Fracture evaluation of crack in dissimilar metal pipe welds of primary heat transport system: tests and analysis

In Indian Advance Heavy Water Reactor (AHWR), Dissimilar Metal Weld (DMW) joints would be used for joining steam drum nozzles (low alloy steel) with downcomer piping (stainless steel). Similar DMW welds are present in piping system of the boiling water reactors (BWR's) and the pressurized water reactors (PWR's) at several locations, and there has been several failures reported near these joints in the existing nuclear power plants. Failure has been mainly due to thermal fatigue, stress corrosion cracking and carbon migration from low alloy steel side to the welds. In view of that, a program consisting of mechanical and micro-structural properties characterization and structural integrity assessment of these joint were carried out. The objective of this program was to investigate the fracture behaviour and verify the existing fracture analysis model (such as, EPRI, LBB.ENG2) for prediction of crack initiation load, maximum bearing load and collapse load of DMW pipe weld. To achieve this objective, DMW joint between low alloy steel and stainless steel pipe has been prepared as per the welding procedure given in ASME IX and acceptance criteria as per ASME Section III. Characterization of weld has been carried out in terms micro-hardness, microstructure, tensile and fracture toughness properties. It has been shown that the fusion boundary between buttering and low alloy steel have higher hardness and minimum fracture toughness compared to other regions of the DMW joints. Minimum fracture toughness of the fusion boundary has been attributed to the metallurgical phenomena occurring due to mismatch in the chemical and physical properties. Fracture tests on full scale pipe weld (12 inches NB Sch. 120) with crack in different regions have been conducted to understand initiation of crack growth and its subsequent growth under monotonic loading. It has been observed that load at the initiation of crack growth is lower by 15-35% than maximum load for a given crack configuration. During the fracture tests of all cracked pipe weld, SS side of pipe weld joint has shown significantly higher deflection compared to that of LAS side. Fracture analysis of DMW joints with crack in different regions have been carried out using LBB.ENG2 and EPRI methods. It has been observed that there is significant influence of selection of tensile properties and J-R curve in estimation of load-load line displacement, crack initiation load and maximum load using LBB.ENG2 and EPRI model. It has been found that estimation of load-load line displacement and maximum load using tensile properties of SS material gives comparable results with that of the experiment. (author)