Influence of microalloying and heat treatment on the kinetics of bainitic reaction in austempered ductile iron

The effect of different contents of alloying additions and austenitizing temperature on the transformation kinetics of austenite in a ductile iron austempered at 300 and 400 degreesC has been investigated in the present study. X-ray diffraction, optical microscopy, and hardness measurements were used to determine the transformation kinetics of low Ni iron, low Mo iron and low Ni, Mo iron during austempering at 300 and 400 degreesC for 1 to 480 min after austenitizing at 850 and 930 degreesC for 120 min. Nickel and molybdenum in used contents are shown to delay the bainitic transformation without the undesirable features. Decreasing the autenitizing temperature is shown to increase the driving force for stage I of reaction but to have only a small effect on stage II kinetics. This shifts the position of the processing window to short periods of time and leads to opening of the processing window, which is closed for higher autenitizing temperatures. A more uniform austempered microstructure can be obtained with a decrease of autenitizing temperature. Decreasing the autenitizing temperature has the disadvantage of reducing the austemperability.