Triferro carbide, Fe₃C

Triferro carbide, Fe₃C, known to the metallurgist as cementite, occurs in meteorites associated with the carbides of nickel and cobalt, as the mineral Cohenite, (Fe, Co, Ni)₃C. It occurs normally in steel, and was first isolated by acting on steel with a solution of potassium bichromate in sulphuric acid. The metal dissolves, leaving the cementite as a black powdery residue.

A variation of this method consists in using bars of steel as anodes in baths of dilute acid, platinum plates suspended in porous cells serving as cathodes.

A better method consists in first preparing a steel very rich in carbide and then effecting the isolation of the latter by dissolution of the free metal in acid. To this end 1000 grams of iron are heated with arc-lamp or sugar carbon (100 grams), to white heat, and poured on to a large iron plate to solidify. After removing any scale, etc., the mass is powdered, digested for several weeks with normal acetic acid, and then with 1/5-normal hydrochloric acid. Any remaining carbon is removed by levigation, and the residual cementite washed with alcohol and ether, and finally dried in a vacuum.

As obtained in this manner, cementite is grey in colour and very brittle; it can be powdered in the hand. Its hardness is 3.2 to 3.3; density at 21° C. 7.396; molecular volume 24.34. It crystallises in pseudo-hexagonal form. The specific heat of cementite, as calculated from data obtained with carbon steels, is 0.1581.

The evidence is admittedly conflicting, but the balance of evidence is distinctly in favour of the view that cementite is an endothermic substance, as indicated by the results of Ruff.

The suggestion has frequently been made that cementite is not a compound, but a solid solution of carbon in iron. This view is not generally accepted, however, as otherwise a whole series of solid solutions containing varying percentages of carbon might be expected to exist, which is not the case.

When damp, cementite is rapidly oxidised in air, yielding a mixture of hydrated oxide and carbonaceous material. It is not altered by exposure to dry air; when very finely divided, it burns in air below 150° C., becomes incandescent in bromine vapour at about 100° C., in chlorine at a somewhat lower temperature, and in sulphur vapour at about 500° C. Although insoluble in concentrated nitric acid, cementite dissolves in nitric acid of density 1.18 to 1.2, yielding a brown-coloured solution, the depth of colour being proportional, other things being equal, to the amount of carbide in solution. By dissolving a given weight of steel in nitric acid, therefore, and comparing the colour obtained with that when the same weight of a steel of known composition is employed, it is possible to estimate with considerable accuracy the amount of cementite in the first steel. This is known as the Eggertz test, and is largely used in steel works.

Cementite is less readily attacked than metallic iron by dilute hydrochloric acid; it is gradually dissolved by a normal solution of the acid, and readily in a concentrated solution, the gaseous products being hydrogen and hydrocarbons.

The molecule of cementite is probably not represented by the simple formula Fe₃C, but by (Fe₃C)_n, where n is some whole number greater than unity. At present there appears to be no method of determining the value to be assigned to n.

When iron containing cementite is maintained at a temperature a little above 700° C. for some time, the carbide dissociates almost completely into у iron and graphite. This is quite in accordance with the view that cementite is an endothermic compound.

A consideration of the influence of cementite upon the metallurgical properties of iron is reserved for later discussion.