Analytical electron microscope study of the dissolution of the Fe3C iron carbide phase (cementite) during a graphitisation anneal of carbon steel

The evolution of microstructure and composition of the Fe3C iron carbide phase (cementite) during a graphitisation anneal of a quenched, medium carbon steel has been studied by analytical electron microscopy, including energy dispersive X-ray spectroscopy (EDX), electron energy loss spectroscopy (EELS) and energy filtered transmission electron microscopy (EFTEM) imaging. During heat treatment at 680 C, dissolution of the cementite particles dispersed in the martensitic matrix was completed within a time period of 1.5 h, during which time graphite nodules began to form. However, a non-graphitic carbon- rich amorphous phase was also detected during this heat treatment. It is postulated that these amorphous particles could be an intermediate stage during the overall graphitisation process.