MICROSTRUCTURE AND FRACTURE OF ALUMINUM AUSTEMPERED DUCTILE IRON INVESTIGATED USING ELECTRON-MICROSCOPY

An unalloyed nodular cast iron, in which the conventional addition of silicon has been replaced by aluminium, has been studied after heat tr eating in the bainite temperature range. Specimens were austenitised a t 950 degrees C for 2 h and then austempered for times up to 5 h at ei ther 300 or 400 degrees C. At 400 degrees C the microstructure produce d for 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 longe r times, transition carbides are precipitated, initially eta carbide i n the ferrite, and after 5 h, Chi carbide at the austenite/ferrite int erfaces by decomposition of the high carbon austenite. Austempering at 300 degrees C produced epsilon carbide in a lower bainitic ferrite to gether with stable, high carbon retained austenite, for all heat treat ment conditions: Chi carbide precipitated at the ferrite/austenite int erfaces after 5 h austempering. The fracture behaviour of samples aust empered at both 300 arzn 400 degrees C is dramatically influenced by t he Chi carbide formation, which results in a mostly brittle cleavage m ode of failure.