One of the first things that can cause confusion when discussing cast iron, or attempting to source it, is the varied number of ways it can be described. Because of its age and wide geographical spread many terms, often local in nature, have grown up. Cast iron, grey cast iron, gray cast iron, flake graphite iron and flake graphite cast iron all refer to the same material. The term grey is used because of the colour of the fracture surface if a piece is broken.
An example of a Flake Graphite Cast Iron Casting from Durham Foundry
Hematite can also be used and refers to cast iron with a specific application in castings subject to heating and cooling cycles or operating at elevated temperatures, such as forge furnace doors and arch plates. It is rather an old fashioned term, referring to cast iron with a high carbon content (4%), a low silicon content (1.6%) and low impurities.
If molten cast iron is allowed to cool normally the carbon comes out of solution and forms flakes of graphite which run through the ferrite/pearlite matrix, hence the alternative term, flake graphite iron. These flakes are at the microscopic level, the ends of which form stress points in the cast iron. Cast irons typically contain 2-4 wt% of carbon with silicon at 1.8-2.4 wt% 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 the white irons where the carbon is present mainly as cementite. The carbon equivalent is defined as:
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