The steel phase diagram above is not actually the equilibrium phase diagram for the iron-carbon system, but due to kinetics Fe3C usually forms. For ferrous alloys with higher carbon contents graphite often (although not always) forms. These high C content alloys are referred to as cast irons, the three common types of which are ‘grey’, ‘spheroidal’ and ‘white’.
Cast irons usually contain some Si or other alloying additions, which often stabilise the graphite phase, such that it precipitates out even when the wt% present is less than 4.3% (the eutectic composition for the Fe-C phase diagram).
Grey Cast Irons:
Usually contain more C or Si than white cast irons, and require a lower cooling rate. They are called ‘grey’ cast irons not because of their colour, but due to the appearance of a fractured surface. Grey cast irons are quite ductile and have unreflective fracture surfaces.
Steps on cooling:
- When the alloy falls below the liquidus, graphite begins to precipitate out. For a simple Fe-C system this means the composition must be hypereutectic, but the addition of Si moves the eutectic composition by stabilising the graphite phase. The graphite precipitates are flake-like with growth occurring in preferred crystallographic directions
- At the eutectic temperature a cementite and γ (austenite) eutectic forms from the remaining liquid phase this is known as ledeburite.
- As the temperature continues to decrease carbon diffuses out of solid solution to the graphite precipitates.
- When the eutectoid temperature is reached, the remaining austenite transforms to pearlite (lamellar cementite (Fe3C) and ferrite (iron with some carbon in solid solution). Some alloying additions may modify this final transformation, for example if enough Ni is present the austenite will not transform to pearlite.
Spheroidal Cast Irons:
These are similar to grey cast irons, but they contain ‘inoculants’ – alloying additions that change the form of the graphite precipitates. These inoculants are usually Mg or Ce (~0.1wt%) and they cause the graphite to grow in spheres rather than flakes.
There are two theories offering an explanation for this. The first describes the Mg or Ce impurities “poisoning” the graphite growth sites, attaching to them and slowing growth in that direction. The second suggests an increase in interfacial energy – the surface energy between the melt and the graphite, such that surface area per volume is minimised.
The cooling steps follow the same route as the grey cast irons with the graphite precipitates growing in spherical shapes.
White Cast Irons:
These contain less Si or C than grey cast irons and undergo faster cooling. This results in cementite forming in favour of graphite. Again the name ‘white’ has little to do with the ordinary appearance of the alloy, but rather refers to the fracture surface. White cast irons are much more brittle than grey cast irons, and so their fracture surfaces are reflective, leading to their classification as ‘white’.
The cooling route depends on the composition of the melt, whether it is hyper – or hypo – eutectic (eutectic composition is at 4.3wt.%C). A hypereutectic composition leads to the cementite precipitating out first; a hypoeutectic composition leads to γ – austenite precipitating out first.
Note – “hypereutectic” has a higher carbon content than the eutectic composition.
“hypoeutectic” has a lower carbon content than the eutectic composition.
The first phase to precipitate out forms dendrites due to non-equilibrium effects; the cooling melt does not always follow the predicted composition on the phase diagram (see the page on the lever rule in the Phase Diagrams and Solidification TLP and the dendritic growth page in the Solidification of Alloys TLP).
When the eutectic is crossed the remaining melt solidifies as an austenite, cementite eutectic (ledeburite). The carbon continues to be ejected from the austenite as the alloy cools, diffusing to the cementite. At the eutectoid temperature the final transformation takes place from austenite to pearlite. In some very quickly cooled white cast irons the austenite may transform to martensite.