Governing factors for delayed hydride cracking in Zr-2.5Nb tubes

DHC tests have been carried out at different temperatures ranging from 100 to 280oC on compact tension specimens from Zr-2.5Nb pressure tubes with different yield strengths and hydrogen concentrations of 3 to 100 ppm. With the increasing supersaturated hydrogen concentration, DHC velocity (DHCV) of the Zr-2.5Nb pressure tubes has increased exponentially to a constant and its threshold stress intensity factor, KIH has also decreased. Thus, DHCV and KIH can be nicely described as a function of the supersaturated hydrogen concentration over the terminal solid solubility for dissolution (TSSD) independent of temperature. Another factor to control DHCV is the yield strength of the Zr-2.5Nb pressure tube: faster DHCV for the Zr-2.5Nb tubes with a higher strength. By normalizing DHCV of the Zr-2.5Nb tubes by the solubility and diffusivity of hydrogen, DHCV is found to have a linear relation only with yield strength, irrespective of temperature. Therefore, delayed hydride cracking of Zr-2.5Nb pressure tubes is governed by their tensile strength and a difference in hydrogen concentration in solution between the crack tip and the bulk regions. A driving force for DHC is discussed from the view points of the concentration gradient and diffusivity of hydrogen. (author)