Cementite

Cementite, also known as iron carbide, is an interstitial compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe₃C. By weight, it is 6.67% carbon and 93.3% iron. It has 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 ferrous metallurgy. While iron carbide is present in most steels and cast irons, it is produced as a raw material in the Iron Carbide process, which belongs to the family of alternative ironmaking technologies

Metallurgy

Orthorhombic Fe₃C. Iron atoms are blue.

The iron-carbon phase diagram

In the iron–carbon system (i.e. plain-carbon steels and cast irons) it is a common constituent because ferrite can contain at most 0.02wt% of uncombined carbon. Therefore, in carbon steels and cast irons that are slowly cooled a portion of the element is in the form of cementite. 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.

While cementite is thermodynamically unstable, eventually being converted to ferrite and graphite at higher temperatures, it does not decompose on heating at temperatures below the eutectoid temperature of the metastable Iron-Carbon phase diagram, which is of the order of 1000 K.

Pure form

Cementite changes from ferromagnetic to paramagnetic at its Curie temperature of approximately 480 K.

Molar volume vs. pressure for cementite at room temperature.

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. As carbon is one of the possible minor light alloy components of metallic planetary cores, the high-pressure/high-temperature properties of cementite (Fe₃C) as a simple proxy for cohenite are studied experimentally. The figure shows the compressional behaviour at room temperature.

Other iron carbides

There are other forms of metastable iron carbides that have been identified in tempered steel and in the industrial [Fischer-Tropsch_process]. These include Epsilon (ε) carbide, hexagonal close-packed Fe_2-3C, precipitates in plain-carbon steels of carbon content > 0.2%, tempered at 100-200 °C. Non-stoichiometric ε-carbide dissolves above ~200 °C, where Hägg carbides and cementite begin to form. Hägg carbide, monoclinic Fe₅C₂, precipitates in hardened tool steels tempered at 200-300 °C. Characterization of different iron carbides is not at all a trivial task, and often X-ray diffraction is complemented by Mössbauer spectroscopy.

External links

• Crystal structure of cementite at NRL
• Crystal Structure of Cementite, Cambridge University
• Alternative Ironmaking Technologies
• L.J.E. Cohen Nature of Carbides of Iron, Bullettin 631 US Bureau of Mines
• Pycalphad: Thermodynamic calculation of cementite http://pycalphad.readthedocs.org/en/latest/examples/CementiteAnalysis.html based on
• Bengt Hallstedt, Dejan Djurovic, Jörg von Appen, Richard Dronskowski, Alexey Dick, Fritz Körmann, Tilmann Hickel, Jörg Neugebauer, Thermodynamic properties of cementite, Calphad, Volume 34, Issue 1, March 2010, Pages 129-133, ISSN 0364-5916,http://dx.doi.org/10.1016/j.calphad.2010.01.004.
• Ester Esna du Plessis The Crystal Structures of Iron Carbides, Ph.D. Thesis, University of Johannesburg, 2006 https://books.google.it/books/about/The_Crystal_Structures_of_the_Iron_Carbi.html?id=AN6QtgAACAAJ&redir_esc=y
• Characterization by Moessabuer Spectroscopy of Iron Carbides obtained by Fisher Tropsch Synthesis http://pubs.acs.org/doi/abs/10.1021/j100221a030
• Formation of Fe₇C₃ and Fe₅C₂ type metastable carbides during the crystallization of an amorphous Fe₇₅C₂₅ alloy http://www.sciencedirect.com/science/article/pii/0022309381900302