Hardness measurements were used to determine the post-irradiation annealing response of A533B class 1 plate steel irradiated to a fluence of 1 x 10¹⁹ n/cm² (E>1 MeV) at 150°C. Rockwell hardness measurements indicated that the material had hardened by 6.6 points on the B scale after irradiation. The irradiation induced hardness increase was associated with a decrease in upper shelf energy from 63.4 J to 5-1.8 J and a temperature shift in the Charpy curve at the 41 J level from 115°C to 215°C. Specimens were annealed after irradiation at temperatures of 343°C (650°F), 399°C (750°F), and 454°C (850°F) for durations of up to one week (168 h). Hardness measurements were made to chart recovery of hardness as a function of time and temperature. Specimens annealed at the highest temperature 454°C recovered the fastest, fully recovering within 144 h. Specimens annealed at 399°C recovered completely within 168 h. Specimens annealed at the lowest temperature, 343°C recovered only ∼70% after 168 h of annealing.

After neutron irradiation, a new feature of black spot damage was found to be superimposed on the unirradiated microstructure. The density of black spots was found to vary from 2.3 x 10¹⁵/cm³ to 1.1 x 10¹⁶/cm³ with an average diameter of 2.85 nm. Following annealing at 454°C for 24 h the black spot damage was completely annealed out. It was concluded that the black spot damage was responsible for 70% of the irradiation-induced hardness.