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.