Graphite formation in iron-carbon alloys.

To achieve formation of the graphite eutectic (austenite graphite) in the stable Fe-C system (gray solidification), the chemical composition of the cast iron must be constituted so that sufficient tendency for graphite formation is possible.

To this end, the graphitization factor K must be adjusted accordingly in dependence of the cooling rate. Graphite diffusion from the melt may be lamellar, rosette-shaped, interdendritically, finely distributed, vermicular, or nodular (s. Graphite form, Graphite classification chart).

Upon further cooling, the austenite is subject to eutectoid decomposition, pearlite is formed; i.e. the structure comprises a pearlitic matrix with embedded graphite. Pearlite-stabilizing additives in cast iron (copper, tin) enable the achievement of a purely pearlitic structure.

On the other hand it is possible to force eutectoid decomposition of austenite into ferrite and graphite according to the conditions of the stable Fe-C system by adjusting the iron composition such that the graphitization factor is very high.

Accordingly, with a low graphitization factor ledeburite formation (chill) occurs.

Additional references:
Eutectic graphite quantity
Eutectic cast iron
Eutectoid temperature