Driving force for delayed Hydride cracking of zirconium alloys

Abstract

The effect of hydrogen concentration on the delayed hydride cracking velocity (DHCV) and the threshold stress intensity factor, KIH of a Zr-2.5Nb tube were examined at test temperatures ranging from 100 to 280°C by subjecting compact tension specimens with a hydrogen concentration of 12 to 100 ppm H to an overtemperature cycle. The DHCV and KIH increased and decreased, respectively, with an increase in the supersaturated hydrogen concentration over the terminal solid solubility for dissolution (TSSD) or ΔC. They then leveled off to constant values at ΔC in excess of the ΔCmax corresponding to a difference of the terminal solid solubility of the hydrogen on cool-down and on heat-up. Further, intentional introduction of an undercooling by 0 to 40°C at the test temperature decreased the DHCV of the Zr-2.5Nb tube, indicating that ΔC between the bulk region and the crack tip governs the DHCV. A new DHC model is proposed where the driving force for DHC is the difference in the hydrogen concentration between the bulk region and the crack tip by preferentially nucleating the hydrides only at the crack tip under an applied tensile stress, due to a hysteresis in the TSS of hydrogen on heat-up and on cool-down. A supplementary experiment was conducted to validate the feasibility of the proposed DHC model.