EQUILIBRIUM DIAGRAM

1. Iron-Iron Carbide Equilibrium
Diagram

2. Iron – Iron carbide equilibrium diagram

3. Fe- Fe3C diagram with critical temperature

4. Summery of critical temperatures in the iron- carbide
equilibrium diagram
Critical points Temperature °C Significant changes
A0 (curi temperature of
cementite)
210 Cementite becomes
paramagnetic
A1 (lower critical temperature) 725 Pearlite begins to
transform into austenite
A2 (curi temperature of ferrite) 768 Ferrite becomes
paramagnetic
A3 (upper critical temperature of
hypo eutectoid steels)
725-910 Completion of ferrite to
austenite transformation
Acm (upper critical temperature
of hypo eutectoid steel)
725-1147 Completion of cementite
to austenite
A4 1395-1492 Completion of austenite to
δ ferrite transformation

5. Iron – Iron Carbide Equilibrium Diagram
 Iron(Fe) and iron carbide (Fe3C) are the components
in the iron-iron carbide equilibrium diagram.
 Steels are an alloy of iron and carbon in which the
carbon content is up to 2.0 percent
 Alloy of iron and carbon in which the carbon content
is more than 2.0 percent, it is known as cast iron.

6. Definition of Structure
 Cementite or iron carbide
 The chemical formula for cementite is Fe3C
 It contains 6.67 percent C by weight
 It is a typical hard and brittle interstitial compound of
low tensile strength but high compressive strength.
 Its crystal structure is orthorhombic.
 It is the hardest structure

7. Definition of structure
 Austenite
 Austenite is γ solid solution
 It is an interstitial solid solution of carbon dissolved in γ
iron
 It is normally not stable at room temperature
 Maximum solubility is 2 percent C at 1129°C
 The average properties are
Tensile strength:150,000 psi
Elongation :10 % in 2 in gage length
Hardness :Rockwell C40
Toughness :High

8. Definition of structure
• Ledeburite
 Ledeburite is the eutectic mixture of austenite and
cementite.
 It contains 4.3 % Carbon and represents the eutectic of cast
iron.
 Ledeburite exists when the carbon content is greater than 2
%, which represents the dividing line on the equilibrium
diagram between steel and cast iron.

9. Microstructure of Austenite

10. Definition of structure
 Ferrite
 Ferrite is the α solid solution
 It is an interstitial solid solution of a small amount of
carbon dissolved in α iron
 It is the softest structure
 The average properties are
Tensile strength:40,000 psi
Elongation :40 % in 2 in gage length
Hardness :less than Rockwell C 0and
less than Rockwell B 90
Toughness :Low

11. Microstructure of Ferrite

12. Definition of Structure
• Pearlite
 Pearlite is eutectoid mixture containing 0.80 percent C
and is formed at 723°C on very slow cooling
 When the steel has the eutectoid composition of 0.8%
carbon, the microstructure consists of alternate layers of
thin, parallel plates of α(ferrite) and Fe3C (cementite).
 This eutectoid mixture is called pearlite.

13. • Pearlite
 The average properties are
Tensile strength:120,000 psi
Elongation :20 % in 2 in gage length
Hardness :Rockwell C20 or BHN 250-300

14. Microstructure of Pearlite
2500X 17,000X

15. Allotropy of Iron
 The pure substances , when in a crystalline form,
remain stable within certain limit of pressure and
temperature. This is known as allotropy.
 The pure iron exists in three allotropic forms
i. Alpha(α)
ii. Gama (γ)
iii. Delta(δ)

16. Allotropy of Iron
 Alpha(α)
 It has a body centred cubic (BCC) structure.
 It has highly ferromagnetic up to 768°C and again heating it become
non magnetic but crystal structure still remain BCC up to 910°C
 The non magnetic α iron earlier known as β iron
• Gama (γ)
 Above 910°C allotropic changes takes place, it is
transformed from α to γ structure
 Iron is changed from BCC structure to Face centred
cubic(FCC) structure.

17. Allotropy of Iron
 Delta (δ)
 Upon heating to 1388°C again allotropic changes takes
place, the γ iron is transformed into δ iron.
 The structure is transformed from FCC into BCC, it is
stable up to 1535°C (melting point of pure iron)
 The BCC structure δ iron has a longer cube edge than
BCC structure of α iron