The cryogenic (−196 °C) toughness of weld metal (WM) in 9 wt% nickel (9Ni) steel joints sharply decreases after welding compared with the tempered base metal. In this study, a novel deep penetration keyhole tungsten inert gas (K-TIG) welding technology was applied in 9Ni steel joint to improve the efficiency of backing welding. At the same time, the pure Ni plate and the molybdenum (Mo) wire were added to form the WM and improve the cryogenic impact toughness of the welded joints. The microstructure, mechanical properties, and fractography of joints were carefully investigated. The results show that adding appropriate amounts of Ni and trace Mo to the WM can improve the cryogenic toughness. The microstructure of WM after adding Ni mainly presents body-centered cubic structure martensite with Ni-rich grain boundaries. The grain size of martensite is refined by adding an appropriate Ni and Mo, which leads to the improvement of cryogenic impact toughness. After K-TIG welding, the tensile strength of WM in all joints is superior to the base metal. The maximum cryogenic impact absorbed energy of 97.3 J in WM was obtained by adding a 0.5 mm thick Ni plate. Besides, the grain size of WM and the cryogenic impact absorbed energy are in good agreement with the Hall-Petch relationship. Based on electron microfractography analysis, it is found that the WM without filler exhibits a brittle fracture mode at the impact test, while the WM filled with Ni exhibits a ductile fracture mode. Moreover, the heat affected zone (HAZ) in all joints demonstrates the mixed fracture mode. The results demonstrate that K-TIG welding can be applied to the backing welding, and the body-centered cubic martensitic WM with high cryogenic toughness can be achieved by adding appropriate Ni, which has great potential in the high-speed construction of industrial vessels.

与回火母材相比，9 wt% 镍 (9Ni) 钢接头中焊接金属 (WM) 的低温 (-196 °C) 韧性在焊接后急剧下降。在这项研究中，一种新颖的深熔透小孔钨极惰性气体 (K-TIG) 焊接技术应用于 9Ni 钢接头，以提高背焊效率。同时加入纯镍板和钼(Mo)丝形成WM，提高焊接接头的低温冲击韧性。仔细研究了接头的微观结构、力学性能和断口形貌。结果表明，在WM中加入适量的Ni和微量Mo可以提高低温韧性。添加Ni后WM的显微组织主要为体心立方结构马氏体，晶界为富Ni。通过添加适当的Ni和Mo来细化马氏体的晶粒尺寸，从而提高低温冲击韧性。K-TIG焊接后，WM在所有接头的抗拉强度均优于母材。通过添加 0.5 mm 厚的 Ni 板获得了 WM 中 97.3 J 的最大低温冲击吸收能量。此外，WM的晶粒尺寸和低温冲击吸收能量与Hall-Petch关系非常吻合。基于电子显微断口分析发现，无填料的WM在冲击试验中表现出脆性断裂模式，而填充Ni的WM表现出韧性断裂模式。此外，所有接头的热影响区（HAZ）都表现出混合断裂模式。结果表明，K-TIG焊可以应用于背焊，