Effect of silicon content on transformation kinetics of austempered ductile iron

Abstract

The present study investigated the effect of austenitising temperature (850, 900, and 950°C) and austempering time (0–7 h) on the volume of retained austenite of a 0.3 wt-%Mn ductile iron containing two different levels of silicon, namely 2.02 wt-% and 3.31 wt-%, and austempered at 360°C. The volume fraction of retained austenite and austenite carbon content results were then correlated with microstructural changes and impact toughness results. It is shown that the austenite stability is of great significance with respect to impact toughness, and that ferrite, when present in acicular form, can increase the mechanical stability of the austenite. It is observed that decreasing the austenitising temperature increases the driving force for the stage I transformation reaction in which mother austenite transforms to high carbon austenite plus acicular ferrite. However, the austenitising temperature has only a small effect on the kinetics of the stage II reaction in which high carbon austenite transforms to bainiticferrite plus carbides. In the low silicon iron, austenitising at 950°C results in a continuous network of intercellular and low carbon austenite which reduces impact properties. Intercellular austenite is attributed to the segregation of manganese and high austenitising temperature which decrease the carbon diffusion rate and delay ferrite nucleation and growth. Decreasing the austenitising temperature to 850°C increases the rate of transformation which results in a more uniform microstructure, stable high carbon austenite, and higher impact toughness. Silicon has the effect of modifying the Fe-C phase diagram such that a higher solution treatment temperature is needed to fully austenitise the iron. Furthermore, a three phase region of austenite-ferrite-graphite is introduced in the Fe-C-Si phase diagram. Consequently, austenitising at low solution treatment temperatures produces structures containing proeutectoidferrite. Increasing the austenitising temperature to 950°C leads to a more uniform acicular microstructure of stable high carbon retained austenite andferrite and results in optimum impact properties. Following short austempering times in irons containing 2.02% and 3.31% silicon, the carbon content of the retained austenite is low and on subsequent cooling to room temperature it transforms to martensite, resulting in low impact values. Optimum properties are obtained at intermediate austempering periods when both the amount of retained austenite and the austenite carbon content are maximum. Extending the austempering time causes the high carbon austenite to decompose to ferrite plus carbides, the stage II reaction, leading to a reduction in impact toughness.