Temperature and hydrogen concentration limits for delayed hydride cracking in irradiated zircaloy

Critical temperatures for delayed hydride cracking (DHC) initiation during cooling (T-RIT) and crack arrest during heating (T-DAT) have been measured in experiments on specimens from a Zircaloy-2 electron beam weld irradiated to a fluence of 3 to 5 X 10(25) n (.) m(-2) (E > 1 MeV) and then hydrided to concentrations of 35 and 55 ppm. The experimental observations are shown to be consistent with a simple model based on previously determined hydrogen terminal solid solubility data for dissolution and precipitation, which describes conditions for sustained hydride precipitation at the crack tip. When plotted against bulk hydrogen concentration in solution, both T-RIT and T-DAT fall below the dissolution solvus temperature and above the precipitation solvus temperature. A key assumption in the model is that, while the local crack tip stress concentration causes local enhancement of the hydrogen concentration in solution, the hydride precipitation solvus is unaffected by stress. The good agreement obtained between measured and predicted critical temperatures provides strong support for this assumption.