Iron containing more than 2.1% C (the solubility limit for carbon in austenite) together with other impurity elements such as silicon and phosphorus, which has been cast from the liquid. Without further treatment it is very brittle and not malleable either hot or cold. Cast iron exists in two main forms, white and grey; terms which describe the appearance of the surface exposed when the metal is fractured.
In white iron the carbon is present as cementite (Fe3C) and as pearlite. Its production is favoured by fast cooling rates and low silicon content. Both irons are brittle – grey iron because of the lack of strength and disposition of the graphite flakes, and white because of the extreme hardness and brittleness of the cementite. Malleable cast irons can be obtained by heat-treatment of white cast irons by converting the combined carbon into free carbon or temper carbon. In the whiteheart process for example, a certain amount of carbon is removed from the surface by oxidation
Cast Iron, Grey
A cast iron in which the carbon is predominately in the form of graphite flakes. The formation of grey cast irons is promoted by high silicon content (highly reducing conditions). It is often stated that early cast irons tend not to be white as the early furnaces would not be sufficiently reducing to reduce enough silicon. However, very early cast iron found on many Roman and later iron smelting site produce some grey cast iron amongst the metallurgical debris. This is because slow cooling also promoted graphite growth over cementite.
Grey cast iron has excellent casting properties and can be machined, but is brittle. It is less easily converted to wrought iron as the carbon in form of graphite flakes and it is more difficult to get the carbon back into the iron so that is is able to diffuse through the metal.
Cast Iron, Mottled
A cast iron in which the conditions were intermediate between those for graphite and cementite formation, so that the metal contains patterned regions of white and grey cast iron.
Cast Iron, White
A cast iron in which the carbon in the iron is in the form of cementite. As a result this alloy is extremely hard and brittle. This form of cast iron was favoured for conversion to wrought iron the carbon diffuses more easily if it is already combined with iron in the form of cementite. The formation of white cast iron is favoured by low silicon content, phosphorus, and fast cooling rates.