The document discusses the iron-carbon phase diagram. It describes three important reactions: 1) The eutectic reaction occurs at 4.3% carbon and 1,147°C, where liquid transforms to austenite and cementite. 2) The eutectoid reaction occurs at 0.76% carbon and 727°C, where austenite transforms to ferrite and cementite to form pearlite. 3) The peritectic reaction occurs at 0.16% carbon and 1,493°C, where liquid and delta-ferrite transform to austenite. The phase diagram is used to explain the microstructures that form in steels with different carbon

1. The Iron–Carbon Phase Diagram
Prof. H. K. Khaira
Professor in MSME Deptt.
MANIT, Bhopal

2. Iron–Carbon Phase Diagram
• In their simplest form, steels are alloys of Iron
(Fe) and Carbon (C).
• The Fe-C phase diagram is a fairly complex
one, but we will only consider the steel and
cast iron part of the diagram, up to 6.67%
Carbon.

3. Fe – C Equilibrium Diagram

4. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license.
Figure 12.33 The iron-carbon phase diagram showing the relationship between the
stable iron-graphite equilibria (solid lines) and the metastable iron-cementite
reactions (dashed lines).

5. Phases Observed in Fe-C Diagram • Phases
1. Ferrite
2. Austenite
3. Cementite
4. δ-ferrite
• And phase mixtures
1. Pearlite
2. Ledeburite

6. Phases Observed in Fe-C Diagram
1. Ferrite
Ferrite is the interstitial solid solution of carbon in alpha iron. It has B.C.C.
Structure. It has very limited solubility for carbon (maximum 0.022% at 727°C
and 0.008% at room temperature). Ferrite is soft and ductile.
2. Austenite
Austenite is the interstitial solid solution of carbon in gamma (γ) iron. It has
FCC structure. Austenite can have maximum 2.14% carbon at 1143°C.
Austenite is normally not stable at room temperature. Austenite is nonmagnetic and soft.
3. Cementite
Cementite or iron carbide (Fe3C) is an intermetallic compound of iron and
carbon. It contains 6.67% carbon. It is very hard and brittle. This intermetallic
compound is a metastable phase and it remains as a compound indefinitely at
room temperature.
4. δ-ferrite
It is a solid solution of carbon in δ-iron. It is stable at high temperatures. It has
BCC structure.

7. Phase Mixtures Observed in Fe-C
Diagram
• 1. Pearlite
The pearlite consists of alternate layers of ferrite
and cementite. It has properties somewhere
between ferrite and cementite. The average
carbon content in pearlite is 0.76%
• 2. Ledeburite
Ledeburite is an eutetcic mixture of austenite and
cementite in the form of alternate layers. The
average carbon content in ledeburite is 4.3%.

8. A few comments on Fe–C system
• Carbon occupies interstitial positions in Fe. It
forms a solid solution with α, γ, δ phases of
iron
• Maximum solubility in BCC α-ferrite is limited
(max. 0.025 % at 727 °C) as BCC has relatively
small interstitial positions
• Maximum solubility in FCC austenite is 2.14 %
at 1147 °C as FCC has larger interstitial
positions

9. A few comments on Fe–C system
• Mechanical properties
– Cementite is very hard and brittle - can strengthen
steels.
– Mechanical properties depend on the
microstructure, that is, amount and distribution of
ferrite and cementite.
• Magnetic properties: α -ferrite is magnetic
below 768 °C, austenite is non-magnetic

10. Fe-C Alloys
• Fe-C alloys can be of two types.
1. Steels
Steels are alloys of iron and carbon containing up
to 2.14% C. Other alloying elements may also be
present in steels.
2. Cast irons
Cast irons are alloys of iron and carbon
containing more than 2.14% C. Other alloying
elements may also be present in cast irons.

11. Important Reactions in Fe-C System
• There are three important reactions taking
place in Fe-C system
1. Eutectic reaction
2. Eutectoid reaction
3. Peritectic Reaction

12. Important Reactions in Fe-C System
• Eutectic reaction
• Eutectic: 4.30 wt% C, 1147 °C
• L (4.30% C) ↔ γ (2.14% C) + Fe3C
• Eutectoid reaction
• Eutectoid: 0.76 wt%C, 727 °C
• γ(0.76% C) ↔ α (0.022% C) + Fe3C
• Peritectic Reaction
• Peritectic: 0.16% C, 14930 C
• δ(0.11% C) + L(0.51%C) ↔ γ (0.16%C)

13. Important Reactions in Fe–C System
Peritectic: 0.16% C, 14930 C
δ(0.11% C) + L(0.51)%C ↔ γ (0.16%C)
Eutectic: 4.30 wt% C, 1147 °C
L (4.30% C) ↔ γ (2.14% C) + Fe3C
Eutectoid: 0.76 wt%C, 727 °C
γ(0.76% C) ↔ α (0.022% C) + Fe3C

14. Eutectic Reaction
• Eutectic reaction:
at 4.30 % C and 1147 °C
L (4.30% C) ↔ γ (2.14% C) + Fe3C
• In eutectic reaction, the liquid solidifies as a phase
mixture of austenite (containing 2.14% C) and
cementite. This phase mixture is known as ledeburite.
• The average carbon content in ledeburite is 4.30%.
• The eutectic reaction occurs at a constant
temperature. This is known as eutectic temperature
and is 1147 °C.

