The initiation of delayed hydride cracking at a notch in coarse grained, random textured zircaloy weld metal

• Main Author: Schofield, John
• Published: University of Manchester 2002
• Subjects: 620.1
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.637516

Zirconium alloys used in water-cooled nuclear reactors are susceptible to a cracking process, known as Delayed Hydride Cracking or DHC, which involves hydrogen diffusion to a stress concentrator, precipitation, growth and fracture of zirconium hydrides. Prompted mainly by CANDU nuclear reactor incidents DHC has been the subject of research effort aimed at defming the conditions for crack initiation and growth and the underlying mechanism. Much of this work has focused upon the Zr-2.5%Nb alloy in a heavily cold worked form as found in CANDU pressure tubes. The Zircaloys, which are generally considered to be less susceptible to DHC, have tended to receive less attention, especially in the irradiated condition. The focus of this thesis is the study of DHC initiation at a notch in a coarse grained, random texture Zircaloy-2 or Zircaloy-4 weld during thermal-mechanical cycling and during cold hold conditions, with the behaviour of an engineering component in mind. The microstructure of such a weld is significantly different to the fme grain, heavily textured microstructure found in CANDU Zr-2.5%Nb pressure tubes. The experiments reported in this thesis consider the conditions necessary to initiate DHC at two notch profiles in irradiated material. The two notch geometries are: a 1 nun deep notch with an included angle of 450 and a root radius of 50J.lm and a Imm deep notch having a root radius of 700J.lm The applied thermal mechanical cycle is considered unique in that the applied load varied with temperature and was increased as the specimens were cooled. Results obtained from testing are used to derive threshold load conditions for DHC initiation. One conclusion from the work is that the time dependent processes involved in DHC initiation are sufficiently fast at cold temperatures (T::S;60°C) to cause cracking during sustained periods (3-6 months) of loading. The work suggests that threshold conditions for cracking may be reduced marginally if cycling involves a sustained period of loading at low temperatures.