The significance of billet pre-heating parameters in hot-extrusion of Zr-2.5Nb pressure tubes for CANDU~1 reactors

摘要

The Zr-2.5Nb pressure tubes for CANDU reactor are produced by hot extrusion of billets. The billets are pre-heated at a temperature in the alpha-beta phase field. To study the effects of the billet pre-heating on the microstructure and properties of the finished tube, pressue tubes were manufactured under controlled pre-heating of initially beta-quenched billets at different alpha-beta temperatures (780degC to 850degC) with different soak times (15 to 300 min), Increasing temperature and soak time in the two-phase region leads to significant reduction in the primary-alpha volume fraction combined with much grain coarsening and oxygen enrichment of the primary alpha-rains. These microstructural changes in the billet material prior to extrusion had a strong influence on the microstructure of the finished tube. The characteristic alpha-grain structures developed in these and some other existing pressure tubes are described and analyzed based on the billet pre-heating aprameters, The test results show that the chagnes in the microstructure due to billet pre-heating could have a strong influence on the mechanical and other relevant properties of the pressure tube material.