The effect of annealing on hardness, microstructure and delayed hydride cracking in Zr–2.5Nb pressure tube material

The effect of annealing at 400 °C on hardness, microstructure and velocity of delayed hydride cracking (DHC) in Zr–2.5 pressure tube material in both the axial and radial directions was investigated. DHC velocity tests were performed at temperatures from 120 to 240 °C on specimens which had been annealed up to 1000 h. The β-phase initially was a thin, continuous film between α-grains providing a path for rapid hydrogen diffusion. Since the β-phase pathways were closely spaced in the axial direction but not in the radial direction, DHC velocity is 1.9 times faster in the axial direction initially. During annealing, the β-phase decomposed to discrete particles, the room temperature hardness dropped 10% and the axial DHC velocity decreased by a factor of five. The radial crack velocity decreased by a factor of three and the ratio of axial to radial DHC velocity decreased to 1.2. The decrease of this ratio corresponded to the large disruption of the β-phase caused by prolonged annealing. The activation energies for DHC velocity in these tests lay between 33 and 47 kJ/mol, suggesting that the existing theories for activation energy are incomplete.