EFFECT OF SILICON CONTENT ON TRANSFORMATION KINETICS OF AUSTEMPERED DUCTILE IRON

The present study investigated the effect of austenitising temperature (850, 900, and 950 degrees C) and austempering time (0-7 h) on the vo lume 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 aust empered at 360 degrees C. The volume fraction of retained austenite an d austenite carbon content results were then correlated with microstru ctural changes and impact toughness results. It is shown that the aust enite stability is of great significance with respect to impact toughn ess, 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 sta ge I transformation reaction in which mother austenite transforms to h igh carbon austenite plus acicular ferrite. However, the austenitising temperature has only a small effect on the kinetics of the stage II r eaction in which high carbon austenite transforms to bainitic ferrite plus carbides. In the low silicon iron, austenitising at 950 degrees C results in a continuous network of intercellular and low carbon auste nite which reduces impact properties. Intercellular austenite is attri buted to the segregation of manganese and high austenitising temperatu re which decrease the carbon diffusion rate and delay ferrite nucleati on and growth. Decreasing the austenitising temperature to 850 degrees C increases the rate of transformation which results in a more unifor m microstructure, stable high carbon austenite, and higher impact toug hness. Silicon has the effect of modifying the Fe-C phase diagram such that a higher solution treatment temperature is needed to fully auste nitise the iron. Furthermore, a three phase region of austenite-ferrit e-graphite is introduced in the Fe-C-Si phase diagram. Consequently au stenitising at low solution treatment temperatures produces structures containing proeutectoid ferrite. Increasing the austenitising tempera ture to 950 degrees C leads to a more uniform acicular microstructure of stable high carbon retained austenite and ferrite and results in op timum impact properties. Following short austempering times in irons c ontaining 2.02% and 3.31% silicon, the carbon content of the retained austenite is low and on subsequent cooling to room temperature it tran sforms to martensite, resulting in low impact values. Optimum properti es are obtained at intermediate austempering periods when both the amo unt 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. (C) 1998 The Institute of Materials.