The transformation behaviour of the β-phase in Zr–2.5Nb pressure tubes

In a CANDU reactor, the fuel bundles and primary coolant are contained within Zr–2.5Nb pressure tubes that are approximately 6.3 m in length, have an internal diameter of 104 mm and a wall thickness of 4.2 mm. The Zr–2.5Nb pressure tubes are nominally extruded at 815 oC, cold-worked 27%, and stress relieved at 400 oC for 24 h, resulting in a structure consisting of elongated grains of hexagonal-close-packed α-Zr, partially surrounded by a thin network of filaments of body-centred-cubic β-Zr. These β-Zr filaments are metastable and contain about 20% Nb. The stress-relief treatment results in partial decomposition of the β-Zr filaments with the formation of hexagonal-close-packed ω-phase particles that are low in Nb, surrounded by a Nb-enriched β-Zr matrix. A temperature–time–transformation (TTT) diagram has been developed for the β-phase in Zr–2.5 wt%Nb pressure tubes. The results show that the morphology and/or physical state of the β-phase has a significant effect on the transformation behaviour compared with a bulk Zr–20 wt%Nb alloy. This means that a specific TTT-diagram is required to describe the behaviour of these engineering components.