The Effects Of Laser Energy Allocation On Weld Formation Of 9%Ni Steel Made By Narrow Gap Laser Welding And Weld Joint Properties Analyze

Recent year,with the improvement of awareness in environmental protection,liquid natural gas(LNG)gets high recognition in society,which is widely used in electricity generation,automobile fuel,residents living and modern industry.As a high purity,high strength and high nickel content steel,9% nickel steel shows excellent toughness and strength under-196℃ low temperature environment.Compared with austenitic stainless steel and aluminum alloys,9% Nickel steel has lower expansion coefficient and higher cost performance.It can be predicted that 9% nickel steel will be largely used in LNG devices as one of the important structural material.The traditional welding method of 9% Nickel steel including:(1)shielded metal are welding,(2)submerge arc welding and(3)flux cored arc welding.There are some main problem occurred during 9% Nickel steel welding.(1)Heat affected zone is consisted of large martensite,it weakens the toughness of weld joints.(2)Large groove results in a high cost in nickel based filling metal.(3)Low penetration for single welding,an integrated weld joint need a large number of welding pass which leads to inefficiency in production.(4)The magnetic blow phenomenon occurred in traditional arc welding.Narrow gap laser welding provides a new method for 9% nickel steel welding avoiding the magnetic blow phenomenon.Besides,it take advantagement in high efficiency,low cost in filling metal and slight thermal formation.However,interlayer defect,lack of fusion and instability during welding proceed tend to occur in narrow gap laser welding.There is few report about narrow gap laser welding applying in 9% nickel steel.Related research mainly concentrates in mechanical properties of weld joint,and the study around improvement of welding parameters and influential factors analyzing remain blank.In this paper,narrow gap laser welding was applied in 9% nickel steel plates for LNG vessel with YLS-10000 fiber laser device and ErNiMo-6 nickel base filling wire.Experimental result showed that weld joint tend to be lack of fusion in along fusion line and form the interlayer defect because of the obvious difference in molten temperature and surficial tension between base metal and filling metal.The energy allocation during welding process is the key factor for good-forming of weld joint,the perfect energy distribution among previous weld bead,side wall and filling wire is essential to avoid the lack of fusion.In this paper,laser power(P),defocusing distance(D)and distance between beam and wire(l)were adopted as the factors for orthogonal test.Side walls fusion ratio,evenness of weld surface and morphology of heat affected zone(HAZ)were chosen as the assessment indicator of weld formation.The link between welding parameters and weld formation and energy allocation model during laser welding were proposed.Analyzing results showed that fusion of side wall and filling metal depended on P and D.Increasing the defocusing distance makes light spot direct interact with side wall,which would reduce the possibility of lack of fusion.When the groove is too large to be totally covered by laser beam by incrasing defocusing distance,it is a good way to turn up laser power to avoid lack of fusion.High evenness of the previous weld bead is so important for reducing the interlayer defects.The collapse of key hole will result in serious fluctuation of weld bead surface.When laser beam scanning through surficial hole of weld bead,the metallic vapor moves to lower height and the melting of feeding wire slows down which causes the instability of energy allocation that is the reason for interlayer defects.If even surface has been achieved,increasing the defocusing distance will benefit to the weld formation.According to the energy allocation model,defectless 9%Ni steel weld bead was achieved by narrow gap laser welding.Weld joint consists of austenite: columnar crystal close to the fusion line and equiaxed grain in the center of weld joint.HAZ change from coarse grain region,double phase region to fine grain region.HAZ is mainly consisted of martensite and none retained austenite is found in this area.The microhardness of weldment is about 200 HV and that of HAZ is about 370 HV.The microhardness value of coarse grain region is far higher than base metal.Tensile testing fracture located in weldment.The tensile strength at room temperature is 650 MPa a and at low temperature(-196℃)is 850 MPa.Both of them are slightly lower than that of base metal.Tests at low temperature show that the impact toughness along the weld central line and fusion line are above 40 J.Dimples are the dominating microstructure of impact fracture,so the fracture is ductile fracture.The result show that the properties of 9% nickel steel narrow gap laser welding joint meet the requirements of CCS standard.The theory proposed by this paper provides a method of improving weld deformation for narrow gap laser welding.Following this theory,16 mm thick 9% Nickel plates narrow gap laser welding was realized.