Underwater laser welding for single pass full penetration without backing was achieved under a water pressure equivalent to that at a depth of 100m under the sea level. For the welding, a 304-type material with a thickness of 4.5 to 6mm and a disk laser with a maximum power of 3kW were used. Nitrogen gas was used to shield the surface of laser irradiation, thereby providing a locally dried environment. We confirmed that the weld penetration became shallower with an increase in the surrounding pressure in both water and air. By appropriately adjusting the gap distance, the molten metal flowed into the back bead, and the full penetration bead was formed without underfill or humping bead under a water pressure. Under high-pressure conditions, an increase in the austenite content in the nitrogen-based weld metal was observed, but no crack was detected. In addition, it was shown that the amount of diffusible hydrogen in melt metals can be reduced by nitrogen gas shielding on the back side. The mechanical properties of the final material, such as tensile strength, bend strength, and hardness, were comparable to those of the base metal.
