Irradiation Embrittlement Behavior of SA508 Gr.4N RPV Steel Model Alloy

The mechanical properties and irradiation embrittlement behavior of SA508 Gr.4N low alloy steel have been characterized systematically using SA508 Gr.4N model alloys. For an evaluation of neutron irradiation embrittlement behavior of model alloy, several irradiation tests were carried out at the research reactors, HANARO and HBWR, up to a fluence level of 1.5 × 10²⁰n/cm² (E>1MeV) at 290 ± 10°C. The master curve method according to ASTM E1921 was adopted to evaluate the fracture toughness in the transition region. Ni and Cr additions resulted in increasing the martensite fraction in low alloy steel by enhancing the hardenability of the steel. Thus, the predominant microstructure of SA508 Gr.4N model alloy is a mixture of tempered martensite and bainite, while SA508 Gr.3 steel shows a typical tempered upper bainitic structure. SA508 Gr. 4N model alloy shows excellent strength and transition behavior compared to commercial SA508 Gr.3 steel. After neutron irradiation, the yield strength and tensile strength of model alloy were increased with an increase in the neutron fluence level. The transition temperature shifts of SA508 Gr.4N model alloy obtained by both Charpy impact and fracture toughness tests were not significantly larger than those of commercial SA508 Gr.3 low alloy steel. It seems that the increased Ni content in the SA508 Gr.4N model alloy did not show significant effects on the irradiation embrittlement behavior owing to the controlled low Mn content. In addition, good fracture toughness of the SA508 Gr.4N model alloy was maintained even after neutron irradiation up to a level of ∼10²⁰n/cm².