Cementite

Some metals are very hard.

This stuff is made of iron. It also has bits of carbon. It helps hold iron together.

It can be hard but also break easily. It can even be found in space.

It is found in iron meteorites. Some people call that space version cohenite.

This material is very important for making steel. It is a special part of many metals.

Cementite is a hard material. It is made of iron and carbon. It is a compound, which means two or more things are joined together.

By weight, it is mostly iron. It has 93.3% iron. The rest is 6.67% carbon. This material is very hard. But it is also brittle. This means it can break easily.

Cementite is found in many steels and cast irons. It forms when metal cools down slowly. Sometimes it forms a pattern called pearlite. This is a mix of cementite and ferrite. Ferrite is another part of the metal.

How it forms depends on heat. If the metal cools at a low temperature, it makes fine colonies. If it cools at a high temperature, it makes large particles. These particles sit at grain boundaries.

You can even find it in space. It is in iron meteorites. This space version is called cohenite. It is named after Emil Cohen. He was a scientist who studied minerals.

Caption: A map of how iron and carbon change with heat.

Cementite is a very hard and brittle material. It is a compound made of iron and carbon. Scientists call it iron carbide. Its chemical formula is Fe3C. This means it is made of iron and carbon joined together. By weight, it is mostly iron. It contains 93.3% iron and 6.67% carbon.

Because it is so hard, it is often called a ceramic. It is a very important part of working with metals.

This material forms in different ways when metal cools. In white cast iron, it forms directly from the liquid melt. In carbon steel, it forms as the metal changes from austenite to ferrite. This happens during slow cooling. It can also form from martensite during a process called tempering. Sometimes, cementite and ferrite mix together. This creates a striped pattern called pearlite.

The name cementite comes from a special theory. Scientists Floris Osmond and J. Werth had an idea about steel. They thought solidified steel looked like cellular tissue. They believed ferrite was the nucleus of the cells. They thought cementite was the envelope around them. Because it wrapped around the iron, they said it "cemented" it.

Heat changes how cementite looks inside the metal. The cooling rate is very important. If the metal cools at a low temperature, it forms fine colonies. If it cools at a higher temperature, it forms coarse particles. These large particles sit at grain boundaries. Fine cementite has more surface area. This helps the metal change faster. Coarse carbides are slower to dissolve during heat treatments.

You can even find cementite in space. A natural version exists in iron meteorites. It contains small amounts of cobalt and nickel. This space version is called cohenite. It was named after Emil Cohen. He was a German mineralogist who described it. There are also other types like Hägg carbide. This can be found in the Wedderburn meteorite as the mineral Edscottite.

Cementite is a chemical compound made of iron and carbon. It is also known as iron carbide. Scientists use the chemical formula Fe3C to describe it. This compound is an intermediate transition metal carbide. It is a very important part of ferrous metallurgy, which is the study of iron and steel. In its pure form, cementite is a hard and brittle material. Because of these qualities, it is often classified as a ceramic.

The composition of cementite is very specific. By weight, the material consists of 93.3% iron and 6.67% carbon. It possesses an orthorhombic crystal structure. This means its atoms are arranged in a specific, repeating geometric pattern. While it is found in most steels and cast irons, it can also be produced as a raw material. This occurs through the iron carbide process, which is part of a family of alternative ironmaking technologies.

Cementite forms through several different processes depending on the metal. In white cast iron, it forms directly from the liquid melt. In carbon steel, the process is more complex. As austenite transforms into ferrite during slow cooling, cementite precipitates out. It can also form from martensite during a heat treatment called tempering. Sometimes, cementite and ferrite mix together in an intimate way. This creates a striped, layered structure known as pearlite.

The name "cementite" comes from a historical scientific theory. Scientists Floris Osmond and J. Werth proposed a unique view of solidified steel. They believed the structure looked like cellular tissue. In their theory, ferrite acted as the nucleus of these cells. They believed cementite acted as the envelope surrounding the cells. Because the carbide wrapped around the iron, they felt it "cemented" the structure together.

Temperature plays a massive role in how cementite behaves. On a metastable iron-carbon phase diagram, cementite does not decompose when heated below the eutectoid temperature of 723 °C. However, it is thermodynamically unstable. At higher temperatures, it eventually converts into austenite if carbon levels are low. If carbon levels are high, it converts into graphite. The physical properties of cementite are quite high. Its room temperature microhardness is between 760 and 1350 HV. It has a bending strength of 4.6 to 8 GPa and a Young's modulus of 160 to 180 GPa.

The shape, or morphology, of cementite affects how steel changes during heat treatments. The coiling temperature and the cooling rate are two major factors. If the metal is cooled at lower temperatures, cementite forms fine pearlitic colonies. If it is cooled at higher temperatures, it precipitates as coarse particles at grain boundaries. Fine cementite has more surface area. This increased surface area helps the metal undergo transformations more quickly. Conversely, coarse carbides dissolve more slowly during annealing. This impacts how the microstructure evolves during heat treatments.

There are several other types of iron carbides to consider. Epsilon (ε) carbide is a hexagonal close-packed Fe2–3C. It precipitates in plain-carbon steels with more than 0.2% carbon when tempered at 100–200 °C. This non-stoichiometric carbide dissolves once temperatures rise above approximately 200 °C. At that point, Hägg carbides and cementite begin to form. Hägg carbide is a monoclinic Fe5C2 that precipitates in hardened tool steels tempered between 200 and 300 °C.

Cementite and its relatives can even be found in space. A natural iron carbide occurs in iron meteorites. This version contains small amounts of nickel and cobalt. It is called cohenite, named after the German mineralogist Emil Cohen. Another variety, Hägg carbide, appears naturally in the Wedderburn meteorite. In that specific meteorite, it is known as the mineral Edscottite. Even in its pure form, cementite can change its magnetic properties. It shifts from ferromagnetic to paramagnetic when heated to its Curie temperature of approximately 210 °C.