Abstract
ABSTRACTThis paper presents the methodology used for manufacturing the Zircaloy-4 tubes required for the Zircaloy-4 mock-up dissolver assembly. The evolution of the microstructure at different stages of production of Zircaloy-4 tubes was characterized using scanning electron microscopy (SEM). Microstructural evolution follows the sequence as dendritic structure (as-cast ingot) → Widmanstätten structure (β-quenched) → bimodal grain size (hot extruded) → heterogeneous deformed structure (pilgered) → partially recrystallized structure (final annealed). High strength and low ductility were obtained for the Zircaloy-4 tubes at pilgered condition due to grain size refinement and work hardening during the pilgering process. Compared with the pilgering stage, the Zircaloy-4 tubes in the final annealed condition exhibited moderate strength and high ductility due to the partially recrystallized microstructure. Autoclaving was carried out to improve the corrosion properties of the pilgered Zircaloy-4 tubes in boiling 11.5 M nitric acid. When exposed to boiling 11.5 M nitric acid for 1000 h, a lowest corrosion rate of 0.003 mpy was obtained for autoclaved Zircaloy-4 tubes. Laser Raman Spectroscopy (LRS) analysis confirmed that the origin of passivity of pilgered and autoclaved Zircaloy-4 tubes was due to the presence of a protective passive film composed of ZrO2.
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