Compound of iron and carbon known as iron carbide and having the approximate chemical formula Fe3C. It is characterized by an orthorhombic crystal structure. Cementite contains 6.67 percent carbon. When it occurs as a phase in steel, the chemical composition is altered by the presence of manganese and other carbide-forming elements. Cementite is hard and brittle. It is magnetic below 1,670º F (910º C).