysis of the underwater and onshore weld joints shows that the rapid cooling of water is conducive to the change of austenite precipitated on ferrite from needle to granular-like and from grain boundary precipitation to intragranular precipitation. At the same time, the austenite precipitated phase is decreased and the grain is refined. The test proves that this welding torch can realize the stable welding of underwater TIG. By selecting the appropriate welding parameters, the weld with similar performance to onshore environment can be obtained, which is of great significance for underwater welding maintenance.

Key words: underwater local dry method, TIG welding, weld formation, duplex stainless steel, microstructure and mechanical properties