Relationship of Yield Strength and Upper-Shelf Impact Energy Changes for Irradiated Reactor Vessel Steel

Abstract

Radiation-induced changes on the strength and toughness properties of reactor pressure vessel steels are known to be dependent on the chemical content and, to a lesser extent, on the product form. The precise relationship between key variables, however, is yet to be established. Emphasis is currently being placed on the development of mechanistic models to provide coherence between the variety of empirical observations presented to date. Results are presented of an investigation of radiation damage trends exhibited by changes in the uniaxial tension and Charpy upper-shelf energy properties. Regression analysis techniques were used to determine trends based on chemistry and neutron fluence. These trends were evaluated in terms of proposed mechanistic models toward developing a broader understanding of radiation damage mechanisms. Radiation-induced transition temperature shift in reactor vessel steels is known to be dependent on the copper and nickel content of the material. In this investigation, Charpy upper-shelf impact energy and ultimate tensile strength changes were found to exhibit similar dependencies on the copper content and neutron irradiation exposure, but neither property change was significantly influenced by nickel content. Based on the apparent difference in radiation behavior trends between transition temperature shift and shelf energy/tensile property changes, a multiple defect radiation damage model was postulated. The first defect type, dependent on copper content, increased the yield strength through a precipitation hardening mechanism. A second defect type, dependent on both copper and nickel content, affected the microcleavage fracture stress through a process such as grain boundary embrittlement. A third defect type, dependent on neutron irradiation only, affected the yield strength by a process such as vacancy hardening. This postulated model provides a reasonable basis for explaining observed differences in radiation behavior.