Analysis of Hydride Effects in Zr-2.5Nb Micro Pressure Tubes
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During operation of a CANDU reactor, corrosion occurs which results in free hydrogen that can diffuse into the tubes. Once the solid solubility of hydrogen in the zirconium matrix is exceeded, the hydrogen will precipitate as a flake-like brittle hydride phase. The natural orientation of the hydride flakes is in the circumferential direction but under a tensile hoop stress the hydrides are able to reorientate themselves to the radial direction. This makes the pressure tubes susceptible to delayed hydride cracking (DHC) which can cause failure of the tubes. A memory effect has been observed to cause hydrides that would otherwise form in the circumferential direction to form in the radial direction. Studies have been previously performed to examine the memory effect however they have not quantified the hydride orientation distribution. This study examined the memory effect in Zr-2.5Nb micro pressure tubes with three different microstructures and textures. A stepped micro pressure tube sample design was hydrided to 100 ppm(wt%) and pressurized to obtain a nominal hoop stress ranging from 65 MPa to 350 MPa. Each of the samples was heated to 350oC to dissolve all of the hydrogen followed by cooling under stress. Samples were then reheated to 350oC for 1 hour and 24 hours and cooled without stress. Almost complete reorientation was observed in typical pressure tube material which had a very fine microstructure and a large portion of basal plane normals in the transverse direction. After reheating, little memory effect was found in material similar to the commercially used pressure tube material. However a clear memory effect was observed in the other two samples. The memory effect was observed in the range of angles where hydrides are naturally present. A second flanged sample design was used to find the dilational strain associated with hydride reorientation from which the normal strain could be calculated. The strain normal to the hydride, εnormal, was calculated to be 0.11 ± 0.05. This study provides a valuable resource that can be used to improve DHC models which are used to determine the useful life of the pressure tubes.