Embrittlement of 9Cr-1MoVNb and 12Cr-1MoVW steels irradiated in HFIR

Abstract Charpy impact specimens of 9Cr-1MoVNb and 12Cr-1MoVW steels and these steels with 2% Ni (9Cr-1MoVNb-2Ni and 12Cr-1MoVW-2Ni) were irradiated in the High Flux Isotope Reactor (HFIR) at 300°C up to 34 dpa and at 400°C to 42 dpa. Nickel was added to study the simultaneous effect of displacement damage and transmutation helium on toughness. Displacement damage is produced by fast neutrons and helium is produced by the reaction of 58Ni with thermal neutrons in the mixed spectrum of HFIR. Irradiation caused large increases in the ductile-brittle transition temperature (DBTT) of all four steels. The saturation in the shift in DBTT with fluence that has been observed in such steels irradiated in fast reactors does not apply to specimens irradiated in HFIR. Shifts in DBTT of over 300°C were observed at 400°C for 9Cr-1MoVW-2Ni and 12Cr-1MoVW-2Ni; shifts of over 200°C were observed for the standard steels. The shifts are the largest ever recorded for this type of steel and are attributed to the high helium concentrations (up to 400 appm).

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