Study On Fracture Behavior And Mechanism Of Nuclear Safe-End Dissimilar Metal Welded Joint

The nuclear pressure vessel safe-end DMWJ is exposed not only to high temperature and high pressure,alternating complex stress,but also to the intense neutron irradiation and the sudden pressure mutation caused by the power change of the reactor,which are the most vulnerable components.So it is important to investigate facture toughness and fracture behavior of DMWJ exposed to the stimulated service environment of PWRs.In the present research,fracture toughness and fracture behavior of DMWJ(SA508-3 ?309L/308L—316L DMWJ)of the PWRs exposed to the simulated service environment of PWR(strain rate and temperature),were investigated by using fatigue testing machine and tensile testing machine combined with the analysis of optical microscopy,scanning electron microscopy and transmission electron microscopy.The main results are as follows:It is found that DMWJ shows significant microstructure heterogeneity.Weld near 316L has the lowest hardness across the DMWJ.During the tensile testing,the DMWJ specimens fracture in the weld-near 316L region,it is suggested that is weakest region of the joints.The main reason is the toughness of weld-near 316L is poor caused by the precipitated large amounts of carbides.With the increasing of tensile stress,carbides debond from the matrix into voids,and final lead to the fracture.It is clear that with the increasing of strain rate,the strength and the fracture toughness of the joints increases,while the elongation decreases,and the fracture toughness increase by 34.73%.The fracture toughness and yield-ultimate strength difference shows linear relation.With the increasing of strain rate,the dislocation density increases,resulting the increasing of yield-ultimate strength difference.The larger the yield-ultimate strength difference is,the more easy the release stress of concentration in the pre-fatigue crack front,which it hindered the main crack propagation,increasing the facture toughness.The results show that with the increasing of the testing temperature,the strength,elongation and the fracture toughness of the joints decreases.With the increasing of temperature,the dislocation density and the deformation twins decreased,resulting in decreasing of yield-ultimate strength difference.With the less yield-ultimate strength difference,the stress concentration in the pre-fatigue crack front is hardly to release,thus the facture toughness decreases.At high temperature,the deformation of joint can be hardly coordinated with deforming twins,which weakens the change of crack propagation direction,therefore the fracture toughness decreases.