Research On The Low-temperature Performance Of S355J2W Steel Welded Joints

S355J2W weathering steel is mainly used in the manufacture of high-speed train bogie.Bogie is a key component which is important for the stability and safety of the train.However,a ductile-brittle transition behavior resulting in a sudden brittle fracture occurs for S355J2W steel at low temperature.The connection of the various parts of the bogie is mainly through welding.The welding defects and inhomogeneity of the microstructures and other factors increase the low-temperature sensitivity of the joints,leading the joints to brittle fracture of low temperature.Therefore,it is necessary to investigate the low-temperature performance of S355J2W steel joints.In this paper,the tensile strength,plasticity and toughness at various temperatures were studied through tensile test,impact test and fracture toughness test.And the ductile-brittle transition behavior of welded joints was studied in depth.The critical events of cleavage fracture of welded joints were discussed by fracture metallographic observation.The grain orientations and crack propagation paths of the base metal and weld were analyzed by EBSD(Electron Backscatter Diffraction).The microstructures and dislocation morphology near the fracture at room temperature and low temperature were analyzed by TEM(transmission electron microscope).The cleavage fracture stress of the welded joints was calculated by using the finite element analysis method combined with the fracture mesoscopic parameters.And the effect of the microstructure on low-temperature toughness was analyzed.The results of low temperature mechanical property and toughness test show that,as the temperature decreases,the strength of welded joints increases and the plasticity and toughness decrease.The base metal owns the best low-temperature toughness and the lowest ductile-brittle transition temperature,while the weld owns the worst low-temperature toughness and the highest ductile-brittle transition temperature.Therefore,the weld is the weakest part of the welded joints.The microscopic fracture mechanism of the welded joints gradually changes from ductile fracture to brittle fracture when the temperature decreases.The critical event of the cleavage fracture of the base metal at-80?-196? is that the microcracks of the grain size extend through the grain boundary and the critical event at-196? transforms into the nucleation of microcracks.The inhomogeneous microstructure of the weld and heat affected zone result in differences in cleavage fracture critical events.The proportion of high angle grain boundaries in the base metal is very high,which can effectively hinders the crack propagation and promote the low temperature toughness.The small angle grain boundaries in the pro-eutectoid ferrite and the ferrite side-plate in the weld have little effect on hindering the crack propagation,while a significant proportion of high angle grain boundary in the acicular ferrite is conducive to enhance the low-temperature toughness.A large plastic deformation of base metal at room temperature induces dislocation multiplication.The dislocation density at-196? is lower than the one that at room temperature.And it is found that the pinning of Nb2C precipitation on the dislocation is significant.Cleavage initiation source of the base metal is grain boundary.Cleavage initiation source of the weld is mostly induced by inclusions.There are two nucleation mechanisms of grain boundary and inclusions in the heat affected zone.The fine microstructure of the base material makes the characteristic microcrack length smaller and brings about a greater cleavage fracture stress.Therefore,the base metal has good low-temperature toughness.The coarse microstructure of the weld and heat affected zones significantly reduce the cleavage fracture stress and poorer low-temperature toughness.The inhomogeneity of the microstructure results in a large discreteness of cleavage fracture stress.