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

This study was aimed at developing low-alloy steels for nuclear reactor pressure vessels by investigating the effects of alloying elements on mechanical and fracture properties of base metals and heat-affected zones (HAZs). Four steels whose compositions were variations of the composition specification for SA 508 steel (class 3) were fabricated by vacuum-induction melting and heat treatment, and their tensile properties and Charpy impact toughness were evaluated. Microstructural analyses indicated that coarse M₃C-type carbides and fine M₂C-type carbides were precipitated along lath boundaries and inside laths, respectively. In the steels having decreased carbon content and increased molybdenum content, the amount of fine M₂C carbides was greatly increased, while that of coarse M₃C carbides was decreased, thereby leading to the improvement of tensile properties and impact toughness. Their simulated HAZs also had sufficient impact toughness after postweld heat treatment (PWHT). These findings suggested that the low-alloy steels with high strength and toughness could be processed by decreasing carbon and manganese contents and by increasing molybdenum content.