On the saturation of the DBTT shift of irradiated 12Cr-1MoVW with increasing fluence

Irradiation of ferritic steels often results in an increase in the ductile-to-brittle transition temperature (DBTT), sometimes well in excess of 1000C, and also a substantial decrease in the upper shelf energy (USE). Two irradiation experiments were conducted to determine the fluence dependence of this degradation in impact properties. Identical subsize Charpy specimens of 12Cr-1MoVW steel were irradiated in the University of Buffalo Reactor (UBR) at 3000C to a fluence of 0.86x1024 n/m2 (E>0.1 MeV) and in the High Flux Isotope Reactor (HFIR) at 300 and 4000C to a fluence of ∝1x1026 n/m2 (E>0.11 MeV). Impact testing indicated that the DBTT was increased by irradiation at 3000C by 36 (UBR) and 1640C (HFIR). Corresponding decreases in the USE were 13 and 33%, respectively. Irradiation in HFIR at 4000C led to an increase in the DBTT of 2170C and a 50% drop in the USE. Tests on aged but unirradiated control materials indicated aging alone has an insignificant effect on the impact properties. The results show that the major portion of the irradiation embrittlement at 3000C occurs for fluences beyond 1x1024 n/m2. Comparison of the results with data in the literature shows that the greatest effect on the impact properties of 12Cr-1MoVW steels results after irradiation in the temperature range of 300 to 4000C. Finally, a possible detrimental influence of small amounts of helium is noted and discussed. (orig.)