A New Fabrication Process for Zr-Lined Zircaloy-2 Tubing

Abstract

A new fabrication process for Zr-lined Zircaloy-2 cladding tubes was developed, including a cold pilgering pass schedule and an appropriate heat treatment. In this study, the effect of tool design in cold pilgering on the quality of tubes was investigated. Cold pilgering tests were performed using tools with different tool curves. Simultaneously, for each tool the plastic strain and stress in the tubes during cold pilgering were simulated using a theoretical plastic deformation model. The investigations indicated that an abrupt change in the strain and stress in the tubes during cold pilgering should be avoided to prevent crack formation in the tubes. The results made possible the use of higher reduction (91%) in cold pilgering. Also, anappropriate heat treatment for the properties of the final tubes in the new process was investigated. It indicated that quenching the tubeshell and the intermediate annealing temperature had a small effect on the mechanical properties. To obtain both nodular and uniform corrosion resistance the appropriate precipitate size was in the range 140 to 170 nm. β-quenching of the tubeshell was not effective for improving the uniform corrosion resistance. An appropriate intermediate annealing temperature was in the range 823 to 953 K for the α+β quenched and for the α-annealed tubeshells. These results led to the new fabrication process for Zr-lined Zircaloy-2 cladding tubes (12 mm outside diameter) in which the final tube was fabricated using only two cold pilgering passes from the tubeshell (63.5 mm outside diameter).