IRON CARBON EQUILIBRIUM DIAGRAM

IRON CARBON EQUILIBRIUM DIAGRAM

Fig. 1 | Iron Carbon Equilibrium Diagram

• Ferrite (α- iron): -

1. Ferrite or α iron is the structural form of pure iron at room temperature.
2. α- iron is soft ductile and B.C.C. structure.
3. Solubility of carbon in ferrite is 0.006% at room temperature and maximum 0.0218% at 723⁰c.

 

• Austenite (γ iron): -

1. The F.C.C. modification of iron is called austenite.
2. It is stable at temperature between 910⁰c and 1400⁰c.
3. Maximum solubility of carbon is 2.1% at 1150⁰c.

 

• 𝛿 – Iron: -

1. Above 1400⁰c γ iron changes to 𝛿 – Iron, this is B.C.C. structure.
2. Maximum solubility of carbon is 0.08% at 1495⁰c.

 

• Cementite: -

1. In Iron carbon alloys, if carbon is excess of the solubility limit form Iron carbide or cementite (Fe₃c).
2. Orthorhombic structure

 

• Pearlite: -

1. Pearlite is a mechanical mixture of 87% ferrite and 13% cementite.

 

• Ledeburite: -

1. It is a eutectic mixture of austenite and cementite.

 

• Eutectoid reaction: -

Solid 1 ⟺ Solid 2 +Solid 3

Austenite ⟺ Ferrite + Cementite

[NOTE: -   When cooling⟹                    ⟸When heating]

 

This reaction occurs at a temperature of 723⁰c and 0.77% of carbon composition.

 

• Eutectic reaction: -

Liquid ⟺ Solid 1 +Solid 2

Liquid Iron ⟺ γ iron + Fe₃C

                       (LEDEBURITE)

[NOTE: -   When cooling⟹                    ⟸When heating]

 

This reaction occurs at a temperature of 1147⁰c and 4.32% of carbon composition.

 

• Peritectic reaction: -

Liquid + Solid ⟺ Solid

Liquid + 𝛿 – Iron ⟺ γ iron

[NOTE: -   When cooling⟹                    ⟸When heating]

 

This reaction occurs at a temperature of 1495⁰c and 0.17% of carbon composition.