Numerical Simulation Of Welding Residual Stress In Typical Materials Used In Nuclear Power Plant

Fusion welding technology has been widely used to assemble the parts in nuclear power plant.As the weakest links in a specific welded structure,the quality and performance of welded joints bear on the safe operation and service life of nuclear power equipment directly.Due to the high operating temperature and pressure,there are always premature failures such as creep rupture and stress corrosion crack in the welded joints.Welding residual stress is a major inducement of premature failure,which must be investigated deeply.With special metallurgical character,the solid-state phase transformation(SSPT)will occur in typical nuclear power materials,for instance,Cr-Mo steels and NiCrMoV steels.The volume change,yield strength variation and plasticity induced by SSPT could influence the evolution and distribution of welding residual stress,especially in the thick and large components.In addition,tempering process should not be neglected during the multi-pass welding.The start-end location effect in hoop structures also deserves attention greatly.Based on the SYSWELD software,a thermal-metallurgical-mechanical computational procedure was developed to simulate welding residual stress,with considering SSPT and tempering effect.In this study,the influence of SSPT and phase transformation mode on welding residual stress in 30Cr2Ni4 MoV was investigated.The temperature and residual stress profile in P92 multi-pass weldment were predicted.Meanwhile,hole-drilling method was employed to measure residual stress pertinently.The influence mechanism of tempering effect on residual stress is clarified by Satoh test.Moreover,a full scale 3-D model and moving heat source were used to simulate the residual stress profile in 2.25Cr-1Mo welded pipe.Diffusive transformation,displacive transformation and tempering process were all considered in the computational approach.The simulation result was verified by sectioning method.A 3-D long plate model was designed to study the influence of start-end location effect on residual stress approximately.The simulation result shows that the SSPT and phase transformation mode could influence the evolution of welding residual stress in 30Cr2Ni4 MoV.The volume expansion induced by SSPT could reduce the longitudinal tensile stress in welding seam.While the decreased degree of tensile stress was relevant to the phase transformation mode.For the practical 30Cr2Ni4 MoV weldments,it is necessary to consider both SSPT and real phase transformation when simulating welding and heat treatment process in order to gain a reliable result.The HD measurement is generally in good agreement with the prediction result,which suggests the developed computational approach could predict residual stress profile of P92 weldments validly.In multi-pass welding condition,the final state of residual stress was determined by the last thermal cycle and weld bead distribution in top layer.The tempering phenomenon was obscure in such a 10 mm thick P92 welded joint due to the high heat input of cover welding.However,Satoh test shown the tempering effect could reduce about 200 MPa(20%)of tensile stress in ideal conditions.For the influence of SSPT,the hoop stress in 2.25Cr-1Mo welded pipe shown a feature like fish scales.The influence of stress equilibrium on axial stress is more obvious than SSPT.With considering the tempering effect,there is slight change of residual stress profile on surface,while the internal stress was relaxed significantly.The material at the start-end location of hoop welding seam experienced austenitizing and SSPT twice,thus there is a clear fluctuation of both hoop and axial stress.In fact,the start-end location effect is inevitable in actual welding process,the stress fluctuation need to be controlled by proper measures to reduce its detrimental effect on mechanical properties of the whole welded joints.