Effect of Precipitation Control on Strength and Impact Toughness in Advanced Pressure Vessel Steel

This paper reports the effect of increase in vanadium (V) content on changes in precipitation reaction and on mechanical properties in SA1017 grade 23 steel (2.25Cr-1.5W-VNbTi) for application to pressure vessels in advanced nuclear reactors. Increase in V content prohibits formation of coarse M₂₃C₆ carbide and allows only fine MX carbo-nitrides precipitation in this steel during tempering. Calculation based on Ashby–Orowan equation suggests that increase in yield strength is mainly attributed to precipitation hardening of fine MX carbo-nitrides and it shows a good agreement with experimental results. Meanwhile, step-cooling heat treatment causing temper embrittlement decreases Charpy impact toughness of steels at sub-zero temperatures, but higher V-alloyed steel shows higher toughness. Because interfaces between MX carbo-nitrides and the matrix act as strong trap sites of segregating P, its segregation toward prior austenite grain boundaries is decreased. Therefore, alloy design of V content for SA1017 grade 23 improves strength without degradation of impact toughness and temper embrittlement.