Cementite is a hard and brittle substance that forms naturally from the smelting of white cast iron. It is formed during the tempering process of steel and is ferromagnetic but becomes paramagnetic at 480K. Cohenite is a similar substance found in meteorites and high iron deposits.
Cementite is a chemical compound whose inclusion hardens steel. Each molecule is composed of three iron atoms bonded to one carbon atom (Fe3C) to form a crystal lattice structure called orthorhombic, in which multiple rectangular prisms arise from the same basic structure and intersect at 90 degree angles. The result is a very hard and brittle substance called iron carbide or cementite.
In its purest form, cementite is classified as an unoxidized ceramic. It is solid and inert and can withstand crushing forces, chemical erosion, abrasion, and temperatures up to 3000 degrees F (1600 C). It forms naturally from the smelting of white cast iron, where it precipitates from iron as carbon to form large particles. Sometimes it thus appears in phase with austenite, an allotrope of iron, which can sometimes cool to form martensite, a steel with a very strong crystal lattice.
The steel is tempered to increase hardness and reduce brittleness creating cementite. The first step in the tempering process is called austenizing, when steel is melted in a solution of iron and carbon, or austenite. The steel cools rapidly and martensite is formed from the austenite. It is then reheated and slowly cooled in a controlled manner and cementite is formed. It is impossible to produce enough energy to carry out the reaction, so the cementite is usually mixed with small amounts of unconverted martensite, bainite, also Fe3C, but with a different crystal structure, and ferrite (iron).
Cementite is ferromagnetic, meaning it exhibits magnetic characteristics with or without a magnetic field, like a refrigerator magnet. At 480K (404 F, 207 C), however, the atomic poles start moving and are no longer aligned. The spins of the molecules are randomized and the magnetization ceases. The substance becomes paramagnetic, which means it is only magnetized if the field is applied from an external source. Even then, the magnetization will be weak because it relies on induced dipoles, and no external force can induce every dipole in every molecule, crystalline structure or not. Indeed, it is the non-linear attraction that gives ferromagnets their strength.
There is a substance very similar to cementite called cohenite. It is also Fe3C, except that it forms a rod-like crystal and contains trace amounts of nickel and cobalt. It occurs naturally in meteorites, and there are places on Earth with very high iron deposits, such as volcanic magma flow trails that occur over coal deposits.
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