SA508 Gr.1A 저합금강의 기계적특성에 미치는 이상영역열처리 및 Pre-tempering 열처리의 영향

To apply the leak-before-break (LBB) concept to the main steam line piping of nuclear power plants, the use of SA508 Gr.1A low-alloy steel is being considered. To increase the LBB safety margin, it is essential to improve the strength and toughness of the material. In this study, intercritical heat treatment (IHT) and pre-tempering were applied to V-added SA508 Gr.1A low-alloy steel, and the effects of heat treatments on mechanical properties and LBB safety margin were analyzed. IHT resulted in the formation of fine grains at the grain boundaries and the decomposition of coarse cementite, which led to an improvement in impact transition toughness and J-R fracture resistance without a significant decrease in strength. The application of pre-tempering promoted the formation of nano-sized precipitates. It increased the strength by approximately 30 MPa or more at room temperature and 286°C, while no significant difference was observed in impact transition toughness and J-R fracture resistance. Both heat treatments increased the LBB safety margin from 1.37 to 1.41 by enhancing yield strength or J-R fracture resistance. However, when IHT and pre-tempering were applied together, J-R fracture resistance increased but yield strength decreased. As a result, there was no increase in the LBB safety margin.