Microstructure and fracture of aluminium austempered ductile iron investigated using electron microscopy

Abstract

An unalloyed nodular cast iron, in which the conventional addition of silicon has been replaced by aluminium, has been studied after heat treating in the bainite temperature range. Specimens were austenitised at 950°C for 2 h and then austemperedfor times up to 5 h at either 300 or 400°C. At 400°C the microstructure producedfor austempering times up to 3 h was typical of austempered ductile iron, consisting of carbide free upper bainitic ferrite together with a stable, high carbon enriched retained, reacted, austenite. For longer times, transition carbides are precipitated, initially η carbide in the ferrite, and after 5 h, χ carbide at the austenite/ferrite interfaces by decomposition of the high carbon austenite. Austempering at 300°C produced e carbide in a lower bainitic ferrite together with stable, high carbon retained austenite, for all heat treatment conditions: χ carbide precipitated at the ferrite/austenite interfaces after 5 h austempering. The fracture behaviour of samples austempered at both 300 and 400°C is dramatically influenced by the χ carbide formation, which results in a mostly brittle cleavage mode offailure.

MST/1968