Effect of Thermomechanical Processing and Heat Treatment on the Properties of Zr-3Nb-1Sn Strip and Tubing
Effect of Thermomechanical Processing and Heat Treatment on the Properties of Zr-3Nb-1Sn Strip and Tubing
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The property response of Zr-3Nb-1Sn (weight percent) strip and tubing to variations in thermomechanical processing and heat treatment has been evaluated under commercial production conditions. When aged below the eutectoid temperature, that is, below 1100°F (593°C), cold-worked Zr-3Nb-1Sn strip and tubing were found to have out-of-reactor steam corrosion properties similar to the Zircaloys, while exhibiting superior tensile, creep, and formability characteristics. Creep was at least 20 times slower in Zr-3Nb-1Sn strip than in Zircaloy strip. Aging at 800°F (427°C) maximized creep strength by precipitating fine ω particles in the β Zr phase when β was present in the form of a network surrounding individual α Zr platelets, that is, a Widmanstatten morphology. The Widmanstatten morphology was developed by either high temperature rolling or solution treatment. Increasing aging temperature and reducing cooling rate increased precipitate size and caused the β phase to agglomerate. These changes improved ductility and formability but decreased strength and corrosion resistance. However, increasing annealing time at 1050°F (566°C) markedly improved corrosion properties. Good corrosion resistance in Zr-3Nb-1Sn is associated with a minimum volume fraction of β, enrichment of β in niobium, a small β-grain size, and a uniform distribution of the β phase. The welding characteristics of Zr-3Nb-1Sn were found to be similar to Zr-2.5Nb. In both alloys, post-weld heat treatment was effective in improving corrosion resistance and weld ductility while maintaining strength above base metal levels.
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