Carbide Precipitation in the Microstructures of Austempered Ductile Irons Containing 0.48% and 4.88% Al

The influence of aluminium on the microstructure of austempered ductile cast iron (ADI) has been studied. The presence 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 temperatures. 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 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 depends on the Al concentration.