Cementite or iron carbide's a chemical compound with the formula Fe3C (or Fe2C:Fe), and an orthorhombic crystal structure. It's a hard, brittle material, normally classified as a ceramic in its pure form, though it's more important in metallurgy. It forms directly from the melt in the case of white cast iron. In carbon steel, it either forms from austenite during cooling or from martensite during tempering. It mixes with ferrite, the other product of austenite, to form lamellar structures calle… (More on Cementite)