Intergranular embrittlement and irradiation hardening due to phosphorus in reactor pressure vessel steels

Nishiyama, Yutaka ; Onizawa, Kunio ; Matsuzawa, Hiroshi*

The effects of grain-boundary P segregation and hardening on the embrittlement in terms of Charpy ductile-to-brittle transition temperature (DBTT) in several neutron-irradiated reactor pressure vessel (RPV) steels with different bulk contents of P have been investigated using a scanning Auger microbe, a local electrode atom probe and positron annihilation spectroscopy. Increasing the neutron fluence promotes intergranular P segregation, particularly in steels with high levels of P. The content of P significantly also affects irradiation hardening due to distinct formation of P-rich precipitates arising from the stabilization of vacancies. It was found that neutron irradiation mitigates an embrittling effect of segregated P, therefore the hardening more strongly affects the DBTT shift than the P segregation. The likelihood of intergranular embrittlement of RPV steels is discussed by these results.