Effect of Defocusing Conditions on Penetration Stability and Low-Temperature Impact Properties in Laser-Arc Hybrid Welding of 9% Ni Steel

One of the most critical issues in the construction of LNG vessels is ensuring the structural integrity of LNG storage tanks under cryogenic service conditions. For this purpose, 9 wt.% Ni steel has been widely used because of its excellent low-temperature performance. However, conventional arc welding processes used for thick steel plates often involve high heat input, which can cause welding deformation and degradation of weld quality. Laser-arc hybrid welding (LAHW), which combines the deep penetration capability of laser welding with the gap-bridging and filler metal supply advantages of arc welding, is a promising process for improving welding efficiency and weld quality. In this study, LAHW was applied to 15-mm -thick 9 wt.% Ni steel to evaluate the effects of defocusing conditions on penetration stability, weld bead formation, microstructure, and low temperature impact energy. Bead on plate and butt welding tests were performed to establish a process window that ensures stable full penetration while suppressing root humping. The experimental results showed that appropriate defocusing conditions enabled full penetration even at relatively low heat input, whereas excessive defocusing increased the tendency for root humping. Tensile and Charpy impact tests were conducted to evaluate the mechanical performance of the LAHW weldments. The impact absorbed energy decreased with decreasing test temperature.