 Iron carbide plates | |
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| Names | |
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| IUPAC name Iron carbide | |
| Other names Cementite | |
| Identifiers | |
3D model (JSmol) | |
| ECHA InfoCard | 100.031.411 |
| EC Number |
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| Properties | |
| Fe3C | |
| Molar mass | 179.546 g/mol |
| Appearance | dark gray or black crystals, odorless |
| Density | 7.694 g/cm3, solid[1] |
| Melting point | 1,227 °C (2,241 °F; 1,500 K)[1] |
| insoluble | |
| Structure[2] | |
| Orthorhombic,oP16 | |
| Pnma, No. 62 | |
a = 0.509 nm,b = 0.6478 nm,c = 0.4523 nm | |
Formula units (Z) | 4 |
| Thermochemistry[3] | |
| 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 theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Cementite (oriron carbide) is acompound ofiron andcarbon, more precisely an intermediate transition metalcarbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has anorthorhombic crystal structure.[4] It is a hard, brittle material,[4] normally classified as aceramic in its pure form, and is a frequently found and important constituent inferrous 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 namecementite originated from the theory ofFloris Osmond and J. Werth, in which the structure of solidified steel consists of a kind of cellular tissue, withferrite as the nucleus and Fe3C the envelope of the cells. The carbide thereforecemented the iron.
In the iron–carbon system (i.e.plain-carbon steels andcast irons) it is a common constituent becauseferrite 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 ofwhite cast iron. In carbonsteel, cementite precipitates fromaustenite as austenite transforms to ferrite on slow cooling, or frommartensite duringtempering. An intimate mixture with ferrite, the other product of austenite, forms alamellar structure calledpearlite.
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 theeutectoid 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]
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Cementite changes fromferromagnetic toparamagnetic upon heating to itsCurie temperature of approximately 480 K (207 °C).[10]
A natural iron carbide (containing minor amounts of nickel and cobalt) occurs iniron meteorites and is calledcohenite after the German mineralogistEmil Cohen, who first described it.[11]
There are other forms ofmetastable iron carbides that have been identified in tempered steel and in the industrialFischer–Tropsch process. These includeepsilon (ε) carbide,hexagonal close-packed Fe2–3C, 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 hardenedtool steels tempered at 200–300 °C.[12][13] It has also been found naturally as the mineralEdscottite in theWedderburn meteorite.[14]
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