Cementite, also known as iron carbide, is an important component of many steel grades and has a major influence on their properties.
Properties and Structure of Cementite
Cementite (Fe3C) is a chemical compound of iron and carbon. This intermetallic compound has an orthorhombic crystal structure and is relatively hard and brittle. In the iron-carbon alloy, cementite is a key factor in increasing the hardness and strength of steel.
Role of Cementite in Steel Production
In steel production, cementite is essential for achieving the desired mechanical properties. By controlling the proportion of cementite in the alloy, the hardness, wear resistance, and strength of the final product can be adjusted. Cementite is typically found in the form of lamellae within the pearlite of steel’s microstructure.
Influence of Cementite on Material Processing
Due to its hard nature, cementite has a significant influence on the processing of steel products. While it increases strength and wear resistance, it can also negatively affect machinability, as it makes the material more brittle and can lead to increased tool wear.
In Practice: What Foundry Technicians Need Cementite For
For the foundry technician, cementite is above all a warning sign when it appears where it doesn’t belong. In gray cast iron with lamellar graphite, the cooling rate in the thin-walled sections of the mold determines whether the carbon precipitates as soft graphite or remains bound as hard, brittle cementite. If a thin rib cools too quickly, a “white structure” forms there, meaning free cementite instead of graphite, and this exact spot can then hardly be machined afterward, because the tool wears prematurely on the hard cementite. In practice, wall thickness distribution and melt inoculation are therefore controlled so that cementite only forms where it is desired, for example in the hard surface layer of chilled-iron rolls. In malleable cast iron, cementite is actually the intended starting condition, which is then specifically converted into temper carbon through the annealing treatment.
Cementite Compared to Other Microstructure Constituents
| Microstructure Constituent | Carbon Content | Hardness (approx.) | Character |
|---|---|---|---|
| Ferrite | ~0.02% C | ~80–100 HV | soft, ductile |
| Pearlite | ~0.8% C (average of ferrite/cementite) | ~200–250 HV | lamellar structure of ferrite and cementite |
| Cementite (Fe₃C) | 6.67% C | ~800–1000 HV | very hard, brittle |
Frequently Asked Questions About Cementite
Is cementite an element or a compound?
Cementite is an intermetallic compound of iron and carbon with the formula Fe₃C, not an alloy in the strict sense and not a pure element.
Why is cementite so hard?
The orthorhombic crystal structure of Fe₃C offers dislocations hardly any slip planes through which the material could deform plastically. This makes cementite hard, but at the same time brittle.
What is the difference between cementite and carbide?
Cementite is a specific carbide, namely iron carbide, Fe₃C. The term carbide generally covers all compounds of carbon with metals, such as chromium carbide or tungsten carbide.
Standard Reference
DIN EN 1561:2024-03 for gray cast iron with lamellar graphite limits the permissible proportion of free cementite (“white structure”) as a casting defect, because it impairs the machinability and mechanical properties of the component.
Further Terms
- Carbide – general term for carbon-metal compounds
- Gray cast iron – cast iron in which cementite is avoided as a casting defect
- Malleable cast iron – makes deliberate use of cementite conversion
- Martensite – contains carbon trapped in solid solution rather than precipitated cementite