Weld defects and precipitates of deposited metal in 9Ni steel welded joint

Three kinds of electrode with different niobium contents were developed and compared, and the influence of alloying elements on microstructure and mechanical properties was summarized. Strong carbide elements such as Nb, Ti, and V were added to form stable precipitates. The existence of intercrystalline precipitates leads to pinning and zigzag of grain boundaries, hinders the propagation of cracks, and enhances the low temperature strength and impact toughness of the materials. The No. 1 and No. 2 ENiCrMo-6 electrodes meet the requirements of LNG (liquified natural gas) equipment. The tensile strength of the deposited metal reaches 687 MPa, while the average impact energy at −196°C is 131 J. Owing to fluctuations in the stress concentration at the junction of grain boundaries, cracks may easily form. MC carbide can retard the crack propagation. With the increase of Nb and other alloys, the strength and hardness increase gradually, but the plasticity and toughness are retarded to a certain extent.