Mechanical Properties And Integrity Assessment Of Welded Joints For Joining The Pipe Nozzle And Safe End In Nuclear Power Plants

Dissimilar metal welded joints(DMWJs)are widely used in primary water systems of pressurized water reactor in nuclear power plants to connect the ferritic steel pipe-nozzles of the pressure vessels(such as reactor pressure vessels,steam generators and pressurizers)with the austenitic stainless steel safe ends.The running history of PWRs shows that the DMWJs are weak components in the primary water systems.Thus,it is critical to investigate the mechanical properties and structural integrity of DMWJs with different manufacturing techniques to enhance the manufacture process as well as to ensure safe service.In this paper,failure assessment diagrams(FAD)of DMWJs were constructed based on R6 option 3,and effects of crack locations and crack sizes on failure assessment diagrams(FADs)have been studied.The mechanical properties and fracture toughness of five DMWJs with different manufacturing processes have been experimentally investigated.In the end,the safety of the five DMWJs was assessed using the constructed FADs.The main work and conclusion were as follows:(1)Limit loads and failure assessment curves(FACs)of three-material(without buttering)and four-material(with buttering)welded joints are basically the same.With moving crack location from pipe nozzle to 316L safe end pipe and increasing crack depth,the failure assessment curves(FACs)shift downward and the safety margin of the cracked structure decreases.It is feasible to connect pipe-nozzle with safe end using simplified manufacture process without buttering layer for the consideration of cost saving.(2)The mechanical properties(yield/ultimate strength,ductility,fracture strain,reduction of area as well as stress-strain curves)of an Alloy52M dissimilar metal welded joint(DMWJ)in nuclear power systems were determined.The results show that the mechanical properties of the two welded joints are basically the same.The mechanical properties of DMWJ without buttering can reach the level of DMWJ with buttering.(3)The experimental results of J-resistance curves and fracture toughness show that interface cracks have the lowest J-resistance curves and fracture toughness,which means they are the weakest regions in the welded joints.Final heat treatment(high-temperature tempering)has little effect on fracture toughness of the welded joints.J-resistance curves and fracture toughness J1c for cracks of 4-material joints at different locations are slightly higher than those of 3-material joints.Overall,the toughness of two joints is basically the same.(4)The experimental results of J-resistance curves and fracture toughness of two welded joints(four-material with repair welding and three-material without repair welding)show that J-resistance curves and fracture toughness of four-material joints are slightly higher than those of three-material joints.J-resistance curves and fracture toughness of the repair welding material are similar to those of original joints,which means the toughness of repair welding material is not lower than that of original joints.(5)The safety assessments of five DMWJs(different manufacturing and repair welding processes)were conducted using constructed FADs for normal,emergent and accident working conditions.The results show that all the cracks(a/t=0.25,a/t-0.5)are safe and have large safety margins.The R6 option 1 FAC and general FAC in ASME code are too conservative.The integrity of safe-end welded joints(without buttering)is similar to that of four-material joints(with buttering).