Weldability And Welding Procedure Research Of 9%Ni Steel For LNG Tank

Weld joint performances are vety important to the safety and quality of large cryogenic storage LNG (liquefied natural gas) tanks. At present, in view of absence of comprehensive research on weld joint performances of 9% Ni steel in China, it is necessary to start further study of 9% Ni steel in order to master weldability of thick-plate 9% Ni steel and to present proper welding technology and welding parameters. As a result, safe operation of LNG storage tanks can be guaranteed and domestic storage building companies will become more competitive in building large cryogenic storage tanks.Based on experiment results, following conclusions can be drawed: (1) Influence of welding current, baking temperature and holding time on the amount of diffusible hydrogen in deposit metal using Lincoln Nyloid 2Φ3.2mm welding rod is elicited to find out proper baking criterion in order to reduce diffusible hydrogen content. (2) Influence of ambient temperature, ambient humidity and preheating temperature on cold craking susceptibility is derived and proper preheating temperature to avoid cold cracking is presumed. (3) Based on thermal simulaion experiment, inflence of heat input and interlayer temperature on HAZ performances and microstructure is analysed, regressive equations of relaionship between HAZ performances (mainly cryogenic toughness and hardness) and welding parameters(heat input and interlayer temperature) were constructed. A interlayer temperture of less than 100℃and a heat input of less than 18 KJ/cm are presumed to obtain high quality weld joints.Based on weldability experiments, 10mm thick and 22.1mm thick 9% Ni steel were welded using AWS A5.11M-97 ENiCrMo-6, ENiCrMo-3 welding rod by SMAW and A5.14-97 ERNiCrMo-3,ERNiCrMo-4 welding wire by SAW respectively. Results showed that it is practicable to apply a preheat temperature of about 75℃, a preheating temperature lower than 100℃and a heat input less than 20 KJ/cm to obtain high quality weld joint for multilayer welding for both welding methods.