유도가열 벤딩된 SA312 TP316 및 SA312 TP304 스테인리스강 곡관의 기계적물성치 평가

Abstract

The nuclear power plants are made up of a number of pipe systems that operating under high temperature and the high pressure. Recently, the piping systems employed pipe bends fabricated by high-frequency induction bending(HFIB). The use of bends can eliminate welds between bend and pipes improve flexibility, and low cost. For such reasons, HFIB process has been gradually increased in the piping systems of power plants and shipbuilding plants. However, HFIB could locally change microstructure of pipe material induced by local heating, straining and rapid cooling. Thus, the material properties of the pipe bend would be different from those of straight pipe and show spatial variation within pipe bend. Therefore, it is necessary to investigate the local material properties of pipe bend fabricated by HFIB. The investigation is important for piping systems applying leak-before-break(LBB) concept because the lower bound material properties should be used for LBB evaluation. In this study, tensile test and J-R fracture toughness test are carried out to investigate the local material properties of SA312 TP316 and SA312 TP304 stainless steels pipe bends fabricated by HFIB. Also, the microstructures were observed at different locations in the pipe bend using optical microscope(OM) and the fracture surfaces were observed from the post-test specimens using scanning electron microscope(SEM). The specimens were machined from various locations in the pipe bend, including intrados, extrados, and crown regions of bend, start and end of bend(transition start and end), and straight pipe. Both tests were conducted at both ambient temperature and operating temperature of piping systems in nuclear power plants(316°C). For both SA312 TP316 and SA312 TP304 stainless steels, the pipe bends always showed higher strength and lower ductility than original straight pipe. This means that the strength was increased and the ductility was decreased by HFIB. Within the center of bend region, the highest strength and lowest ductility appeared at intrados region rather than extrados region. For J-R fracture toughness test, J-R curves of the pipe bends are lower than original straight pipe at both ambient and operating temperature of piping systems. In particular, extrados of bend region appeared the lowest J-R curve other than locations.