Mechanical properties of modified 9Cr–1Mo (T91) irradiated at ⩽300 °C in SINQ Target-3

Specimens of martensitic steel T91 were irradiated in the Swiss spallation neutron source (SINQ) Target-3 in a temperature range of 90–300 °C to displacement doses between 3 and 9.8 dpa. Tensile tests were performed at 22, 250 and 350 °C, and small punch (SP) tests were conducted in a temperature range of −186 to 22 °C to derive the change of the ductile–brittle transition temperature (ΔDBTTSP) of the steel after irradiation. The tensile test results demonstrate that the irradiation hardening increases with dose. The uniform elongation falls to less than 1%, while the total elongation is greater than 5% in all cases. All the tensile samples broke in a ductile fracture mode. In the present dose range the irradiation hardening does not saturate and increases even more rapidly at doses above about 6 dpa. The SP tests indicate that the DBTTSP of 0.25 mm thick T91 discs is about −153 °C for the unirradiated condition. After irradiation the DBTTSP increases significantly to −35 °C at 9.4 dpa, corresponding to an estimated DBTTCVN shift of 295 °C; and meanwhile the upper energies decrease. The ΔDBTTSP has a linear dependence on helium content. Analyses of the data indicate that the radiation hardening and the occurrence of intergranular fracture mode in the higher dose SP tests are dependent on gas content.