Abstract
The influence of austempering temperature and salt bath agitation on microstructure and mechanical properties of austempered ductile iron was studied. The as-cast specimens of ductile iron were subjected to heat treatments consisting of austenitisation at 900 °C for 1 h followed by austempering at temperatures ranging from 230 to 450 °C in a salt bath with and without agitation. The volume fraction of retained austenite increases with increasing austempering temperature to about 395 °C and decreases at higher austempering temperatures. The austempering at lower temperatures leads to the formation of finer microstructures. The agitation of the salt bath leads to the formation of the higher volume fraction of ausferrite at the expense of the retained austenite compared to that of the specimens prepared without salt bath agitation. Vickers hardness decreases with increasing austempering temperature and increasing volume fraction of the retained austenite. Room temperature impact energy increases with increasing austempering temperature to about 395 °C and then decreases at higher temperatures. The impact energy increases with increasing volume fraction of the retained austenite. The proposed mathematical models are of industrial importance for prediction of Vickers hardness, impact energy and volume fraction of the retained austenite as functions of austempering temperature and agitation or no agitation of the salt bath.

Key words
austempered ductile iron, heat treatment, microstructure, mechanical properties

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