Study On The Microstructural And Textural Developments During The Fabrication Of Zirconium Tubes In Nuclear Power

Zirconium alloys were extensively used as the in-core structure component andclad fuel cladding for its low thermal neutron capture cross-section, reasonablemechanical properties and adequate corrosion resistance in high temperature water inreactor. However, due to the anisotropic characteristics of zirconium alloy, it has limitedslip system. The texture and microstructure of zirconium alloy have an importantinfluence on the mechanical properties, hydride orientation, stress corrosion crackingand irradiation growth.For the tube deformation, the seamless tube manufacturing method named aspilgering was applied four times from the hot-extruded tube to the final tube. And boththe intermediate and final annealing treatments were performed to reduce the internalstress and improve the microstructure as well. In order to optimize the tube processingparameters, it is necessary to understand the influence of hot extruding temperature, thedegree of cold working, intermediate annealing temperatures and final annealingtemperatures on microstructure and crystallographic texture. In this paper, themicrostructure and crystallographic texture during the processing of Zr-Nb tubesfabrication were investigated using electron channeling contrast (ECC), X-raydiffraction (XRD) and electron backscattered diffraction (EBSD) technology. Thefollowing conclusions can be drawn from the present study:①The microstructure consisted of almost approximate equiaxial grains and thedispersive second phase particle when the samples were extruded above the phasetransformation temperature. Moreover, the temperature has a slight effect on the meansize of equiaxed grains. On the other hand, the texture evolutions of tubes extruded at650℃and630℃have a wide difference in thepreferred orientations: the former has<1010> direction paralleling to the axial direction, with the basal poles distributingrandomly in the radial-tangential plane; the later has <1120> direction paralleling to theaxial direction, and the basal poles tend to concentrate more toward tangential direction.②The evolution of microstructure during the tube reduction are concluded in thefollowing sequence: heterogeneous deformation (first pilgered)—partiallyrecrystallized (first annealed)—heterogeneous deformation (second pilgered)—fullyrecrystallized (second annealed)—heterogeneous deformation (third pilgered)—fullyrecrystallized (third annealed)—heterogeneous deformation (final pilgered)—fully recrystallized (final annealed). In general, the pilgering operations involvedheterogeneous deformation—the grains were fragmented, and during annealing,involving partial or complete recrystallization leading to the grains get refined.③Textural developments during process steps were generalized as developments in twoaspects: on the one hand, deformation (both hot extrusion and pilgering) strengthened<1010> paralleling to axial direction, while annealing enhanced <1120>. On the otherhand, the basal poles gradually shift from tangential direction to the radial directionwith the increasing Q value. The basal pole figure does not change significantly duringannealing. In part, the tendency can be observed that the basal poles concentrate moretoward the normal or radial direction, respectively; in addition, the pole density issimultaneously somewhat decreased.In addition,there exist texture gradient through thetube wall.