H. K. D. H. Bhadeshia

In its crystalline, liquid and glassy states, iron has an affinity for carbon, whether to form a solution over a wide range of compositions, as graphite or diamond, or in the form of compounds with narrowly defined compositions, such as cementite. It is possible, therefore, to find equilibria between iron and graphite, iron and diamond and iron and cementite, represented conventionally by the respective binary, two-phase diagrams. Such diagrams identify domains, for example in temperature and composition space, where either a single phase or a combination of phases is stable. However, the term {\em stable} is a tenuous concept, because there might be something else also consisting of Fe and C, which may be more stable. Instead of considering just two phases together, if we now put iron, graphite and cementite in mutual contact at ambient pressure then the cementite eventually must give way to the more stable equilibrium between graphite and iron. All equilibria in this sense are metastable; even the constituents of atoms will all decay eventually if the Universe keeps on expanding.

Some 50 million tonnes of cementite is produced annually within about 1.6 billion tonnes of steel, adding enormously to the quality of life. This is because it is hard at ambient temperature, as we shall see, due to its crystal structure that has a much lower symmetry than all the forms in which the iron occurs. Its metastability mostly does not matter over the time scale and conditions of normal life.

The name

The name has its origins in the theory of Osmond and Werth, in which the structure of solidified steel consists of a kind of cellular tissue, the iron constituting the nucleus and the carbide the envelope of the cells. The carbide was therefore envisioned to cement the iron.

In mineralogy, the carbide is known as cohenite (Fe,Ni,Co)3C, after the German mineralogist Emil Cohen, who was investigating material of meteoric origin. The impact of carbon-containing meteorites with the moon, is speculated to lead to a reduction of the iron-containing minerals on its surface; the resulting reaction with the carbonaceous gases generated by the impact to produce cementite (Jull:1975). Cementite is in fact of much wider interest than in metallurgy alone, within subjects spanning from astrophysics, planetary science, Lunar processes, and biomedicine to name but a few.

How was its chemical composition established given that the nature of carbon inside steel could not have been understood in the very early days of metallography? In 1878, Müller dissolved some steel in dilute sulphuric acid to leave behind a black residue which when analysed contained 6.01-7.38 wt% carbon. Müller referred to this as amorphous iron. Comprehensive experiments done independently by Abel around 1883 were published in 1885 in a report, on the state of carbon within steel. This confirmed "the correctness of the conclusions based on earlier experiments, that the carbon in cold-rollled steel exists in the form of a definite iron carbide, approximating the formula Fe3C or to a multiple of that formula". In the same experiments, hardened steel (presumably martensitic) "appeared to have the effect of preventing or arresting the separation of carbon, as a definite carbide".

Structure of cementite

Cementite has an orthorhombic unit cell and the common convention is to set the order of the lattice parameters as a=0.50837 nm, b=0.67475 nm and c=0.45165 nm. Note that the order in which the lattice parameters are presented here is consistent with the space group Pnma. There are twelve atoms of iron in the unit cell and four of carbon, as illustrated below. Four of the iron atoms are located on mirror planes whereas the other eight are at general positions (point symmetry 1).

The lattice type is primitive (P). There are n-glide planes normal to the x-axis, at (1/4)x and (3/4)x involving translations of (b/2)+(c/2). There are mirror planes normal to the yy-axis and a-glide planes normal to the z-axis, at heights (1/4)z and (3/4)z with fractional translations of a/2 parallel to the x-axis. The space group symbol is therefore Pnma.

crystal structure of cementite

The crystal structure of cementite, consisting of twelve iron atoms (large) and four carbon atoms (small, hatched pattern). The fractional z coordinates of the atoms are marked. Notice that four of the iron atoms are located on mirror planes, whereas the others are at general locations where the only point symmetry is a monad. The pleated layers parallel to (100) are in ...ABABAB... stacking with carbon atoms occupying interstitial positions at the folds within the pleats, with all carbon atoms located on the mirror planes. There are four Fe3C formula units within a given cell.

