Numerical Simulation Of Residual Stress In P91 Steel Boiler Pipe Based On Solid Phase Transformation Effect

P91 steel,with good high temperature mechanical properties and corrosion resistance,is widely used in ultra(ultra)critical power plant boiler pipes.Multilayer multi-pass welding is a common connection method for boiler piping installation.Due to the alloy composition characteristics of P91 steel,solid phase transformation will occur during welding,which will affect the formation and distribution of welding residual stress,especially the multi-layer multi-pass welding.Welding residual stress,as the main factor causing failure of boiler pipes,is an important index to evaluate the safety of equipment.Therefore,it is of great significance to study the formation process and distribution law of residual stress.In this paper,based on SYSWELD finite element software,the finite element calculation method of "thermal-metallurgical-mechanical" coupling considering martensitic transformation is established for P91 steel.The influence of volume change,yield strength change and phase change plasticity on residual stress caused by solid phase transition is investigated by Satoh test.The numerical simulation of P91 steel plate butt multi-layer welding is used to illustrate the effect of thermal cycling load of rear weld on residual stress of first weld.The formation process and distribution rule of residual stress were explained by numerical simulation of multi-pass welding of P91 steel boiler pipe.The stress,metallographic and hardness distributions on the outer surface of the pipe are obtained by X-ray diffractometer,metallographic analyzer and portable hardness meter.The influence of different welding interlayer temperature and holding time during heat treatment on the welding residual stress of P91 steel boiler pipe is studied by numerical simulation.The numerical simulation results show that the change of yield strength caused by solid phase transformation has a great influence on the residual stress of P91 steel,and the volume change will restrain the tensile stress caused by metal shrinkage.Phase change plasticity can weaken the influence of volume change and produce relaxation effect.In multi-layer plate welding,the overall tensile stress of the welded joint is low after considering the effect of thermal load on the structure of the first pass.In multi-pass welding of pipeline,the toroidal stress along the direction of diameter reduction rapidly changes from compressive stress to large tensile stress,and the closer the inner wall of pipeline,the smaller the tensile stress.The axial stress distributes in the direction of "compression-tension-compression" along the decrease of diameter.Martensitic transformation can only significantly eliminate the residual tensile stress of the current bead.The higher the interlaminar temperature,the lower the residual tensile stress of the welded joint,and even the compressive stress.However,the increase of the interlaminar temperature can not reduce the circumferential tensile stress in the fine-grained zone.Postweld heat treatment can effectively reduce the residual stress,after heat treatment,the residual stress decreases by about 60%～70%.Residual stress gradually decreases with the prolongation of holding time.When holding time exceeds a certain value,the degree of reduction of residual stress is not obvious.The test results show that the average hardness of the weld metal of P91 boiler pipe is about 255 Hv and that of the base metal is about 185 Hv.The metallographic structure of the weld metal is coarse lath martensite,and the heat affected zone is a mixture of partially quenched martensite and partially tempered martensite.The base metal is tempered martensite with fine grains and uniform distribution.The measured results of residual stress agree well with the numerical simulation results.The results of measurement verify the reliability of the numerical simulation.The research results in this paper are helpful to predict and improve the distribution of residual stress in welding,and provide theoretical basis for optimizing welding process and heat treatment process.