Study On Submerged Arc Welding Process And Welded Joint Stress Corrosion Behavior Of N610CF Steel

N610CF steel has obtained good mechanical properties after controlled rolling and cooling process.With good comprehensive properties,this steel is widely used in hydropower penstock and other structures.At the same time,due to the extensive use of this steel and the higher requirements for production efficiency in hydropower industry,double wire submerged arc welding is beginning to be used in the welding of N610CF.At present,the research on double wire welding process of N610CF steel is still in infancy.Therefore,the research on the welding process is of great significance.After welding,there will be residual stress in the joint,which will affect its performance.How to obtain the accurate magnitude and distribution of residual stress will be an important link in the research of joint stress.In addition,the joint will be exposed to various corrosive media during use,and there is a risk of stress corrosion under the action of welding residual stress and working stress.Therefore,the research on double wire submerged arc welding process and joint stress corrosion behavior of N610CF steel is of great significance.In this paper,three different welding speeds（i.e.different welding line energy）were used to weld N610CF steel plate by double wire submerged arc welding.The microstructure and mechanical properties of the joint were studied by means of metallographic test,hardness test and tensile test.The results show that the microstructure types of joints with different welding speeds are basically the same,but the final microstructure proportion and grain size are different due to different linear energy and cooling speed.Among the three groups of joints,the joint forming and comprehensive mechanical properties are better at the welding speed of 70cm/min,which can effectively reduce the softening degree of heat affected zone.The finite element simulation method is used to study the welding temperature field and stress field,and compared with the measured results of thermal cycle acquisition test and X-ray stress test.The distribution law of the simulation is in good agreement with the experimental results.The temperature field simulation results show that the simulated thermal cycle curve is consistent with the measured thermal cycle curve,and the simulation results can effectively predict the joint structure.With the increase of welding speed,the area of molten pool and peak temperature decrease,and the cooling speed of joint increases.The simulation results of stress field show that the change of welding speed（i.e.the change of linear energy）has little effect on the residual stress.The transverse tensile stress reaches+400MPa and the longitudinal tensile stress reaches+450MPa.With the superposition of weld beads,there is an obvious superposition effect of transverse tensile stress.For double-sided multi-layer multi pass welding,the welding sequence has a significant effect on the transverse residual stress of the joint.Alternating welding sequence can effectively reduce transverse residual tensile stress and welding angular deformation.The electrochemical test results show that with the increase of potential,the joint appears passivation,over passivation and re passivation in three corrosion solutions.The joints have obvious corrosion tendency in the three solutions.In Na₂SO₄solution,the self corrosion potential is the lowest and corrosion is the most likely to occur.In Na NO₃solution,the self corrosion current is the highest and the corrosion rate is the fastest.The stress corrosion tendency of the joint is studied by slow strain rate tensile test.The results show that the welded joint has a certain stress corrosion tendency only in Na Cl solution and Na NO₃solution,but the fracture mode of the slow tensile specimen in the three solutions has changed from ductile fracture to brittle fracture to a certain extent,especially in Na NO₃solution.The results of four point bending immersion test show that the joint has good stress corrosion resistance and no crack initiation in three solutions.The sample will form passive film in all three solutions.Due to the uniform corrosion of the joint in Na Cl solution and Na₂SO₄solution,the corrosion will not deepen after the formation of passive film,while the joint is pitted in Na NO₃solution.Under the action of stress,stress concentration is formed at the pitting pit,resulting in the destruction of passive film and deepening corrosion.