FEM Analysis Of The Effect Of Solid Phase Transformation On Residual Stress In Martensite Stainless Steel Welding

With the development of oil, chemical industry, metallurgy, electric power as wellas ocean exploitation, welding technology is more and more applying in these areasinvolved in the equipment production and manufacturing. Welding structures arerequired to serve safely in corrosive medium conditions. However, due to the residualstress in welding structures, stress corrosion cracking is emerged easily. Thereforesafety evaluation and eliminating the residual stress of welding parts to improve theservice life is great significance.This paper simulated the welding process and analyzed the temperature field,stress field, phase transformation and welding deformation results by applying finiteelement method, based on2Cr13martensite stainless steel widely used in actualproduction. The key research of this paper is the effect of phase transformation inwelding process on the distribution of residual stress. For comparison purpose，SUS304austenitic stainless steel was also chosen to be simulated with the same weldingconditions, and simulation results were compared between2Cr13martensite stainlesssteel and SUS304austenitic stainless steel.The simulation of martensite and austenitic stainless steel applied the same doubleellipsoid heat source model and the same mechanical boundary conditions. Asequentially uncoupled analysis had been performed where a thermal analysis was firstcompleted to solve for thermal profiles. A mechanical analysis had been subsequentlyexecuted which reeds in the temperature profiles and solves for displacements, strainand stresses. At the same time, for the sake of calculation accuracy, temperaturedependent thermal–physical and mechanical properties were considered.Need to be noted that effects on welding residual stress of the volumetric change,the yield strength change and transformation plasticity due to austensite-martensitetransformation were investigated. In this study, the Koistien-Marburger relationship wasused to describe the martensite transformation and plastic transformation was computedfrom the evolution mechanism of Greenwood-Johnson.Simulation results show that microstructure of weld and near weld zone(includingcoarse grained region and fine grained region) in matensite stainless steel basically is martensite using TIG welding technology. Microstructure in heat affected zone that isthe zone experienced temperature more than Ac1but below Ac3away from weld centerline is mixed microstructure of martensite and initial phase.Longitudinal residual stress of martensite stainless steel is that the biggestcompression stress is formed in weld center. With the distance from weld center,compression stress is smaller, and compression stress gradually becomes tensile stress.The biggest tensile residual stress appears in the HAZ close to base material area. Needto be noted that a smaller width of tensile residual stress is formed in welding joint of2Cr13martensite stainless steel. The most important is a certain width compression isformed in weld and near weld zone.The longitudinal residual stress of SUS304austenitic steel is obviously differentfrom2Cr13martensite stainless steel. In the welding joint of austenitic stainless steel,the longitudinal residual stress is tensile stress in weld and near weld zone. And thewidth of tensile residual stress is wider than2Cr13martensite stainless steel.The reason of obviously difference of longitudinal residual stress betweenmartensite stainless steel and austenitic stainless steel in weld zone is the phasetransformation. Phase transformation in2Cr13martensite stainless steel during coolingof welding process can bring volume expansion effect. The volume expansion can offsettensile stress produced in cooling. For austenitic stainless steel, there is no phasetransformation in FZ and HAZ. So residual stress distributions follow the general rule.That the welding longitudinal residual stress in FZ and HAZ is usually tensile stress.Phase transformation during welding process in2Cr13martensite stainless steelreducing tensile residual stress and making appear compression stress in weld and nearweld zone has a positive sense for improving stress corrosion of welding joint.Especially in the area of near weld zone, because of grain growth and element diffusion,stress corrosion crack is easy to produce under tensile stress and corrosive medium. Thefinal compression stress is help to avoid stress corrosion crack fundamentally. ForSUS304austenitic stainless steel, stress corrosion crack is easy produced because oftensile stress in FZ and HAZ.The research results of this paper lay a certain foundation for the research of theeffect of phase transformation on welding residual stress and deformation. And the results have an important significance on exploration of applying phase transformationto change residual stress in sensitive area to improve the safety operation ability ofwelding joint.