In order to improve the strength and hardness of iron, a small amount of carbon is often added to pure iron. Due to the interaction of iron and carbon, the following five basic structures can be formed: ferrite, austenite, cementite, pearlite and Body.

1. Ferrite The interstitial solid solution formed by dissolving carbon in a-Fe is called ferrite, which is represented by the symbol F. It still maintains the body-centered cubic lattice structure of a-Fe. Because of its small lattice gap, the ability to dissolve carbon is very poor. The maximum wc at 727°C is only 0.0218%, and it drops to 0.0008% at room temperature. Due to the small amount of dissolved carbon, the mechanical properties of ferrite are similar to those of pure iron, that is, the plasticity and impact toughness are better, but the strength and hardness are lower.

2. Austenite The interstitial solid solution formed by dissolving carbon in y-Fe is called austenite, represented by the symbol A, which maintains the face-centered cubic crystal structure of y-Fe. Because of its larger lattice gap, the ability to dissolve carbon is stronger than ferrite. The carbon mass fraction wc at 727°C is 0.77%, and at 1148°C the wc reaches 2.11%. The strength and hardness of austenite are low, but it has good plasticity. It is the ideal structure for most steels for high temperature pressure processing.

3. Cementite Cementite is an interstitial compound with a complex rhombic structure composed of iron and carbon, represented by the chemical formula Fe3C. The mass fraction wc of carbon in cementite is 6.69%, the hardness is very high, and the plasticity and toughness are almost zero. It mainly exists as a strengthening phase in iron-carbon alloys.

4. Pearlite Pearlite is a mechanical mixture composed of ferrite and cementite, represented by the symbol P. Under slow cooling conditions, the wc in pearlite is 0.77%, and the mechanical properties are between ferrite and cementite, appearing in lamellar form, and have good comprehensive mechanical properties.

5. Leedenite Leedeite is an iron-carbon alloy with a carbon mass fraction wc of 4.3%. When slowly cooled to 1148℃, the eutectic structure of austenite and cementite will be crystallized from the liquid phase at the same time, with the symbol Ld Express. When cooled to 727°C, austenite transforms into pearlite, so the ledeburite is composed of pearlite and cementite at room temperature, called metamorphic ledeburite, represented by the symbol Ld'. Because there is a large amount of cementite in the ledeburite, its performance is similar to that of cementite, that is, it has high hardness and poor plasticity.

It can be seen that iron-carbon alloys have different crystal structures under different temperature conditions, which is the basis for heat treatment of steel.