Microstructure and phase transformation of wear resistant ductile iron grinding balls by continuous cooling process

The hardenability of wear resistant ductile iron austenitized at different temperatures was analyzed by Jominy end-quenching test, and continuous cooling transformation (CCT) curves were determined by Gleeble 1500D. The temperature variety curves were recorded by thermocouples during the test, and the cooling rate curves were obtained. It was indicated that there were significant influences of austenitizing temperature and Jominy distance (DJ) on the cooling rate. All specimens had undergone three different heat transfer mechanisms including film boiling mechanism, nucleate boiling mechanism and natural convection mechanism. Meanwhile, the cooling rate also had a distinct influence on the microstructure characteristics of ductile iron. The results of hardness test and microstructure observation indicated that the specimen austenitized at 830 °C had the most suitable hardness distribution, with high hardness on the quenching end and high toughness inside. That means that the grinding balls present function gradient characteristic can be obtained after continuous cooling. In addition, as to the specimen austenitized at 780 °C, the hardness was lower for the partially austenitized mixture microstructure. As to the specimen austenitized at 880 °C, the coarsing of the microstructure occurred, which affected the mechanical properties significantly.