Carbon plays a dominant role in the structure formation of iron. Iron (Fe) and carbon (C) form a cubic space lattice or 3D lattice. The corner points of this lattice contain iron atoms.
The carbon atoms can take up two positions. They are either surface-centred in the middle of each cube surface and the result is called gamma mixed crystal (top animation). In the case of alpha mixed crystal the carbon atom is spatially-centred inside the cute. Delta mixed crystal plays a secondary role, but it also has spatially-centred carbon atoms, however, it is only used in high-alloyed steels.
Carbon is the most important alloying element in iron. The quantity of carbon contained in the iron is decisive with regard to the hardness of the material and therefore its subsequent usability. At this point it is important to note that far more carbon can be absorbed in the gamma mixed crystal. Iron and carbon form a chemical compound called cementite (Fe3C).
Figure showing partial iron-carbon diagram
The iron-carbon diagram (also called the iron-carbon phase or equilibrium diagram) is a graphic representation of the respective microstructure states depending on temperature (y axis) and carbon content (x axis). The actual iron-carbon diagram is far larger than the part shown here.
At this point we only consider the area of the steel with a carbon content up to 2% maximum. Iron with higher carbon concentration exists – at this point the partial diagram of cast iron would follow, but this is not of interest for our consideration here.
The melt essentially cools via the austenite to ferrite phases – i.e. from gamma to alpha mixed crystal. If this process is viewed from the viewpoint of the crystal structure, the carbon atoms attempt to move from the surfaces to the centre of the crystal. But this position can only be taken by a single atom. The other carbon atoms are released and form cementite (Fe3C).
There are different microstructures in the solid state:
- Ferrite: contains hardly any carbon.
- Ferrite + perlite: As the carbon concentration increases, cementite is released. This becomes a new microstructure constituent and forms a ferrite-perlite microstructure.
- Perlite: As the carbon concentration increases further, the ferrite fraction reduces steadily. From a carbon concentration of 0.8…0.85 %, only perlite is left.
- Perlite + cementite: If the carbon concentration continues to increase, cementite deposits on the grain boundaries of the perlite. If examined under a microscope, these thickened grain boundaries can be seen. Cementite is a very hard and brittle microstructure component. Therefore, in many steel grades the cementite separation must be prevented. This is done by fast cooling. If the formation of cementite is not prevented, material can spall under mechanical loads.