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Stoichiometry of cementite

The carbon atoms in cementite are located in interstitial sites; any deficit from the 3:1 Fe:C atom ratio is attributed to vacant interstices that normally are occupied by carbon atoms, as inferred from lattice parameter changes, originally pointed out by Petch (1944). The specific volume of cementite that is in equilibrium with ferrite at ambient temperature is found to be greater than that calculated using its measured lattice parameters, indicating those vacant carbon sites, i.e., a deviation from the stoichiometric composition (Kayser:1997). Similar conclusions have been reached by measuring phase fractions and lattice parameters in rapidly cooled Fe-C alloys containing large carbon concentrations (Battezzati:2005). Indeed, the detailed changes in three lattice parameters of cementite quenched from different temperatures, have been shown to be consistent qualitatively with corresponding parameters calculated using ab initio methods where carbon-specific sites are left unoccupied (Leineweber:2015).

The Figure below shows the thermodynamically assessed phase boundaries between cementite θ and ferrite α or austenite γ. Cementite has traditionally been depicted as a line compound in phase diagram calculations, but it has been shown that a thermodynamic model that permits its free energy to vary in a manner consistent with experimental data (Gohring:2016), is able to reproduce the equilibrium γ+θ/θ and α+θ/θ phase boundaries. The fact that ferrite can precipitate from cementite that was equilibrated at elevated temperatures, proves that there is an increase in the amount of carbon within cementite at low temperatures (Okamoto:1975).

Any deviations from stoichiometry must be small, because as demonstrated by Cottrell (1993), the bond energy between a carbon atom and iron is greater than that between two iron atoms. Therefore, any deficit of carbon would lead to a reduction in cohesion. Any extra carbon beyond the 3:1 Fe:C ratio would need to be accommodated in less-favoured interstices within the cementite lattice.

stoichiometry of cementite

(a) The composition of cementite that is in equilibrium with austenite or with ferrite in an Fe-C alloy. The data are due to Leineweber et al., determined by measuring the lattice parameters of cementite following quenching from the appropriate temperature. (b) Free energy curve of cementite as a function of chemical composition (referred to γ-Fe and graphite). After Gohring et al.

Circumstances can be engineered to make the cementite deviate from the stoichiometric carbon concentration; the decarburisation of pure cementite (Stuckens:1961), which leads to changes in the volume of the unit cell and in the Curie temperature of cementite, is an example. The deviation tends to be small, typically Fe3C1-x with x≅0.02 There are reports that very small particles of cementite in the structure of iron alloys studied by the atom probe technique exhibit deviations from stoichiometry, but these results should be treated with caution because at small size, the surface energy plays a role in determining the composition of the cementite in equilibrium with the surroundings.

The atom probe permits the composition of cementite to be measured directly using time-of-flight mass spectroscopy. There are, nevertheless, difficulties in measuring the carbon concentration of cementite (Kitaguchi:2014). It has not yet been possible to demonstrate small deviations from stoichiometry using such high-resolution methods. However, using conventional atom probe field ion microscopy, extremely small (4 nm) cementite particles in severely deformed mixtures of ferrite and cementite, have been shown to contain only 16\,at\%\ of carbon, a concentration that recovers to the 25 at% when the mixture is annealed to reduce the defect density and coarsen the cementite (Hong:1999). It is argued that the deformation introduces defects such as vacancies into the cementite, leading to the reduction in carbon concentration. However, it is important to note that the particles containing such a large deviation from stoichiometry were not proven to retain the orthorhombic crystal structure.

Thermal properties

The average thermal expansion coefficient of polycrystalline cementite changes from 6.8×10-6 K-1 to 16.2×10-6 K-1 as the sample is heated to beyond the Curie temperature, (Umemoto:2001).

thermal expansion of cementite

The linear thermal expansion coefficient of polycrystalline cementite as a function of temperature and magnetic state. Adapted using data from Umemoto et al. (2001).

