Carbides of iron

Carbides of the Iron Group Metals. The carbides of iron, nickel, cobalt, and manganese have lower melting points, lower hardness, and different stmctures than the hard metallic materials. Nonetheless, these carbides, particularly iron carbide and the double carbides with other transition metals, are of great technical importance as hardening components of alloy steels and cast iron. 

Steel, as is well known, differs from iron by the presence of a certain amount of carbon, which induces the iron, when cold, to persist in its allotropic state. This appears to be due to a carbide of iron mixed with the excess of iron in the steel. The compound has been found as a meteoric mass it has been named cohenite, and has the formula FegC. On treating steel with dilute acetic acid, the same substance remains as a black powder. Its formula is similar to that of manganese carbide, MngC. 

Carbides of iron are thermodynamically unstable with respect to decomposition to iron and carbon. When hexagonal carbide is heated in an inert atmosphere the following sequence of reactions is observed ... 

When nitrides are treated with carbon monoxide at moderate temperatures, two reactions occur. The first may be termed a completion reaction in which carbon enters the interstitial phase until C + N increases to about 0.5. This reaction is more rapid than the second, the substitution reaction in which carbon replaces nitrogen. Similar reactions are observed when carbides of iron (Hagg carbide or cementite) are... 

A carbide of iron occurs as Cohenite (Fe, Co, Ni)3C, in meteorites in distorted crystals, probably belonging to the cubic system.4 It possesses a metallic lustre, is tin-white in colour when pure, becoming bronze-yellow upon exposure to air. Hardness 5 5 to 6 density 6 977.5... 

The results of this work are in agreement with the work of Halle and Herbst, and of Pichler and Merkel in so far as it also describes the formation Fe2C (Hagg) as a product of low-temperature carbiding of iron catalysts. Kolbel and co-workers report (in agreement with Pichler and Merkel) that copper and alkali accelerate the carbide formation. 

Table 3 Reaction Sequences for Nitrides, Carbonitrides and Carbides of Iron.

The name given to the carbide of iron, FejC. This is an extremely hard and brittle constituent. 

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