Abstract
Blocky alpha can be systematically generated during heat treatment of certain as-welded Zircaloy-4 microstructures. The volume fraction depends upon the exposure time and temperature of the postweld heat treatment. It is necessary to understand the transformation kinetics of the fast-cooled as-welded structure to determine the microstructures present after extended periods of high-temperature operation. An experimental test campaign was devised to observe the evolution of blocky alpha and to understand its transformation kinetics. A number of postweld heat treatments were performed at between 610°C and 760°C to form microstructures with various volume fractions of blocky alpha. Following microstructural analysis, the data were subsequently used to construct a time–temperature transformation (TTT) diagram, capable of predicting blocky alpha formation. The TTT diagram shows that blocky alpha is not expected to form below 600°C in any realistic timeframe and is therefore not an issue at typical civil pressurized water reactor operational conditions.
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