MORPHOLOGY CHARACTERISTIC AND GROWING MANNER OF AUSTENITE IN HYPEREUTECTIC DUCTILE IRON DURING SOLIDIFICATION PROCESS
过共晶球铁凝固过程中奥氏体的生长方式与形貌特征

The high-temperature solidification structures in hypereutectic ductile iron are shown clearly by the color etching technique, and the morphology characteristic and growing manner of austenite during solidification process are analysed. The results reveal that the forming conditions of both the primary austenite dendrite and austenite shell dendrite are mainly related to cooling rate and undercooling of melt. With increasing the casting modulus A. the counts of dendrites reduce, the second arm space increases remarkably and the morphology of dendrites in hypereutectic ductile iron will be transformed from primary austenitic dendrite into shell dendrite. When M,. less than 0.3 cm the measured second arm space is in agreement well with that calculated theoretically. A ring like austenite shell has formed around the primary graphite sphere. At the earlier stage of eutectic, the enveloped or nor- enveloped frame austenite shell lias formed. There is no austenite shell outer the nodular graphite formed in the later stage of eutectic. The austenite shell is a part of the austenite dendrites.