Low-Temperature Toughness Enhancement of 9% Ni Steel Girth Welds in LNG Storage Tanks via a TIP–TIG Welding Process

In this study, a novel TIP–TIG (TT) automatic welding method was used to weld the girth seams of liquefied natural gas (LNG) tanks composed of 9% nickel steel. The microstructure of the joints was characterized via electron microscopy and x-ray diffraction. The tensile strength and ductility of the joints were evaluated via tensile testing. The low-temperature toughness of the joints was investigated via low-temperature impact testing at (-)196 °C. This increased the toughness and strength of the TT-welded joint compared to the corresponding values of joints welded using conventional submerged arc welding (SAW). A comparison with the SAW-welded joints revealed that the TT-welded joint presented high strength and excellent low-temperature toughness, which can be primarily attributed to the low heat input. Compared with SAW, TT welding resulted in a lower joint dilution ratio, smaller heat-affected zone, and lower degree of grain coarsening. In addition to fine austenite, as the joint microstructure and several uniformly distributed carbides in the weld seam. These properties hindered the dislocation slip. However, they promoted precipitation strengthening and grain refinement and provided excellent low-temperature toughness to the TT-welded joints. Notably, TT welding does not require weld-bead cleaning between passes. Additionally, owing to improvements in the degree of automation for TT welding, the welding time can be further matched with that of SAW. TT welding with narrow-gap grooves reduces the required amount of filler metal by more than 50%. Therefore, the TT welding process is significantly advantageous and can replace SAW for girth–seam welding of LNG storage tanks.