Shown below are diffraction data (Reed:1997,Wood:2004,Litasov:2015) for each of the lattice parameters of cementite as a function of temperature. The parameter a is most sensitive to the change from the ferromagnetic to paramagnetic state, with a contraction evident as the temperature is raised within the ferromagnetic range. An increase in the amplitude of thermal vibrations in an anharmonic interatomic potential causes expansion, but the spontaneous magnetisation leads to a contraction, and this latter effect dominates the a parameter below TC, leading to the observed Invar type effect, although it is known that the analogy with the Invar effect in austenite is tenuous. The orthorhombic structure is preserved through the transition at TC. It is not clear why the a parameter is particularly affected by the magnetic transition.

thermal expansion of cementite

Neutron and X-ray diffraction data on the three lattice parameters a, b and c of cementite as a function of temperature. Data from (Wood:2004) (small circles with error bars), (Reed:1997) (filled circles) and (Litasov:2015) (crosses) . The dashed line in each case identifies the Curie temperature. The calculated pressure dependencies of the lattice parameters are as follows (Gorai:2018): Δa=0.0041×P, Δb=0.00578×P and Δc=0.00374×P Å, where the pressure P is in GPa.

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Magnetic properties

Cementite at ambient pressure and room temperature is a metallic ferromagnet that becomes paramagnetic beyond the Curie temperature TC. The very first measurement was by Wologdine in 1909, in which particles of cementite suspended between magnetic poles were seen to collapse as the temperature was increased, giving TC=180°C. Smith in 1911 indicated changes in magnetometer readings due to cementite contained in steel to be between 180-250°C, claiming that actual Curie temperature to be around 240°C. Honda in 1915 put this value at 210°C. The 0 K is calculated to be about 1.86 μB.

There is a calculated transition from ferromagnetic to nonmagnetic at 25 GPa pressure and 300 K. The term nonmagnetic is used here because it is not clear whether the magnetic collapse corresponds to a loss of spin correlation or to a transition from a high-spin to a low-spin state. There is a volume contraction of 2-3% following the transition to the paramagnetic state. The structure, with its orthorhombic symmetry, is magnetically anisotropic, with [001], [010] being the easiest and second easiest, and [100] the hardest magnetisation directions.

Preparation of cementite

Samples of bulk, pure cementite are difficult to prepare given that cementite in contact with iron is less stable than the corresponding equilibrium between graphite and ferrite. The largest samples have been manufactured by mechanical alloying in experiments by Umemoto et al. (Umemoto:2001). Powders of iron and graphite in the correct stoichiometric ratio, are milled together, resulting in a solid solution, as indicated by very broad (≅15°) X-ray diffraction peaks in locations typical of body-centred cubic iron. The mechanically alloyed powder was then spark plasma sintered under vacuum at 50 MPa pressure for 300 s at 1173 K, inducing the formation of cementite (Figure below) (Umemoto:2001). The density achieved was 7.5 g cm-3, which is less than the measured value for pure cementite of 7.662 g cm-3 (Ishigaki:1927), indicating a degree of porosity in the sintered samples.

preparation of cementite

(a) A sample of cementite, courtesy of Professor Minoru Umemoto of Toyohashi University. (b) Reaction of 80 wt% Fe and 20 wt% graphite for ten minutes at the temperatures and pressures indicated. Selected data from Tsuzuki (1984).

The sintering step has been unnecessary in other work where cementite was obtained directly during the milling process (Chaira:2009,Joubori:2014,Joubori:2018). This might be explained by the fact that Umemoto et al. (2001) milled their powders for a much longer time. A comparison of the {110}α X-ray diffraction peaks obtained in the two studies is shown below. The broadening is caused by strain due primarily to dislocations locked within the powder, indicating a much larger defect density in the samples of the Umemoto study. Carbon prefers to be located at dislocations rather than in cementite (Kalish:1970); this explains the necessity for the sintering stage in the Umemoto study.

mechanical alloying of cementite

A comparison of the {110}α X-ray peaks from the experiments of Umemoto et al. (2001) and Joubouri et al. (2018) - the latter has been corrected to the Co Kα wavelength to permit the comparison.

