Cementite

The hard, brittle component of iron-carbon alloys, containing about 6.6% carbon, corresponding to the phase Fe3C and crystallising in the orthorhombic system. It is soluble in molten iron but the solubility decreases in austenite to about 0.9% at the eutectoid temperature. Pearlite, the eutectoid of ferrite and cementite, is the most common component containing cementite in ancient and historic steels.