15. Eutectoid Reaction
• Eutectoid reaction:
at 0.76 %C and 727 °C
γ(0.76% C) ↔ α (0.022% C) + Fe3C
• In eutectoid reaction, the austenite transforms into a phase
mixture of ferrite (containing 0.76% C) and cementite. This
phase mixture is known as pearlite.
• The average carbon content in pearlite is 0.76%.
• The eutectoid reaction occurs at a constant temperature.
This is known as eutectoid temperature and is 727°C.
• Eutectoid reaction is very important in heat treatment of
steels.

16. Microstructure of Eutectoid Steel
In the micrograph, the dark areas are
Fe3C layers, the light phase is α- ferrite
Pearlite nucleates at austenite grain
boundaries and grows into the grain

17. Pearlite Formation
Pearlite nucleates at austenite grain boundaries and grows into the grain
Growth direction

18. Peritectic Reaction
• Peritectic reaction:
at 0.16% C and 14930 C
δ(0.11% C) + L(0.51%C) ↔ γ (0.16%C)
• In peritectic reaction, the liquid and δ iron
transforms into austenite (containing 0.16%
C).
• The peritectic reaction occurs at a constant
temperature. This is known as peritectic
temperature and is 1493°C.

19. Development of Microstructure in
Iron - Carbon alloys

20. Iron-Carbon (Fe-C) Phase Diagram
1. Eutectic (A):
L
T(°C)
1600
Adapted from Fig. 10.28,
Callister & Rethwisch 3e.
L
1400
+ Fe3C
2. Eutectoid (B):
+ Fe3C
1200
+L
AA
(austenite)
1000
800
+Fe3C
B
727°C = T eutectoid
600
120 m
Result: Pearlite is
alternating layers of
400
0
(Fe)
L+Fe3C
1148°C
Fe3C (cementite)
• 2 important points
+Fe3C
1
0.76
and Fe3C phases
2
3
4
4.30
5
6
6.7
C, wt% C
Fe3C (cementite-hard)
(ferrite-soft)
20

21. Microstructure of Eutectoid steel
• In eutectoid
steel, pearlite is formed
at eutectoid
temperature.
• The austenite gets
converted into pearlite
which is a mechanical
mixture of ferrite and
cementite..
• This tranformation
occurs at 727o C (at
constant temperature)

22. Microstructure of Eutectoid Steel
• When alloy of eutectoid composition (0.76 wt % C) is
cooled slowly it forms pearlite, a lamellar or layered
structure of two phases: α-ferrite and cementite (Fe3C).
• The layers of alternating phases in pearlite are formed
for the same reason as layered structure of eutectic
structures: redistribution of C atoms between ferrite
(0.022 wt%) and cementite (6.7 wt%) by atomic
diffusion.
• Mechanically, pearlite has properties intermediate to
soft, ductile ferrite and hard, brittle cementite.

23. Microstructure of Hypoeutectoid
Steel
Compositions to the left of eutectoid (0.022 0.76 wt % C) is hypoeutectoid (less than
eutectoid) alloys. Microstructure change is
γ→α+γ→α+P
1. First ferrite is formed when temperature
comes down below Ae3 temperature.
γ→α+γ
2. The amount of ferrite increases with
decrease in temperature till eutectoid
temperature.
3. Remaining austenite changes to pearlite at
eutectoid temperature.
α+γ→α+P

24. Microstructure of Hypoeutectoid Steel
T(°C)
1600
Adapted from Figs. 10.28 and 10.33
L
1400
1000
+ Fe3C
800
727°C
600
pearlite
+ Fe3C
1
0.76
400
0
(Fe)C0
L+Fe3C
1148°C
(austenite)
2
3
4
5
6
6.7
C, wt% C
100 m
pearlite
Fe3C (cementite)
1200
(Fe-C System)
+L
Hypoeutectoid
steel
proeutectoid ferrite
Adapted from Fig. 10.34, Callister & Rethwisch 3e.
24

25. Microstructure of Hypoeutectoid
Steel
Hypoeutectoid steels contain proeutectoid ferrite (formed
above the eutectoid temperature) plus the pearlite that
contains eutectoid ferrite and cementite.

