Influence of the size of the prior β grains on the microstructure of β quenched Zircaloy-4

Abstract

This work deals with the influence on Zircaloy-4 of annealing in the β domain and subsequent quenching in view of extrusion. The effect of the size of the prior β grains on the microstructure is investigated. The results indicate that for prior β grain sizes up to 1.6 mm, the β quenched Widmanstätten microstructure is distributed between “parallel platelets” and “basket-weave”. However, sizes larger than 2 mm promote the formation of “basket-weave”. Double annealing in the β domain leads to the formation of large prior β grains with no need to anneal for a long time. Indeed, protracted annealing may lead to exaggerated growth. Efficient double annealing requires some conditions: rate of cooling after the first β annealing a few °C.s−1 (air cooling), heating rates of 1 or 2 °C.s−1 during the second rise in temperature. The second cooling should be somewhat quicker (possibly 12 °C.s−1 and above) to obtain the thin, intricate lamellae favored for extrusion.