Numerical Simulation Of Residual Stress In P92 Steel Pipe Based On Solid Phase Transformation Effect

With the increase of steam pressure and temperature of Ultra-supercritical unit boilers,boiler components also put forward higher requirements for the high temperature performance of materials.As a new type of ferritic heat-resistant steel,P92 steel is widely used in important pressure-bearing parts such as main steam pipelines,superheaters,and headers because of its excellent high temperature performance.However,in the process of use,some welding problems of P92 steel gradually appear,such as creep fracture under high-temperature and high-pressure working conditions,and easy to produce type IV crack tendency in the heat-affected zone.As an important factor leading to the failure of boiler pipelines,welding residual stress is an important index for evaluating the safety performance of equipment,so it is of great practical significance to study the distribution law and formation process of residual stress.Based on SYSWELD welding software,this paper develops and optimizes the coupling calculation method of "temperature-structure-stress" considering the phase transition effect of solid-state with P92 steel as the research object,simulates the multilayer multi-pass welding process of flat plate and pipeline,and analyzes the distribution law and formation process of welding residual stress.At the same time,experimental verification was carried out,and the residual stress,hardness distribution and micromorphology of the welded structure on the outer surface of the pipe were measured by X-ray diffractometer,hardness tester and metallographic microscope.After the welding finite element model is verified,the welding process(welding speed,preheating temperature and interlayer temperature)of P92 steel is optimized by combining the response surface method,and the influence of post-weld heat treatment on residual stress under different tempering and holding times is explored.The numerical simulation results show that the volumetric strain and plastic strain caused by the solid-state phase change effect have significant effects on the residual stress and welding angle deformation,and should be fully considered when calculating the residual stress.The simulation results of flat multilayer welding show that the residual stress peak appears on the upper and lower surfaces of the weld,and the upper heat source has an elimination effect on the residual stress of the lower weld.The distribution law of residual stress of pipe welding is that the axial compressive stress gradually increases in the direction of the outer surface to the inner surface,and is transformed into tensile stress in the middle of the pipeline,and the closer to the inner surface of the pipeline,the smaller the tensile stress.In the upper part of circumferential,a large compressive stress area is formed on the outer surface,a tensile stress area is formed in the middle of the pipe,and a larger range of compressive stress is formed on the inner surface.The results of the response surface method on the optimization of welding process show that changing the welding speed has the most significant effect on the residual stress.In the test temperature range,the reduction of residual stress by increasing the interlayer temperature was more significant than that of increasing the preheating temperature,and the reduction of residual stress in the range of 150 °C to 200 °C was more significant.Post-weld heat treatment can significantly reduce the residual stress of welding,and the maximum elimination rates of transverse and longitudinal are 66.8% and 64.0%,respectively,but after the holding time exceeds 2 h,the residual stress reduction of heat treatment is not obvious.The experimental measurement results show that: the residual stress distribution obtained by finite element simulation is consistent with the experimental measurement results,verifying the accuracy of the welding model.The weld and most of the heat affected zone of P92 steel pipe are quenched martensite,with high hardness,average hardness of 436 HV;the base metal remains tempered martensite,with low hardness,average hardness of 236 HV;the heat affected zone is a mixed structure of quenched martensite and tempered martensite,the initial hardness is close to the weld,and the hardness away from the weld decreases rapidly.The research conclusions of this paper can provide some help for the prediction,defect assessment and life evaluation of welding residual stress of boiler pipes,and also provide theoretical basis for the optimization of welding process and heat treatment process in actual industrial production.