1. Iron – Iron Carbide Phase Diagram
MME 293: Lecture 08
Iron – Iron Carbide Phase Diagram
Department of MME
BUET, Dhaka

2. Today’s Topics  The iron – carbon phase diagram
 Details of iron – iron carbide phase diagram
 Classification of iron – carbon alloys
Reference:
1. SH Avner. Introduction to physical metallurgy, 2nd Ed., Ch. 7.

3. Allotropic Transformation of Pure Iron
Liquid
d (delta) iron (BCC)
g (gamma) iron (FCC)
a (alpha) iron (BCC)
nonmagnetic
magnetic
1535°
1401°
910°
768°
Temperature, °C
Time
Cooling Curve of Pure Iron

4. Iron –Carbon Phase Diagram
Iron Carbide, Fe3C
 An interstitial compound, containing wt.% carbon.
 The compound dissociates into iron and carbon, if given the opportunity:
Fe3C = Fe + 3C
 Fe3C is a metastable compound, because such decomposition takes a very long time at room temperature.
 Because of the presence of Fe3C, this diagran is not a true equilibrium diagram.
Fe3C
6.67
Temperature, °C
Iron
Carbon
wt. % carbon

5. Iron – Iron Carbide Phase Diagram
Temperature, °C
1535°
1401°
1492°
910°
723°
1130° Fe
wt. % carbon
Fe3C

6. Temperature, °C wt. % carbon
0.10
0.50   
0.18  
2.0
4.3  
0.025
0.80
0.008
6.67   Fe
wt. % carbon
Fe3C

7. Temperature, °C wt. % carbon
Delta (d) iron
Liquid
Austenite (g )
Ferrite (a)
Cemenite Fe
Fe3C

8. Temperature, °C wt. % carbonL
austenite + L
L + cementite
austenite
austenite + ferrite
austenite + cementite
ferrite
cemenite
ferrite + cementite Fe
Fe3C

9. The Invariant Reactions
Temperature, °C
wt. % carbon
d -iron + L
Peritectic Point 
austenite Fe
Fe3C
The Peritectic Reaction @ C
Delta-iron 0.10%C Liquid 0.50%C Austenite 0.18%C
cooling
heating

10.  Temperature, °C wt. % carbon liquid austenite + cementite Fe Fe3C
Eutectic Point
liquid 
austenite + cementite Fe
Fe3C
The Eutectic Reaction @ C
Liquid 4.3%C ( Austenite 2.0%C + Cementite 6.67%C )
Ledeburite (an eutectic mixture)
cooling
heating

11.  Temperature, °C wt. % carbon austenite ferrite + cementite
Eutectoid Point 
ferrite + cementite
The Eutectoid Reaction @ 723 C
Austenite 0.8%C ( Ferrite 0.025%C + Cementite 6.67%C )
Pearlite (an eutectoid mixture)
cooling
heating Fe
Fe3C

12. The Major Phases Temperature, °C wt. % carbon Fe Fe3C delta iron
liquid
lede-burite
austenite
cemenite
pearlite
ferrite Fe
Fe3C

13. Iron – Iron Carbide Phase Diagram
Temperature, °C
wt. % carbon
liquid
austenite + liquid
liquid + cementite
austenite
austenite + ferrite
austenite + cementite
pearlite
cemenite
ferrite
ferrite + pearlite
pearlite + cementite Fe
Fe3C

14. Characteristics of Major Phases
Temperature, °C
wt. % carbon
Ferrite (a)
 An interstitial solid solution of carbon dissolved in BCC a-iron.
 Carbon solubility – wt.% max. at 723 C, wt.% min. at 0 C.
 The softest structure that appears on the iron – iron carbide diagram.
 Average properties: 40,000 psi TS, 40 % elong. in 2 inch, < RC 0 or < RB 90 hardness.
lede-burite
ferrite Fe
Fe3C

15. Austenite (g) Temperature, °C wt. % carbon Fe Fe3C
 An interstitial solid solution of carbon dissolved in FCC g-iron.
 Carbon solubility – 2.00 wt.% max. at C, 0.80 wt.% min. at 723 C.
 Not stable at room temperature; can be made stable under certain conditions.
 Average properties: 150,000 psi TS, 10 % elong. in 2 inch, RC 40 hardness, high toughness.
austenite Fe
Fe3C

16. Cementite (Fe3C) Temperature, °C wt. % carbon Fe Fe3C
 An interstitial intermetallic compound of iron carbide with an orthorhombic structure.
 Its chemical formula is Fe3C and contains 6.67 wt.% carbon.
 The hardest and brittle structure that appears on the iron – iron carbide diagram.
 Average properties: 5,000 psi TS, high compressive strength.
cemenite Fe
Fe3C

17. Pearlite Temperature, °C wt. % carbon Fe Fe3C
 The eutectoid mixture of fine plate-like lamellar mixture of ferrite and cementite.
 Formed from austenite that contains 0.80 wt.% carbon during slow cooling at 723 C.
 Average properties: 120,000 psi TS, 20 % elong. in 2 inch, RC 20 hardness.
pearlite Fe
Fe3C

18. Ledeburite Temperature, °C wt. % carbon Fe Fe3C lede-burite
 The eutectic mixture of austenite and cementite.
 Formed from liquid that contains 4.30 wt.% carbon during slow cooling at 1130 C.
 Not stable below 723 C, where austenite of ladeburite transformed into pearlite. The structure is then called “transformed ladeburite.” Fe
Fe3C

19. Carbon Solubility in Iron
Austenite
 FCC structure
 4 iron atoms/unit cell
 Dense atomic packing (74 %)
Ferrite
 BCC structure
 2 iron atoms/unit cell
 Loose atomic packing (68 %)
 Iron expands when changes from austenite to ferrite during cooling.
Why does austenite absorb more carbon than ferrite ?
 Radius of an average carbon atom is 0.70 (x10-8) cm.
 Radius of maximum void in FCC austenite is 0.52 (x10-8) cm, while that in BCC ferrite is 0.36 (x10-8) cm.
 The distortion of ferrite lattice by carbon atom is much greater than in case of austenite.
 Austenite has a greater solubility of C atoms than ferrite

20. Classification of Iron –Carbon Alloys
Temperature, °C
liquid
Steels are Fe-C alloys that pass through austenite zone during cooling
austenite
eutectic
eutectoid
ferrite
ferrite + pearlite
pearlite + cementite
0.8
2.0
4.3
STEELS
CAST IRONS
hypo - eutectoid
hyper - eutectoid
hypo - eutectic
hyper - eutectic