Cracking of zircaloy as a result of unusual localized texturing

Abstract

Microcracks in rolled Zircaloy have been observed at an angle of 35 deg to the plate normal in relatively fine-grained bands within a matrix of coarser grains. The fine-grained regions show a different crystallographic orientation than the matrix when observed under polarized light. This duplex “crack-prone” structure results from a rolling schedule of many light passes. A fewer number of heavier passes alleviates the problem. Starting with a single Zircaloy ingot, thermomechanical processing is varied according to three different schedules: light, normal, and heavy reductions per rolling pass. Samples obtained at several stages during each of the three rolling schedules are examined for crystallographic texture and fatigue strength. A computer program is used to generate inverse pole figures, histograms of basal pole inclination vs volume percent of grains and/numbers from X-ray diffraction data. The /numbers define the effective fraction of basal poles aligned in the directions of interest. These results are used to illustrate the texture changes and their influence upon the cracking sensitivity of Zircaloy during the various stages of reduction, and thermal treatments of the three rolling schedules. A significant effect on cracking sensitivity due to hot and cold rolling pass schedules is shown, and a tendency for light hot rolling passes to align basal poles normal to the rolling plane is demonstrated.