Investigation On Microstructure And Properties Of 9% Nickel Steel Joint By Narrow-Gap Welding With Fiber Laser For LNG Storage Tanks

9% Nickel steel is a kind of low-carbon martensitic cryogenic steel,which is widely used for LNG storage tanks with excellent low-temperature toughness and weldability.At present,there exists many limitations in welding 9% Nickel steel by traditional arc welding methods,such as low efficiency,big deformation,high cost of welding consumables and arc magnetic blow.In this paper,YLS-10000 fiber laser and ERNiMo-8 filler wire were used for the narrow-gap laser welding of thick 9% Nickel steel for LNG tanks,and the microstructure and mechanical properties of the joint were analyzed and tested.Firstly,the groove size of narrow-gap welding was optimized on the basis of laser autogenous welding results of 9% Nickel steel.Then,the effects of welding parameters,such as spot diameter,laser-arc distance,laser power,wire feed rate and welding speed,on the weld formation of 9% Nickel steel by narrow-gap laser welding was studied.The optimized welding parameters are as follows: spot diameter 1.88 mm,laser-arc distance 0 mm,laser power 3.0 kW,wire feed rate 2.0 m/min,welding speed 0.42 m/min.Optimized welding parameters can effectively decrease weld defects,such as lack of fusion,pores and cracks.Secondly,16 mm/25 mm thick 9% Nickel Steel were narrow-gap welded with optimized groove size and welding parameters,and sound joints with good side-wall fusion were obtained.The weld consists of 1 layer laser autogenous welding and 5 layers/8 layers laser welding with filler wire.The width of weld is 3.5 mm approximately.Thirdly,the microstructure and composition of the joint were analyzed by optical microscope,SEM and EDS.The main microstructure of weld zone is ? solid solution,with a small amount of Mo-rich precipitated phase distributing in the austenite grain boundary.Crystalline forms on two sides of weld is columnar crystal,while center is equiaxed crystal.The HAZ is clearly divided into coarse grain zone and fine grain zone,and both of their microstructure are martensite and a small amount of residual austenite.The transition of alloying elements in partial melting zone near fusion line finally lead to the microstructure transforming from M(martensite)to M+A(martensite + austenite).At last,mechanical properties of the welding joint were evaluated by microhardness tester,tensile test and-196? low-temperature impact test.The average hardness of HAZ is about 340 HV,which is much higher than that of the base metal 230 HV and the weld zone 200 HV.Fractures occur on weld zone of tensile test specimens,demonstrating the tensile strength is slightly lower than that of the base metal.The low-temperature impact energy of the joint increases as the distance from the weld center increase,and all mean values are over 70 J.The fracture morphology is tiny dimple pattern with features of ductile fracture.