Cementite, also known as iron carbide, is an interstitial compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard, brittle material, normally classified as a ceramic in its pure form, though it is more important in ferrous metallurgy. While iron carbide is present in most steels and cast irons, it is produced as a raw material in the Iron Carbide process, which belongs to the family of alternative ironmaking technologies. (source)