An EFTEM study of the dissolution of cementite during the graphitisation annealing of a quenched medium carbon steel

Abstract

Graphitisation annealing at 680°C of a quenched medium carbon (0.38wt%C) steel with high contents of both Al (1.44wt%) and Si (1.82wt%) has resulted in the formation of graphite nodules ∼2-5νm in diameter in a very short time (∼2 hours). This could potentially improve machinability by an alternative and more commercially viable alloying route to, for example, the addition of lead. The overall graphitisation process involves the dissolution of the Fe3C iron carbide phase (cementite) as well as formation of the graphite nodules. The evolution of microstructure and composition of cementite during the annealing treatment has been studied by analytical electron microscopy, including energy loss spectroscopy (EELS) and energy-filtered transmission electron microscopy (EFTEM) imaging. The formation of a non-graphitic, carbon-rich amorphous phase was identified during the heat treatment. This amorphous phase could be a previously undetected intermediate stage during the overall graphitisation process, the nodular graphite evolving from it. © 2006 IOP Publishing Ltd.