Relationship Between Irradiation Hardening and Embrittlement of Pressure Vessel Steels

Based on a large body of test and power reactor data, empirical relationships between irradiation strengthening and embrittlement are derived. It is shown that the Charpy V-notch (C_v) 41-J indexed transition temperature increases and the upper-shelf energy decreases systematically with increases in the yield stress. The transition temperature shifts are related to two mechanisms: (1) increases in the maximum temperature of elastic-cleavage fracture, and (2) decreases in the slope of the C_v energy versus test temperature curve associated with reductions in the upper-shelf energy. The cleavage shift contribution, which is usually dominant, can be predicted from the initial temperature of fracture at general yield and the change in ambient temperature static yield stress. In developing this simplified cleavage fracture model, it is shown that: (a) yield stress changes are independent of temperature and strain rate; (b) the increase in yield stress with decreasing temperature is independent of the strain rate, irradiation, and metallurgical state; and (c) the microcleavage fracture stress is independent of irradiation and temperature. A semi-empirical procedure for estimating the shift contribution due to upper-shelf energy decreases and the total temperature shift at 41 J, based on the observation of an approximately constant temperature interval of the transition regime, is proposed, along with a method for forecasting the entire irradiated C_v curve.