cementite (Fe3C)
/ si-MEN-tite /
Cementite is the hard, brittle, ceramic-like compound of iron and carbon, Fe3C. It is the grit in steel: very hard, so it strengthens, but brittle, so too much of it makes iron snap. It is what all that carbon in steel ends up as, once the iron can no longer keep the carbon dissolved.
More precisely, cementite is iron carbide, Fe3C, a fixed-composition intermediate compound with 6.7 wt% carbon (25 atomic percent) and a complex orthorhombic crystal structure. It is very hard (above 800 Vickers) and essentially non-ductile. It is technically metastable, meaning that given enough time, and with some silicon present, it slowly decomposes toward graphite plus iron, but in ordinary steels it is stable for all practical purposes.
Cementite is one of the two phases in pearlite, its hard lamellae alternating with soft ferrite, and it appears whenever carbon exceeds ferrite's tiny solubility. Its hardness is the source of a high-carbon steel's strength and wear resistance, while its brittleness is why very high-carbon steels and white cast iron are hard but fragile. As a continuous network around grain boundaries (proeutectoid cementite in a hypereutectoid steel) it can badly embrittle the steel.
Pearlite is ferrite plus cementite lamellae; white cast iron is cementite-rich, so it is hard and brittle.
Cementite hardens steel, but a continuous network of it embrittles it.
Cementite is metastable (the true equilibrium phase is graphite), and though it hardens steel, a continuous cementite network embrittles it; it is a compound, not a solid solution.