Effect of Intercritical Heat Treatment on J-R Fracture Resistance of SA508 Gr.1A Low-Alloy Steels

SA508 Gr.1A low-alloy steel is used for main steam line piping in nuclear power plants. The yield strength and J-R fracture resistance of the piping material must be high in order to apply the leak-before-break concept. In this study, intercritical heat treatment (IHT) was applied to two SA508 Gr.1A low-alloy steels having different chemical compositions, namely, samples C and P; the latter containing Mo and V. Their microstructures, tensile properties, impact properties, and J-R fracture resistances were evaluated, and the effects of IHT on the mechanical properties of the steels were analyzed. After IHT, fine grains formed at grain boundaries and coarse cementite decomposed, which greatly improved toughness without reducing the strength. Further, a model to predict the J-R fracture resistance (J_Ic) of the SA508 Gr.1A low-alloy steel was developed by considering the microstructural and mechanical factors that affect the J-R fracture resistance. According to the J_Ic results of 12 kinds of SA508 Gr.1A low alloy steel, subjected to different heat treatments, the J_Ic was linearly proportional to the number of effective grains (N) contained in the plastic zone, which showed that N is the main factor affecting J_Ic. Furthermore, the yield and tensile strengths were considered to compensate for the strength difference owing to the different chemical compositions. The J_Ic prediction model was derived considering the effective grain size, plastic zone size, and tensile properties. The predicted J_Ic values agreed well with the test J_Ic values.