Study On Effect Of Nb And Precipitation On Strengthening And Toughning Of 9Ni Steel Welding Material For LNG Storage Tank

China is a big country of coal energy consumption which conducting energy structural adjustment to ease environmental pressures with imminent supply of clean energy.Liquefied natural gas(referred to as LNG)has advantages of small volume,large combustion and no pollution.Demand of LNG is growing strongly.LNG storage equipment requires material in ultra-low temperature service environment with adequate strength and toughness reserves.At present,domestic LNG tanks with 9Ni steel has been fully achieved localization while nickel-based welding materials still dependents on imports with immature welding technology.Therefore,it is significance for LNG equipment industry autonomy and national energy security to do research on welding materials in domestic.9Ni steel and welding technology used in LNG storage tank has been researched at home and abroad.In this paper,research has summarized the influence of Nb on microstructure and mechanical properties of deposited metal with self-developed 9Ni steel electrode.The mechanism of strengthening and toughening of deposited metal was studied by tensile,bending,hardness and impact test,combined with scanning electron microscopy and thermodynamic calculations.The results showed: weld was austenitic by using nickel-based electrode.Nb had effect of reducing dendritic width and refining grain.Precipitated grain boundaries precipitate in the form of carbides in intergranular,which had a hindrance to crack propagation.With Nb content increased,size and number of precipitate got rised which resulted in reduced plasticity and toughness.Self-developed No.2 electrode(1.30% Nb)had excellentmechanical properties.Deposited metal tensile strength was 705 MPa,yield strength was over 478 MPa and low temperature impact toughness was 116 J which met requirements of LNG tank equipment.The adaptability of 9Ni steel welding consumables was studied by using shielded metal arc welding and submerged arc welding respectively.The microstructures of arc welding joints were analyzed by metallographic microscope and scanning electron microscope.Mechanical properties were measured using tensile,bending,hardness and impact tests while solidification theory of austenite-martensite crystal interface in joint fusion zone was studied.The results showed:self-developed electrode and domestic 9Ni steel had good welding process adaptability.The weld structure was austenite,heat affected zone wasmartensite,and fusion zone was a narrow planar crystal structure.Weld strength was higher than base metal,side bend intact without cracks with a good combination of weld and base metal interface.The maximum hardness of heat affected zone was 360HV5.Weld to heat affected zone impact toughness gradually increased.Weld average impact toughness was 115 J which meet requirements of low temperature toughness.Submerged arc welding seam organization was austenite and a lot of precipitates which contributed to improvement of weld strength.Joint had good bending performance.The hardness of the heat affected zone was 339HV5 which was highest of joint.Weld average impact toughness was 94 J and fracture analysis results showed that joints were tough fracture.