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Iron carbide
	; Iron carbide plates
Names
	IUPAC name Iron carbide
	Other names Cementite
Identifiers
CAS Number	12011-67-5 ;
3D model (JSmol)	Interactive image;
ECHA InfoCard	100.031.411
EC Number	234-566-7;
CompTox Dashboard (EPA)	DTXSID10894255 ;
	InChI InChI=1S/C.3Fe Key: TXAHJXBWFZQNQY-UHFFFAOYSA-N;
	SMILES [Fe]=[C]=[Fe].[Fe];
Properties
Chemical formula	Fe3C
Molar mass	179.546 g/mol
Appearance	dark gray or black crystals, odorless
Density	7.694 g/cm3, solid
Melting point	1,227 °C (2,241 °F; 1,500 K)
Solubility in water	insoluble
Structure
Crystal structure	Orthorhombic, oP16
Space group	Pnma, No. 62
Lattice constant	a = 0.509 nm, b = 0.6478 nm, c = 0.4523 nm
Formula units (Z)	4
Thermochemistry
Heat capacity (C)	105.9 J·mol−1·K−1
Std molar; entropy (S⦵298)	104.6 J·mol−1·K−1
Std enthalpy of; formation (ΔfH⦵298)	25.1 kJ·mol−1
Gibbs free energy (ΔfG⦵)	20.1 kJ·mol−1
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Cementite (or iron carbide) is a 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.^[4] It is a hard, brittle material,^[4] normally classified as a ceramic in its pure form, and is a frequently found and important constituent in ferrous metallurgy. While cementite is present in most steels^[5] and cast irons, it is produced as a raw material in the iron carbide process, which belongs to the family of alternative ironmaking technologies. The name cementite originated from the theory of Floris Osmond and J. Werth, in which the structure of solidified steel consists of a kind of cellular tissue, with ferrite as the nucleus and Fe₃C the envelope of the cells. The carbide therefore cemented the iron.

Metallurgy

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.^[6] Therefore, in carbon steels and cast irons that are slowly cooled, a portion of the carbon is in the form of cementite.^[7] Cementite forms directly from the melt in the case of white cast iron. In carbon steel, cementite precipitates from austenite as austenite transforms to ferrite on slow 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 austenite (low carbon level) and graphite (high carbon level) at higher temperatures, it does not decompose on heating at temperatures below the eutectoid temperature (723 °C) on the metastable iron-carbon phase diagram.

Mechanical properties are as follows: room temperature microhardness 760–1350 HV; bending strength 4.6–8 GPa, Young's modulus 160–180 GPa, indentation fracture toughness 1.5–2.7 MPa√m.^[8]

The morphology of cementite plays a critical role in the kinetics of phase transformations in steel. The coiling temperature and cooling rate significantly affect cementite formation. At lower coiling temperatures, cementite forms fine pearlitic colonies, whereas at higher temperatures, it precipitates as coarse particles at grain boundaries. This morphological difference influences the rate of austenite formation and decomposition, with fine cementite promoting faster transformations due to its increased surface area and the proximity of the carbide-ferrite interface. Furthermore, the dissolution kinetics of cementite during annealing are slower for coarse carbides, impacting the microstructural evolution during heat treatments.^[9]

Pure form

Cementite changes from ferromagnetic to paramagnetic upon heating to its Curie temperature of approximately 480 K (207 °C).^[10]

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.^[11]

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.^[12]^[13] It has also been found naturally as the mineral Edscottite in the Wedderburn meteorite.^[14]

References

^ ^a ^b Haynes, p. 4.67
^ Herbstein, F. H.; Smuts, J. (1964). "Comparison of X-ray and neutron-diffraction refinements of the structure of cementite Fe₃C". Acta Crystallographica. 17 (10): 1331–1332. Bibcode:1964AcCry..17.1331H. doi:10.1107/S0365110X64003346.
^ Haynes, p. 5.23
^ ^a ^b Smith & Hashemi 2006, p. 363
^ Verhoeven, John D. (2007). Steel Metallurgy for the Non-Metallurgist. ASM International. p. 35. ISBN 978-1-61503-056-9.
^ Ashrafzadeh, Milad; Soleymani, Amir Peyman; Panjepour, Masoud; Shamanian, Morteza (2015). "Cementite Formation from Hematite–Graphite Mixture by Simultaneous Thermal–Mechanical Activation". Metallurgical and Materials Transactions B. 46 (2): 813–823. Bibcode:2015MMTB...46..813A. doi:10.1007/s11663-014-0228-3. S2CID 98253213.
^ Smith & Hashemi 2006, pp. 366–372
^ Bhadeshia, H. K. D. H. (2020). "Cementite". International Materials Reviews. 65 (1): 1–27. Bibcode:2020IMRv...65....1B. doi:10.1080/09506608.2018.1560984.
^ Alvarenga HD, Van Steenberge N, Sietsma J, Terryn H (Feb 2017). "The Kinetics of Formation and Decomposition of Austenite in Relation to Carbide Morphology". Metall Mater Trans A. 48: 828–840. doi:10.1007/s11661-016-3874-z.
^ Smith, S.W.J.; White, W.; Barker, S.G. (1911). "The Magnetic Transition Temperature of Cementite". Proc. Phys. Soc. Lond. 24 (1): 62–69. Bibcode:1911PPSL...24...62S. doi:10.1088/1478-7814/24/1/310.
^ Buchwald, Vagn F. (1975) Handbook of Iron Meteorites, University of California Press
^ Hägg, Gunnar (1934). "Pulverphotogramme eines neuen Eisencarbides". Zeitschrift für Kristallographie - Crystalline Materials. 89 (1–6): 92–94. doi:10.1524/zkri.1934.89.1.92. S2CID 100657250.
^ Smith, William F. (1981). Structure and properties of engineering alloys. New York: McGraw-Hill. pp. 61–62. ISBN 978-0-07-0585607.
^ Mannix, Liam (2019-08-31). "This meteorite came from the core of another planet. Inside it, a new mineral". The Age. Retrieved 2019-09-14.

Bibliography

Haynes, William M., ed. (2016). CRC Handbook of Chemistry and Physics (97th ed.). CRC Press. ISBN 9781498754293.
Smith, William F.; Hashemi, Javad (2006). Foundations of Materials Science and Engineering (4th ed.). McGraw-Hill. ISBN 978-0-07-295358-9.

External links

Crystal structure of cementite at NRL
Hallstedt, Bengt; Djurovic, Dejan; von Appen, Jörg; Dronskowski, Richard; Dick, Alexey; Körmann, Fritz; Hickel, Tilmann; Neugebauer, Jörg (March 2010). "Thermodynamic properties of cementite (Fe₃C)". Calphad. 34 (1): 129–133. doi:10.1016/j.calphad.2010.01.004.
Le Caer, G.; Dubois, J. M.; Pijolat, M.; Perrichon, V.; Bussiere, P. (November 1982). "Characterization by Moessbauer spectroscopy of iron carbides formed by Fischer–Tropsch synthesis". The Journal of Physical Chemistry. 86 (24): 4799–4808. doi:10.1021/j100221a030.
Bauer-Grosse, E.; Frantz, C.; Le Caer, G.; Heiman, N. (June 1981). "Formation of Fe₇C₃ and Fe₅C₂ type metastable carbides during the crystallization of an amorphous Fe₇₅C₂₅ alloy". Journal of Non-Crystalline Solids. 44 (2–3): 277–286. Bibcode:1981JNCS...44..277B. doi:10.1016/0022-3093(81)90030-2.