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Three batches of Zircaloy-2 tubing were beta-quenched prior to the final cold-rolling, cold rolled 80 percent, and annealed at 475 to 575°C. A fourth batch was β-quenched in the final size. For comparison, standard tubing was included in all tests performed. The second-phase particles were studied by means of optical and scanning electron microscopy. Corrosion testing was carried out at 400°C and in high-temperature (475 to 500°C) high-pressure steam. The mechanical tests comprised tension, burst, and creep testing under internal pressure.
Beta-quenching instead of an intermediate or the final anneal results in significant structural changes. The most striking features are the formation of a Widmanstätten structure consisting of plates of α-phase and the reprecipitation of much finer second-phase particles in the plate boundaries. Cold-rolling of β-quenched hollows followed by a final anneal in the α-range will give an equiaxed structure, but the size and distribution of the second phase obtained in β-quenching will not be markedly changed.
The weight gain at 400°C increases slightly as a result of β-quenching in intermediate or final size. In high-pressure steam at 475 to 500°C, on the other hand, such β-quenching has a dramatic beneficial effect on the corrosion resistance. At 500°C, for instance, specimens from tubing β-quenched in intermediate size show weight gain values of 55 to 95 mg/dm2, whereas specimens from standard tubing take values in the range 360 to 4280 mg/dm2. The explanation of this improvement seems to be the existence of small second-phase particles formed in α-plate boundaries as a result of dissolution and reprecipitation during β heat treatment and quenching. Experiences from boiling water reactor (BWR) channel corrosion of β-quenched Zircaloy-4 indicate that the 475 to 500°C high-pressure steam test forecasts very well the in-reactor resistance of Zircaloys to nodular corrosion under BWR conditions.
The short-term strength as measured in tension and burst testing is improved by β-quenching of hollows or finished tubes, whereas such treatment results in a slight drop in ductility, especially for tubing β-quenched in the final size. The 400°C transverse creep strength is increased by the introduction of β-quenching prior to the final cold-rolling. The improvement is caused mainly by small second-phase particles, formed during β-quenching, which give rise to precipitation hardening.
Zircaloys, cladding tubes, beta-quenching, nodular corrosion, second-phase particles, creep, zirconium, nuclear industry
Senior research metallurgist, Sandvik AB, Sandviken,
Senior research metallurgist, ASEA-ATOM, Västerås,