Study On The Weldability And Related Simulation Technology Of 9% Ni Steel For LNG Storage Tanks

As global liquefied natural gas (LNG) consumpution incereases fast in recent years, demands to build more large LNG equipments are becoming strong in many countries. Presently, 9% Ni steel has replaced Ni-Cr stainless steel in building large storage tanks and pressure vessels for its excellent cryogenic toughness, becoming the mostly-used material for large LNG storage tanks construction. In this paper, 9% Ni steel was studied based on fundamental theories like phase transformation kinetics and thermodynamics theory, metal physics theory, elasto-plasto mechanics theory, and so on. Using physical simulation and numerical simulation, mechanical properties and cold crack susceptibility of 9% Ni steel for different welding conditions were investigated. Welding temperature field, stress-strain field and phase transformation field were simulated using SYSWELD simulation software to optimize welding parameters and predict microstructures and mechanical properties.Orthogonal regression and thermal simulation technology were used to investigate the influences of heat input and interlayer temperature on the microstructure and mechanical properties of heat affected zone (HAZ) of 9% Ni steel. Results showed that 9%Ni steel HAZ is mainly constructed of martensite, with very little retained austenite at low cooling rate. No considerable decrease in HAZ hardness was observed for a large range of cooling rates. No softening happened in HAZ for field welding. The predominant factor influencing cryogenic toughness of super-critically reheated coarse-grained HAZ (SRCGHAZ) and inter-critically reheated coarse-grained HAZ (IRCGHAZ) is heat input, which should be kept low with a little higher interlayer temperature to achieve high cryogenic toughness of the welding joint. IRCGHAZ is the primary local brittle zone within the HAZs. HAZ mechanical property (mainly HAZ hardness and toughness prediction) models which were coincident with actual experimental values were presented.Based on welding heat transfer theory, heat elastoplastticity theory and phase transformation theory, the three fields (temperature field, microstructure field and stress-strain field) were simulated using SYSWELD software taking into mutual influences and nonlinear relationships between temperature and mechanical properties.Assisted by numerical simulation, slit-Y cracking test was conducted to investigate the cold cracking susceptibility of 9% Ni steel. Results showed that cold cracking susceptibility of 9%Ni steel HAZ was generally low. Using nickel-based welding material, low section fracture ratio can be achieved for different preheat temperatures, ambient temperatures and ambient humidities. A preheat temperature of 75℃is recommended for welding in severe environment.