Why carbon is only so soluble in iron to a limited extent

Iron-carbon diagram

Crystal lattice

Since, as already mentioned, cast iron cannot be forged, it is imperative to make the material malleable so that further processing can take place. This happens through freshening. The resulting steel can now be forged and is therefore easier to process further.

If the metal is melted, i.e. in a liquid state, there are no lattice-like structures. The atoms can move freely in all directions. In order to achieve the desired crystal structures, it is necessary for the metal to cool down slowly and completely at room temperature. Intermittent heat treatments have a positive effect on the formation of the crystal structures. Iron forms a cubic space lattice. The iron atoms are located at the corner points of this lattice.

In low concentrations, carbon does not form a solid chemical bond with iron, but is stored in the gaps in the iron crystal lattice. The amount of carbon that can dissolve in the iron depends on the modification of the iron, since the different crystal lattice forms of iron (e.g. body-centered and face-centered cubic) have interstitial spaces of different sizes.

The delta mixed crystal, which has body-centered atoms, plays a subordinate role, but can only play a role in high-alloy steels. These crystals form in the temperature range from 1,536 ° C (melting point of pure iron) to 1,392 ° C.