Alloy designing study for improving mechanical properties of SA 508 steel for nuclear reactor pressure vessel

This study is concerned with the development of low-alloy steels for nuclear reactor pressure vessel by investigating the effects of alloying elements on mechanical properties of base metals and heat affected zones (HAZ's). Four kinds of steels with different alloying elements 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 lower carbon content and higher molybdenum content, the amount of fine M2C carbides greatly increased, while that of coarse M3C carbides decreased, thereby leading to the improvement of tensile properties and impact toughness. Their simulated HAZ's also had sufficient impact toughness after post-weld heat treatment. These findings suggested that low-alloy steels with high strength and toughness could be processed by reducing the amount of carbon and manganese and by raising the amount of molybdenum. (author)