Iron (Fe) – Iron carbide or Cementite (Fe3c) Phase Diagram - I

The left side of the diagram is pure iron. steels are alloys of Iron (Fe) and Carbon (C). The Iron carbon alloys containing upto 1.7 % carbon called steels and those containing more than 1.7 % carbon called Cast Iron. 

• α-Iron (ferrite) – Exists from room temperature to 910^oC.

                 Interstitial solid solution of Carbon in BCC Iron (Fe).

                 It has a Body Centred Cubic structure.

                 The maximum solubility of C is 0.022 wt% at 723°C.

                 Transforms to FCC γ-Iron (austenite) at 910 °C

•  γ-Iron (austenite) - Exists between 910^oC to 1395 ^oC.

                  Interstitial solid solution of Carbon in FCC Iron (Fe)

                  It has a Face Centred Cubic structure.

                  The maximum solubility of C is 1.7 wt % at 1130 °C. 

                  Transforms to BCC δ-ferrite at 1395 °C

• δ-Iron - Exists between 1395 °C to 1539 °C

                  solid solution of C in BCC Fe

                  The same structure as α-ferrite i.e. Body Centred Cubic structure but has longer cube edge.

                  Stable only at high T, above 1395 °C Melts at 1539 °C

• Fe₃c    (iron carbide or cementite)

                  It is 6.67% carbon and 93.3% iron by weight.

                 This intermetallic compound is hard and brittle.

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