Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 5M

Year: May 2015

The iron – carbon system, phase transformations

A study of iron-carbon system is useful and important in many respects. This is because (1) steels constitute greatest amount of metallic materials used by man (2) solid state transformations that occur in steels are varied and interesting. These are similar to those occur in many other systems and helps explain the properties.

Iron-carbon phase diagram shown in figure is not a complete diagram. Part of the diagram after 6.67 wt% C is ignored as it has little commercial significance. The 6.67%C represents the composition where an inter-metallic compound, cementite (Fe3C), with solubility limits forms. In addition, phase diagram is not true equilibrium diagram because cementite is not an equilibrium phase. However, in ordinary steels decomposition of cementite into graphite never observed because nucleation of cementite is much easier than that of graphite. Thus cementite can be treated as an equilibrium phase for practical purposes.

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 oC. At 1400 oC austenite changes to BCC phase known as δ-ferrite, which finally melts at 1536 oC.

As shown in figure-16, and mentioned earlier, Fe-C system constitutes four invariant reactions:

• peritectic reaction at 1495 oC and 0.16%C, δ-ferrite + L ↔ γ-iron (austenite)
• monotectic reaction 1495o C and 0.51%C, L ↔ L + γ-iron (austenite)
• eutectic reaction at 1147 oC and 4.3 %C, L ↔ γ-iron + Fe3C (cementite) [ledeburite]
• eutectoid reaction at 723 oC and 0.8%C, γ-iron ↔ α–ferrite + Fe3C (cementite) [pearlite]