Study On Cryogenic Fracture Toughness And Microstructure Of 9Ni Steel Flux Cored Arc Welding Joint

9Ni steel is a kind of ferritic steel with excellent cryogenic toughness,which is widely used in LNG carrier cargo containment system,LNG tanks and other areas.In recent years,high-efficiency flux cored arc welding(FCAW)is gradually used to weld 9Ni steel.However,the study on the fracture toughness and allowable value of CTOD at low temperature for 9Ni steel FCAW joint is not enough.Further research is needed to improve the reliability on evaluating the toughness of 9Ni steel flux cored arc welding joint.In this paper,the low temperature performance of 9Ni steel joint is studied through mechanical test and microstructure observation.The research area is mainly the weld center of the joint and the coarse grain zone of the heat affected zone,considering the effect of different thermal cycles on the mechanical properties and microstructure.The CTOD test of the welded joint is mainly designed according to BS7448 standard.The test results show that the fracture toughness of the welded joint decreases with the decrease of temperature.The weak area of the FCAW joint is located in the weld metal and the heat-affected zone near the fusion line.The CTOD value of the weak area at-193 °C can meet the standard in the DNVGL(Det Norske Veritas Germanischer Lloyd)rules,where the minimum CTOD value should not less than 0.15 mm.The weld metal has better toughness at low temperature with higher heat input.The weld metal is nickel-based alloy.The matrix structure is austenite,and there are a lot of Laves phase and carbides at the grain boundary of austenite.The crack propagation path will across the precipitated phase in room temperature and in low temperature.For the joint with higher heat input,the grain size of austenite is larger and precipitated phase is less due to sufficient element diffusion,which means the joint has better fracture toughness in weld metal.The heat affected zone is divided into several parts,and the fracture toughness in different locations of the heat affected zone is tested with small size specimens.The results show that when the crack propagation part located in the fusion line area,the specimens have the lowest fracture toughness.When the location of crack tip away from the fusion line,the grain size of martensite decreases and the retained austenite content increases,and the fracture toughness of the region increases obviously.The inter-critically reheat CGHAZ(IC CGHAZ)which is the weakest area in the CGHAZ,appears unstable crack propagation in the CTOD test.In the unstable crack propagation region,the angle between crack path and lath martensite bundle decreases,the defense effect of lath martensite bundle to crack propagation decreases,and the M-A component at grain boundary promotes crack propagation,making the unstable crack propagation phenomenon appear in the specimen.The critical CTOD of 9Ni steel joint is influenced by temperature and thickness and other factors.At present,the standards about CTOD allowable value of 9Ni steel joint are not unified,and the allowable values of 9Ni steel flux-cored joints under different service conditions are not considered enough.In this article,D-B model and finite element analysis are used to simulate the CTOD values of HAZ under different temperatures and loads.Besides,the critical CTOD values at different temperatures by CTOD test,and the compare between the two kinds of value is made.The results show that the temperature change has a significant effect on the CTOD critical values and calculated values.When the allowable value of CTOD is adjusted according to the load temperature and other conditions in the actual environment instead of using a fixed value,it may be more reasonable to judge whether the toughness of 9Ni steel joint is up to the standard.