It has been proposed, based on evidence from Mössbauer spectroscopy, that there are intermediate stages between the formation of the solid solution during milling, and that of cementite. The process may first involve transition carbides such as Hägg (Fe2C) and ε-carbide, followed by cementite (Matteazzi:1991). Cementite can be made directly from Hägg carbide through the reaction Fe+Fe2C\rightarrow Fe3C (Hofer:1950). Alternatively, powdered cementite can be made by heating Hägg carbide, which is richer in carbon, in a nitrogen stream at 800°C for some 20 min (Herbstein:1964). The resulting sample may contain traces of free iron and amorphous carbon. Cementite also forms when a mixture of iron and graphite heated under a pressure of less than 5 GPa at about 1000°C, (Figure) (Tsuzuki:1984). Cementite powders have been made traditionally by electrochemical extraction from steel containing cementite (Rokhmanov:1997).

A clever method (Yamamoto:2018) for fabricating a "single crystal" of cementite is to incorporate electrolytically extracted cementite particles into a resin which then is subjected to a 10 Tesla magnetic field for some 24 h with the composite periodically rotated in the field to magnetically align the particles as the resin solidifies. This enabled the magnetocrystalline anisotropy of the cementite to be determined experimentally.

Iron can be converted into cementite by exposing it to a carburising gas mixture, if the activity of carbon relative to graphite is maintained at greater than one. Graphite is deposited preferentially unless the surface of the iron is contaminated with blocking atoms such as sulphur, in which case cementite is precipitated (Grabke:2001). It has been demonstrated that cementite can be made by carburising magnetite (Fe3O4) at 1073 K with carbon monoxide (Kim:2013). It is speculated that cementite produced in this manner could be used in an electrical arc furnace to produce iron while at the same time reducing carbon dioxide emissions.

Nanoparticles of cementite can be prepared by the thermal decomposition of Fe(CO)5 (iron pentacarbonyl). These fine particles may be of use in biomedicine for delivery of drugs to specific locations within the body, with the localisation achieved by an external magnetic field (Ramanujan:2009). Elemental-iron particles have been proposed for this purpose but they tend to oxidise (Shultz:2009). Cementite is more corrosion and oxidation resistant,\footnote{The mechanism of oxidation, i.e. the formation first of Fe3O4 followed by Fe2O3 remains identical to that of metallic iron (Galwey:1974). while retaining sufficient ferromagnetism to implement the delivery mechanism. Dispersions of polymer coated cementite nanoparticles have been manufactured by subjecting a gaseous mixture of C2H4/Fe(CO)5/C5H8O2 to a continuous wave CO2 laser pyrolysis (Morjan:2009).

Cementite powder containing pores about 20 nm in size from an aqueous mixture of iron chloride, colloidal silica and 4,5-dicyanoimidalzole. The dicyanoimidalzole is the source of carbon when the mixture is heated to 700°C to produce the powder of cementite which also contains amorphous silica. The silica is then removed by solution in sodium hydroxide, leaving the porous cementite with a high specific surface area. This cementite was demonstrated to be catalytically active in the decomposition of ammonia into a mixture of hydrogen and nitrogen. Cementite apparently has greater stability under harsh conditions than metallic iron, and is safer with respect to the danger of explosions associated with fine metallic powders (Kraupner:2010). Cementite has in fact been shown to exhibit catalytic activity even in the classical Firscher-Torpsch process for converting gaseous components into hydrocarbon liquids (Shultz:1956).

Effect of Silicon on Cementite Precipitation

Other Aspects of Cementite

Other Sources on Carbides

Research publications

Review: theory of carbide precipitation


Carbide chemical compositions


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