cementite: Definition from Answers.com

Iron alloy phases v • d • e
Ferrite (α-iron, δ-iron) Austenite (γ-iron) Pearlite (88% ferrite, 12% cementite) Bainite Martensite Ledeburite (ferrite-cementite eutectic, 4.3% carbon) Cementite (iron carbide, Fe₃C)
Steel classes
Carbon steel (≤2.1% carbon; low alloy) Stainless steel (+chromium) Maraging steel (+nickel) Alloy steel (hard) Tool steel (harder)
Other iron-based materials
Cast iron (>2.1% carbon) Ductile iron Wrought iron (contains slag)

Orthorhombic Fe₃C. Iron atoms are blue.

Cementite or iron carbide is a chemical compound with the formula Fe₃C (or Fe₂C:Fe), and 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 metallurgy.

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. An intimate mixture with ferrite, the other product of austenite, forms a lamellar structure called pearlite. Much larger lamellae, visible to the naked eye, make up the structure of Damascus steel.

Cementite changes from ferromagnetic to paramagnetic at its Curie temperature of approximately 480 K.^[1]

A natural iron carbide (containing minor amounts of nickel and cobalt) occurs in iron meteorites and is called cohenite after the German mineralogist Emil Cohen, who first described it.^[2]

References

^ S.W.J. Smith; W. White; S.G. Barker (1911). "The Magnetic Transition Temperature of Cementite". Proc. Phys. Soc. London (Institute of Physics) 24 (1): 62-69. doi:10.1088/1478-7814/24/1/310.
^ Vagn F. Buchwald, Handbook of Iron Meteorites, University of California Press 1975.

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