Cast irons typically contain 2-4 wt% of carbon with a high silicon concentrations and a greater concentration of impurities than steels. The carbon equivalent (CE) of a cast iron helps to distinguish the grey irons which cool into a microstructure containing graphite and and the white irons where the carbon is present mainly as cementite. The carbon equivalent is defined as:

$$CE \hbox{(wt\%)} =C + {{Si +P}\over{3}} \numeqn$$

A high cooling rate and a low carbon equivalent favours the formation of white cast iron whereas a low cooling rate or a high carbon equivalent promotes grey cast iron.

During solidification, the major proportion of the carbon precipitates in the form of graphite or cementite. When solidification is just complete, the precipitated phase is embedded in a matrix of austenite which has an equilibrium carbon concentration of about 2 wt%. On further cooling, the carbon concentration of the austenite decreases as more cementite or graphite precipitates from solid solution. For conventional cast irons, the austenite then decomposes into pearlite at the eutectoid temperature. However, in grey cast irons, if the cooling rate through the eutectoid temperature is sufficiently slow, then a completely ferritic matrix is obtained with the excess carbon being deposited on the already existing graphite.

White cast irons are hard and brittle; they cannot easily be machined. When they are hypoeutectic ()

$$\eqalign{ \hbox{Liquid}\rightarrow & \underbrace{\gamma-\hbox... ...ow \cr & ~~~~~~~~~~~~\hbox{pearlite} ~~~~~~~~~ + {\rm Fe_3C} }$$

castiron132101500: Schematic representation of the iron-carbon phase diagram showing the eutectic and eutectoid reactions.

Grey cast irons are softer with a microstructure of graphite in transformed-austenite and cementite matrix. The graphite flakes, which are rosettes in three dimensions, have a low density and hence compensate for the freezing contraction, thus giving good castings free from porosity.

The flakes of graphite have good damping characteristics, good machinability but are stress concentrators, leading to poor toughness.

The addition of minute quantities of magnesium or cerium poisons preferred growth directions and leads to isotropic growth resulting in spheroids of graphite. This spheroidal graphite cast iron has excellent toughness and is used widely, for example in crankshafts.

Austempered Ductile Cast Irons

The latest breakthrough in cast irons is where the matrix of spheroidal graphite cast iron is not pearlite, but bainite (). This results in a major improvement in toughness and strength. The bainite is obtained by isothermal transformation of the austenite at temperatures below that at which pearlite forms.