Effect of silicon content on impact properties of austempered ductile iron

Abstract

This study investigates the effect of austenitising temperature (850°C, 900°C, 950°C) and austempering time (0 to 7 h) on the impact energy values of three irons containing increasing silicon contents (2.02%, 2.65%, 3.31%) and around 0.3% manganese, austempered at 360°C. It was shown that increasing the silicon content modifies the Fe-C phase diagram such that (a) a higher solution treatment temperature is required to fully austenitise the iron, and (b) the resulting austenite dissolves less carbon. Increasing the austenitising temperature simultaneously with the silicon content restores a high carbon austenite, a fully ausferritic structure and high impact properties. On the other hand, the low silicon iron austenitised at 950°C results in a microstructure containing a continuous network of intercellular austenite. This is attributed to the manganese whose segregation is more evident in the iron containing 2.02% silicon. For a given silicon content, increasing the austenitising temperature increases the carbon content dissolved in the austenite which in turn reduces the free energy controlling the rate at which the austempering reaction proceeds. Either short or extensive holding in the austempering bath leads to undesirable structures and low mechanical properties. Intermediate austempering periods resulted in structures containing ferrite needles, a large volume of high carbon stable austenite and a better combination of mechanical properties.