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What is Cementite?

By Angela Brady
Updated May 17, 2024
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Cementite is a chemical compound whose inclusion hardens steel. Each molecule is made of three iron atoms bonded to one carbon atom (Fe3C) to form a crystal lattice structure called orthorhombic, where multiple rectangular prisms arise from the same base 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 a non-oxide ceramic. It is solid and inert, and can withstand crushing force, chemical erosion, abrasion, and temperatures up to 3000 degrees F (1600 C). It forms naturally by the melting of white cast iron, where it precipitates out of the iron as carbon to form large particles. It sometimes appears this way in phase with austenite, an allotrope of iron, which can sometimes cool to form martensite, a steel with a very strong crystal lattice.

Steel is tempered to increase hardness and reduce brittleness by creating cementite. The first step in the tempering process is called austenizing, when the steel is melted into a solution of iron and carbon, or austenite. The steel is rapidly cooled, and martensite forms from the austenite. It is then heated again, and cooled slowly in a controlled manner, and cementite is formed. It is impossible to produce enough energy to run the reaction to completion, so the cementite is usually mixed with small amounts of unconverted martensite, bainite, which is also Fe3C, but with a different crystal structure, and ferrite(iron).

Cementite is ferromagnetic, which means it displays magnetic characteristics with or without a magnetic field, like a refrigerator magnet. At 480K (404 F, 207 C), however, the atomic poles begin to move around and are no longer aligned. The spins of the molecules become randomized, and magnetization ceases. The substance becomes paramagnetic, which means it is only magnetized if the field is applied by an outside source. Even then, the magnetization will be weak because it relies upon induced dipoles, and no outside force can induce every dipole in every molecule, crystalline structure or not. In fact, 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 it forms a rod-like crystal and contains trace amounts of nickel and cobalt. It occurs naturally in meteorites, and on Earth is places with very high iron deposits, like volcanic magma flow trails that happen over coal deposits.

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Discussion Comments

By anon350085 — On Oct 01, 2013

Cementite is "used with" ferrite to form peralite which helps to make steels stronger and more weldable. It is great for structure and pressure vessel steels such as S355J2+N or A516 70 N

By jcraig — On Aug 29, 2011

@titans62 - It is really hard to find any good information on the internet about what cementite is or what it is used for. I'm supposed to be writing about it for a class assignment.

From what I've been able to find, cast iron is classified as iron with a certain percentage of carbon in it. I believe that cementite is a type of cast iron with a crystalline structure.

It doesn't sound like cementite usually makes up an entire piece of metal, though. Instead what it sounds like to me is that it is mixed into another alloy to give it more strength.

If someone else here knows how cementite is used, please help us out.

By titans62 — On Aug 28, 2011

I don't know if I'm really clear on what cementite is. Is it a type of iron alloy, or is cementite an additive made from iron and carbon that is used to strengthen steel?

By JimmyT — On Aug 27, 2011

Once cementite is made, what can it be used for? It sounds like it would be extremely strong, but the article also says it is brittle, which makes me think it wouldn't be ideal for something like construction.

Is cementite very malleable? Could it be used for sheet metal or wire? How strong is the magnetic field of cementite? I was wondering if maybe there was some sort of use for it in magnets.

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