DELAYED HYDRIDE CRACKING BEHAVIOR FOR ZIRCALOY-2 PLATE

The delayed hydride cracking (DHC) behavior for Zircaloy-2 plate was characterized at temperatures ranging from 300 to 550-degrees-F. Specimens with a longitudinal (T-L) orientation exhibited a classic two-stage DHC response. At K values slightly above the threshold level (K(th)), crack-growth rates increased dramatically with increasing K values (stage I). The K(th) value was found to be 11 and 14 ksi square-root in at 400 and 500-degrees-F. At high K values (stage II), cracking rates were relatively insensitive to applied K levels. Stage II crack growth was a thermally activated process described by an Arrhenius-type relationship with an activation energy of 65 kJ/mol. This energy level agreed with the theoretical activation energy for hydrogen diffusion into the triaxial stress field ahead of a crack. Above a critical temperature (300-degrees-F), an overtemperature cycle was required to initiate DHC. The magnitude of the thermal excursion required to initiate cracking was found to increase at higher test temperatures. Specimens with a transverse (L-T) orientation showed a very low sensitivity to DHC because of an unfavorable crystallographic orientation for hydride reorientation. Metallographic and fractographic examinations were performed to understand the DHC mechanism.