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A508 Gr 4N has improved fracture toughness because of the addition of 3% nickel, compared to typical low alloy steels which have less than 1% nickel. However, there is an expectation in much of the recent literature, based mostly on low-alloy steels with nickel below 1%, that irradiation embrittlement will increase with increasing nickel (Ni) content. In contrast, the raw irradiation test data show that ASTM A508 Grade 4N containing up to 3.7% nickel, 0.1% Cu and 0.01% P does not show enhanced irradiation embrittlement. A simple statistical fit to irradiation dose and irradiation temperature was developed to make direct comparisons to other low-alloy steels. Since the A508 Gr 4N data showed little discernible effect of Cu in the raw data, the damage may be classified as 'matrix' damage. The peak irradiation embrittlement of A508 Gr 4N is no greater than that of A508 Gr 2, a 0.7% Ni forging material tested under similar conditions with similar limits on Cu and P. At high dose (80 mdpa) the average embrittlement of A508 Gr 4N is slightly higher (33%) than the lower nickel materials. This trend also occurs for low copper A533B and A302B plate material. The irradiation temperature dependence of embrittlement in A508 Gr 4N is nearly the same as other low copper low-alloy steels tested over a wide range of temperatures. The increase in Charpy transition temperature in A508 Gr 4N is due to radiation hardening, and the ratio of TTS to yield strength increase in 3 Ni steels is nearly identical to that observed for conventional low-alloy steels with lower nickel. A very detailed statistical fit was made to the overall data on A508 Gr 4N to evaluate the sensitivity of embrittlement to minor elements and to compare to results from the US surveillance test data, which is at low flux. The fit includes the effect of dose, irradiation temperature, flux, and composition. The embrittlement increases lightly at high flux, consistent with unstable matrix defects (UMD). Cu and P each cause increased embrittlement in A508 Grade 4N steel, and the magnitude of the effects are similar to that for A508 Gr 2 steels irradiated in surveillance tests. Hence, it is concluded that the higher nickel A508 Grade 4N steels show nearly the same irradiation response as conventional low alloy steels with lower nickel for Cu levels up to about 0.11% and P levels up to about 0.01%

Evaluation of Irradiation Embrittlement of A508 Gr 4N and Comparison to Other Low-Alloy Steels