Improvement of cryogenic toughness for 9% Ni steel keyhole TIG butt-welded joints with a Ni interlayer

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.