Instrumented impact properties of some advanced nuclear reactor pressure vessel steels

Steels used to construct nuclear reactor pressure vessels are low-alloy ferritic steels. These steels should have good impact properties, i.e., low transition temperature and high upper shelf energy, both before and during service conditions. The most important service condition is the neutron irradiation. Extensive research and development was conducted to develop such steels. Instrumented impact testing was conducted on three advanced pressure vessel steels and, for comparison, a conventional pressure vessel steel. Both microstructures and fracture surfaces were examined using optical and scanning electron microscopic (SEM) techniques. In general, the advanced steels showed much better impact properties (lower ductile-brittle transition temperature and higher upper shelf energy) than the conventional steel. Loadtime traces showed that increase in the fracture energy was mainly due to increase in the fracture propagation energy rather than the initiation energy. Improvement in the toughness level of the advanced steels compared to that of the HSST steel was related to the difference in chemical composition, microstructure, and fracture surface morphology.