Steel is most commonly used metal by human on the earth for all kinds of applications.

The solid state transformation that occurs in steel is in variety with various properties makes it very useful for different application purposes.

Iron carbide diagram shows all allotropic modification of iron n steel for various temperatures and various compositions.

The Fe-Fe3C is characterized by five individual phases and four invariant reactions.

Five phases that exist in the diagram are: α–ferrite (BCC) Fe-C solid solution, γ-austenite (FCC) Fe-C solid solution, δ-ferrite (BCC) Fe-C solid solution, Fe3C (iron carbide) or cementite - an inter-metallic compound and liquid Fe-C solution.

Four invariant reactions that cause transformations in the system are namely eutectoid, eutectic, monotectic and peritectic. As depicted by left axes, pure iron upon heating exhibits two allotropic changes.

One involves α–ferrite of BCC crystal structure transforming to FCC austenite, γ-iron, at 910 degree Celsius. At 1400 degree Celsius austenite changes to BCC phase known as δ-ferrite, which finally melts at 1536 degree Celsius.

Fe-C system constitutes four invariant reactions:

• peritectic reaction at 1495 degree Celsius and 0.16%C, δ-ferrite + L ↔ γ-iron (austenite)

• monotectic reaction 1495 degree Celsius and 0.51%C, L ↔ L + γ-iron (austenite)

• eutectic reaction at 1147 degree Celsius and 4.3 %C, L ↔ γ-iron + Fe3C (cementite) [ledeburite]

• eutectoid reaction at 723 degree Celsius and 0.8%C, γ-iron ↔ α–ferrite + Fe3C (cementite) [pearlite]