The influence of aluminium on the microstructure of austempered ductile cast iron (ADI) has been studied. The presences of well distributed graphite in the matrix, and a relatively high volume fraction of retained austenite, are responsible for the good mechanical properties of the Aluminium alloyed austempered ductile irons including excellent wear resistance, higher resistance to thermal shock, better graphitising tendency and higher resistance to oxidation at high temperature. A number of specimens of different composition have been made by green sand casting and gravity die casting. The transformation to a bainitic microstructure during austempering under different conditions was then examined for the most successful of the experimental casts. Austenitising temperature of 920C, and austempering temperatures of 350C at different holding times have been used. Microstructures have been examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Furthermore, it was found that isothermal transformation at 350C for different soaking times gave a typical bainitic microstructure. Extension of isothermal transformation time leads to precipitation of carbides which also depended on the Al concentration.

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