Effects of alloying elements on mechanical and fracture properties of basemetals and simulated heat-affected zones of SA 508 steels

This study was aimed at developing low-alloy steels for nuclear reactor pre ssure vessels by investigating the effects of alloying elements on mechanic al and fracture properties of base metals and heat-affected zones (HAZs). F our steels whose compositions were variations of the composition specificat ion for SA 508 steel (class 3) were fabricated by vacuum-induction melting and heat treatment, and their tensile properties and Charpy impact toughnes s were evaluated. Microstructural analyses indicated that coarse M3C-type c arbides and fine M2C-type carbides were precipitated along lath boundaries and inside laths, respectively. In the steels having decreased carbon conte nt and increased molybdenum content, the amount of fine M2C carbides was gr eatly increased, while that of coarse M3C carbides was decreased, thereby l eading to the improvement of tensile properties and impact toughness. Their simulated HAZs also had sufficient impact toughness after postweld heat tr eatment (PWHT). These findings suggested that the low-alloy steels with hig h strength and toughness could be processed by decreasing carbon and mangan ese contents and by increasing molybdenum content.