<p>Although hot wire TIG welding improves the speed of deposition compared to TIG welding, it still falls short compared to submerged arc welding. However, adding hydrogen to the protective atmosphere of hot wire TIG welding can further enhance the cladding efficiency. Nevertheless, hydrogen can also deteriorate joint performance and cause defects such as hydrogen cracks. In this study, we investigated the effects of hydrogen on the properties and low-temperature toughness of 9Ni steel joints welded by TIP-TIG for LNG storage tanks. The microstructure and mechanical properties of the joints were analyzed using optical microscopy, scanning electron microscopy, and a universal testing machine. The results showed that the TT weld hydrogenated in a protective atmosphere exhibited good strength and low-temperature toughness. The optimal proportion of mixed gas was found to be 98% Ar + 2% H₂. At 2% hydrogen content, the width of the weld dendrite and the grain size of the coarse grain region were relatively small, and the fracture surface displayed large and deep dimpling and high cleavage step, resulting in the highest low-temperature toughness. However, when the hydrogen content increased to 8%, micropores were generated in the weld, disrupting the material’s continuity and reducing the low-temperature impact energy absorption of the weld, thereby damaging the low-temperature toughness of the molten metal.</p>