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

This work deals with the influence on Zircaloy-4 of annealing in the beta domain and subsequent quenching in view of extrusion. The effect of the size of the prior beta grains on the microstructure is investigated. The results indicate that for prior beta grain sizes up to 1.6 mm, the beta quenched Widmanstatten 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 beta domain leads to the formation of large prior beta 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 beta annealing a few degrees C.s(-1) (air cooling), heating rates of 1 or 2 degrees C.s(-1) during the second rise in temperature. The second cooling should be somewhat quicker (possibly 12 degrees C.s(-1) and above) to obtain the thin, intricate lamellae favored for extrusion.