Definition - What does Free Carbon mean?
In metallurgy, free carbon refers to elemental carbon present in a metal in an uncombined state. Free carbon affects the physical and chemical properties of metals.
In steel, pig iron and cast iron production, free carbon is considered an impurity. It must be removed from raw iron, and alloying elements such as manganese, nickel, chromium and vanadium are added to produce different grades of steel.
Free carbon also known as excess carbon.
Corrosionpedia explains Free Carbon
Free carbon is the part of the total carbon in steel or cast iron that is present in elemental form as graphite or temper carbon. In steels none of the carbon is present as free carbon; it is all dissolved in the iron.
As free carbon is a major impurity, the pig iron becomes brittle and hard. Pig iron may be made into steel (with up to about 2% carbon) or wrought iron (commercially pure iron) through oxidizing some or all of the carbon, together with other impurities. The hardness of the steel depends on its carbon content: the higher the percentage of carbon, the greater the hardness and the lesser the malleability. Regardless of the heat treatment, higher carbon content reduces weldability. In carbon steels, higher carbon content also lowers the melting point.
Liquid or solid iron dissolves carbon quite readily. Smelting results in an alloy that contains too much carbon produced steel. The free carbon and other impurities are removed in a subsequent step. Blowing oxygen through molten pig iron lowers the carbon content of the alloy and changes it into steel.
In the smelting process, a chemical reaction takes place so that undesirable elements like free carbon and sulfur are separated from iron. Importantly, smelting releases oxygen which reduces free carbon and other impurities. Liquid pig iron is subsequently treated in steelmaking furnaces to remove carbon and impurities through preferential oxidation.