Delayed hydride cracking behavior for zircaloy-2 plate

Abstract

The delayed hydride cracking (DHC) behavior for Zirealoy-2 plate was characterized at temperatures ranging from 300 to 550 °F. Specimens with a longitudinal ( T-L) orientation exhibited a classic two-stage DHC response. At Kvalues 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√in at 400 and 500°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°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.