The effect of texture on delayed hydride cracking in Zr-2.5Nb alloy

Pressure tubes for Candu(R) reactors are made of Zr-2.5Nb alloy. They are produced by a hot extrusion followed by a cold-work which results in a material with a pronounced crystallographic texture with basal plane normals of the hexagonal structure around the circumferential direction. Under certain conditions, this material is susceptible to a cracking mechanism known as Delayed Hydride Cracking (DHC). This work investigated the susceptibility to DHC in this pressure tube material, in terms of crystallographic texture acid grain shape. The results are presented in terms of the crack velocity obtained on different planes and directions of the pressure tube. They show that the cracking is more difficult when the crack is propagating at right angles to crystallographic basal planes (which are close to the precipitation habit plane of hydrides) than parallel to the basal plane. However, if the cracking plane is oriented parallel to pre-existing hydrides (hydrides formed as a result of manufacturing process), the crack propagates along these hydrides easily, even if the hydride habit planes are not oriented favourably.