Abstract
The effects of austempering time and temperature on the microstructure and tribological behaviour of austempered SG Al(2.2%) cast iron have been studied. An austenitizing temperature of 900 °C, austempering temperatures of 250–400 °C and austempering times of 5–240 min were used: lower and upper bainitic structures were obtained by austempering within this temperature range. The results show that the influence of aluminium (A1) on bainitic reaction is similar to silicon (Si) in conventional SG cast irons: aluminium is a strong graphitizer and produces a bainitic structure with a high wear resistance. The transformation kinetics were investigated using X-ray diffraction and microstructural studies were undertaken using optical and scanning electron microscopy. Dry sliding wear tests were conducted on a modified Amsler-type machine from which it was found that the austempering of SG Al iron significantly increases the wear resistance. The relationship between volume fraction of retained austenite and wear (weight loss) revealed that optimum tribological behaviour can be obtained by varying the austempering temperature and time. The results show that the hardness and wear resistance decrease with an increase in austempering time and temperature. The observed changes in tribological behaviour can be correlated with the kinetics and austempered microstructure of the irons.
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