High-Resolution Metallography of a Coarse Microstructure: Graphite Formation in the Solid-State in Steel

The equilibrium form of carbon in iron and steel is graphite. In higher carbon cast irons graphite is normally formed from the liquid state. In lower carbon steels, carbon invariably exists in the form of metastable cementite, due to slow graphitization kinetics in the solid, but in fact, graphite formation is also more likely to be prevented by alloying to stabilize the microstructure for service. Relatively few high-resolution studies of graphite formation have thus been made in the past; this is also because of probable difficulties associated with specimen preparation of a relatively coarse aggregate microstructure. More recently, new investigative techniques and methods have allowed closer examination of graphite formation in steels. Examples are given of HRTEM, EELS, ELNES, EFTEM, and FIB/FEGSEM recently applied to observations of graphite nucleation and growth in the solid state in steel. This has importance, for example, in the potential development of more economic and user-friendly machining steels, which would rely upon internal lubrication by graphite particles in the microstructure.