Brittle fracture resistance of reactor pressure vessel steels in the initial state

The authors investigate the influence of chemical and structural inhomogeneity on the brittle fracture resistance (BFR) of VVER vessel materials in the initial state (without irradiation). It is proposed to replace the brittle fracture resistance assessment using the critical brittleness temperature TC for the BFR assessment using the brittle-viscous transition temperature TT. Consideration was given to calibration charts used for studying the TT dependence on the grain size and heat treatment. A comparison of the TC and TT values in the experimental industrial 15H2NMFA-A steel billets shows that the TC values are significantly lower than the TT values: – at the lower level of conservatism, the difference between TC and TT is 22 °C; – at the upper level of conservatism, this difference is 24 °C. The array data on the critical brittleness temperature and the ductile-to-brittle transition temperature of impact test samples of 15H2NMFAA (for VVER-1000) and 15H2NMFA grade 1 (for VVER-1200) steels were statistically processed. The industrial shell samples were manufactured at the "Energomashspetsstal" plant (Kramatorsk, Ukraine). It was found that, in the metal of VVER-1000 vessel surveillance specimens with the copper content – less than 0.06%, heat treatment has a significant effect on the TT value, which changes from –99 to –28°C; – from 0.07 to 0.12%, heat treatment has a significant effect on the TT value, which changes from –60 to –40°C