Evaluation of the delayed hydrogen cracking behavior and the hydrogen diffusion coefficient for different microstructures of the Zr-2.5%Nb alloy

Abstract

The susceptibility of delayed hydride cracking (DHC) in the Zr-2.5%Nb alloy was evaluated in six microstructures produced from an extruded tube of Zr-2.5%Nb, which underwent different thermomechanical treatments, divided into two separate groups: Low temperature samples (LT) were heat-treated below the monotectoid temperature in the α-Zr + β-Nb field, and included pressure tube sample of CANDU-type material obtained through two different cold deformation methods, rolling and drawing, and stress-relieved at 400 °C for 24 h, and heat-treated samples at 600 °C/4 h. High-temperature samples (HT) were heat-treated in the β-Zr field at 900 °C/3 h, and two different cooling sequences up to room temperature. The increase in the ultimate tensile strength (UTS) and hardness due to metallurgical processing in LT materials made them more susceptible to DHC, reducing the stress intensity, KIH, from 11.8 to 8.5 MPa m0.5, together with an increase in crack propagation velocity from 1.6 10−8 to 4.5 10−8 m/s. In situ hydrogen diffusion experiments were performed at ANTARES, the cold neutron imaging facility at the FRM-2 reactor, on LT materials. These experiments demonstrated that the recrystallization treatment-induced discontinuity of the β-Zr phase in the parent material has a significant impact on hydrogen diffusion. It results in a 35% reduction in the diffusion coefficient compared to the parent material and a decrease in the terminal solid solubility (TSS) was observed. This resulted in a slight increase in KIH (7%), an increase in the hydride incubation time, and a decrease of about 20% in the crack propagation velocity. Under identical testing conditions, HT specimens were not susceptible to DHC phenomenon.