Microstructure evolution and tensile properties of Zr-2.5Wt%Nb pressure tubes processed from billets with different microstructures

摘要

Starting with identical ingots, billets having different microstructures were obtained by three different processing methods for fabrication of Zr-2.5wt%Nb pressure tubes. The billets were further processed by hot extrusion and cold Pilger tubereducing to the finished product. Microstructural characterization was done at each stage of processing. The effects of the initial billet microstructure on the intermediate and final microstructure and mechanical property results were determined. It wasfound that the structure at each stage and the final mechanical properties depend strongly on the initial billet microstructure. The structure at the final stage consists of elongated alpha zirconium grains with a network of metastable beta zirconiumphase. Some of this metastable phase transforms into stable beta niobium during thermomechanical processing. Billets with quenched structure resulted in less beta niobium at the final stage. The air cooled billets resulted in a large amount of betaniobium. The tensile properties, especially the percentage elongation, were found to vary for the different methods. Higher percentage elongation was observed for billets having quenched structure. Extrusion and forging did not produce any characteristicdifferences in the properties. The results were used to select a process flow sheet which yields the desired mechanical properties with suitable microstructure in the final product.