Embrittlement of reduced-activation ferritic/martensitic steels irradiated in HFIR at 300 °C and 400 °C

Miniature tensile and Charpy specimens of four ferritic/martensitic steels were irradiated at 300 °C and 400 °C in the high flux isotope reactor (HFIR) to a maximum dose of ≈12 dpa. The steels were standard F82H (F82H-Std), a modified F82H (F82H-Mod), ORNL 9Cr-2WVTa, and 9Cr-2WVTa-2Ni, the 9Cr-2WVTa containing 2% Ni to produce helium by (n,α) reactions with thermal neutrons. More helium was produced in the F82H-Std than the F82H-Mod because of the presence of boron. Irradiation embrittlement in the form of an increase in the ductile-brittle transition temperature (ΔDBTT) and a decrease in the upper-shelf energy (USE) occurred for all the steels. The two F82H steels had similar ΔDBTTs after irradiation at 300 °C, but after irradiation at 400 °C, the ΔDBTT for F82H-Std was less than for F82H-Mod. Under these irradiation conditions, little effect of the extra helium in the F82H-Std could be discerned. Less embrittlement was observed for 9Cr-2WVTa steel irradiated at 400 °C than for the two F82H steels. The 9Cr-2WVTa-2Ni steel with ≈115 appm He had a larger ΔDBTT than the 9Cr-2WVTa with ≈5 appm He, indicating a possible helium effect.