Effects Of Environments On Fracture Toughness And Fracture Behavior Of Nuclear Safe-end Dissimilar Metal Welded Joint

The nuclear safe-end located in primary water systems pressure boundary of pressurized water reactors(PWRs) and most connected by dissimililar metal welded joints(DMWJs) is regarded to be the key attention structure. The DMWJ is exposed not only to high temperature and high pressure, alternating complex stress, but also to the intense neutron irradiation, which is the most vulnerable components. So it’s important to investigate facture toughness and fracture behavior of DMWJ exposed to the simulated service environment of PWRs.In this paper, SA508-3-309L/308L-316L DMWJ of the PWRs was studied as the experimental material. The fracture toughness and fracture behavior of DMWJ, exposed to the simulated service environment of PWR (hydrogen, strain rate and temperatures), were investigated by using fatigue testing machine and tensile testing machine combined with the analysis of optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The main contents and results are as follows:The results show that DMWJ has a significant microstructure heterogeneity. The heat flow and element migration during welding process lead to form complex heat affected zone(HAZ) and fusion zone(FZ) in the interface regions between materials. Various morphology of ferrite exist in the weld. Weld has the lowest hardness, strength and ductility across the DMWJ. During the tensile test, all the DMWJ specimens fracture in the buttering layer, which means that buttering layer is weakest part of the joints. The main reason is due to the lath morphology, local low strength mismatches and maximum residual stress in the buttering layer.The results show that the strength, elongation and facture toughness of DMWJ decreased significantly with increasing temperature, the final facture position is still the buttering layer. At the evaluated temperature, the second phase particles are more likely to become a crack source, which make the crack propagation easier. While the dislocation density is reduced, resulting in decreased yield-ultimate strength difference. The smaller yield-ultimate strength difference is not conducive to release the stress concentration in the pre-fatigue crack front, thus reducing the facture toughness.The strain rate has little influence on the tensile properties of as-received DMWJ specimens. However the fracture toughness of as-received DMWJs substantially increases with the increase of loading rate. Hydrogen induces a slight reducing in strengh and ductility. Under the action of enhancing localized planar deformation in austenite caused by hydrogen, the fracture toughness of H-charged DMWJ sharply decrease. After hydrogen charging the fracture surface of DMWJ specimens appear cleavage plane, interface cracking. The interaction between hydrogen and dislocations exist a critical strain rate. When the strain rate is lower than the critical strain rate, hydrogen has greater impact on the fracture behavior. When the strain rate is higher than the critical strain rate, the influence of hydrogen is gradually reduced. Therefore, with decreasing strain rate, the loss of elongation and fracture toughness of the joint are increased.