Metallographic Observations of Delayed Hydride Cracking in Zr-2.5Nb

Zr-2.5Nb is an alloy used to make pressure tubes in CANDU reactors for its combination of corrosion resistance, good mechanical properties, and low neutron capture cross-section.Studies are undertaken of life-limiting phenomena for Zr-2.5Nb to ensure that these phenomena are well understood and that pressure tubes are not operated in a compromised state.One such phenomenon is Delayed Hydride Cracking (DHC) [1,2].DHC is a time-dependent cracking mechanism based on diffusion of hydrogen in the presence of a tensile stress to the tip of a flaw, followed by nucleation, growth and fracture of a brittle hydride phase along appropriately oriented crystal planes.Repetition of these processes, aided by a thermal cycle, can lead to crack propagation in zirconium alloys.DHC can be prevented by ensuring there are no good crack initiation sites, that the hydrogen concentration is not sufficient to precipitate hydrides, or that crystal planes susceptible to cracking are not under high tensile stress.In the present work, eight simulated flaws were machined into the inside surface of a section of pressure tube containing 38 µg/g of hydrogen.The tube assembly was thermally cycled, with an internal pressure higher