Cementite is iron carbide with the formula Fe3C, and an orthorhombic crystal structure. It is a hard, brittle material, essentially a ceramic in its pure form. 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. Cementite contains 6.67% Carbon by weight; thus above that carbon content in the Fe-C phase system, the alloy is no longer steel or cast iron, as all of the available iron is contained in cementite. Cementite mixes with ferrite, the other product of austenite, to form lamellar structures called pearlite and bainite. Much larger lamellae, visible to the naked eye, make up the structure of Damascus steel. Fe3C is also known as cohenite, particularly when found mixed with nickel and cobalt carbides in meteorites.

Cementite Crystal StructurePhotomicrograph of Pearlite Structure
(Dark bands are cementite)
Cementite crystal structure Pearite photomicrograph

at the Sign of the Three Planes