26. Relative amounts of proeutectoid
phase (α or Fe3C) and pearlite?
• Relative amounts of
proeutectoid phase (α or Fe3C)
and pearlite can be calculated
by the lever rule with tie line
that extends from the
eutectoid composition (0.76 %
C) to α – (α + Fe3C) boundary
(0.022 % C) for hypoeutectoid
alloys and to (α + Fe3C) – Fe3C
boundary (6.7 % C) for
hypereutectoid alloys.
• Fraction of total α phase is
determined by application of
the lever rule across the entire
(α + Fe3C) phase field.

27. Example for hypereutectoid alloy with
composition C1
Fraction of pearlite: WP = X / (V+X) = (6.7 – C1) / (6.7 – 0.76)
Fraction of proeutectoid cementite: WFe3C = V / (V+X) = (C1 – 0.76) / (6.7 – 0.76)

28. Amount of Phases in Hypoeutectoid Steel
T(°C)
1600
L
1400
(austenite)
1000
800
600
pearlite
Wpearlite = W
+ Fe3C
r s
727°C
RS
400
0
(Fe)C0
+ Fe3C
1
0.76
W = s/(r + s)
W =(1 - W )
L+Fe3C
1148°C
W ’ = S/(R + S)
WFe3C
=(1 – W ’)
pearlite
2
3
4
5
6
(Fe-C
System)
Fe3C (cementite)
1200
+L
6.7
C, wt% C
100 m
Hypoeutectoid
steel
proeutectoid ferrite
Adapted from Fig. 10.34, Callister & Rethwisch 3e.
28

29. Microstructure of Hypereutectoid
Steel
Compositions to the right of eutectoid (0.76 2.14 wt % C) is hypereutectoid (more than
eutectoid) alloys.
γ → γ + Fe3C → P + Fe3C
1. First cementite is formed when
temperature comes down below Acm
temperature.
γ → γ + Fe3C
2. The amount of cementite increases with
decrease in temperature till eutectoid
temperature.
3. Remaining austenite changes to pearlite
at eutectoid temperature.
γ + Fe3C → P + Fe3C

30. Microstructure of Hypereutectoid Steel
T(°C)
1600
L
1400
+L
1000
+Fe3C
800
600
400
0
(Fe)
pearlite
+Fe3C
0.76
Fe3C
L+Fe3C
1148°C
(austenite)
1 C0
2
3
4
5
6
Fe3C (cementite)
1200
6.7
C, wt%C
60 mHypereutectoid
steel
pearlite
proeutectoid Fe3C
Adapted from Fig. 10.37, Callister & Rethwisch 3e.
30

31. Microstructure of hypereutectoid
steel

32. Amounts of Phases Hypereutectoid Steel
T(°C)
1600
L
1400
1200
+L
1000
+Fe3C
W =x/(v + x)
v x
800
600
pearlite
400
0
(Fe)
Wpearlite = W
V
X
+Fe3C
0.76
WFe3C =(1-W )
1 C0
W = X/(V + X)
WFe
3C’
L+Fe3C
1148°C
(austenite)
2
3
4
5
6
Fe3C (cementite)
Fe3C
6.7
C, wt%C
60 mHypereutectoid
steel
=(1 - W )
pearlite
proeutectoid Fe3C
Adapted from Fig. 10.37, Callister & Rethwisch 3e.
32

33. Example Problem Steel
For a 99.6 wt% Fe-0.40 wt% C steel at a
temperature just below the eutectoid,
determine the following:
a) The compositions of Fe3C and ferrite ( ).
b) The amount of cementite (in grams) that
forms in 100 g of steel.
c) The amounts of pearlite and proeutectoid
ferrite ( ) in the 100 g.
33

34. Solution to Problem
a) Use RS tie line just below
Eutectoid
b)
Use lever rule with
the tie line shown
WFe 3C
R
R S
1600
T(°C)
1200
C0 C
CFe 3C C
0.40 0.022
6.70 0.022
L
1400
+L
1000
+ Fe3C
800
727°C
R
0.057
S
+ Fe3C
600
400
0
Amount of Fe3C in 100 g
L+Fe3C
1148°C
(austenite)
Fe3C (cementite)
C = 0.022 wt% C
CFe3C = 6.70 wt% C
C C0
1
2
3
4
C, wt% C
5
6
6.7
CFe
3C
= (100 g)WFe3C
= (100 g)(0.057) = 5.7 g
34

35. Solution to Problem
c) Using the VX tie line just above the eutectoid
and realizing that
C0 = 0.40 wt% C
C = 0.022 wt% C
Cpearlite = C = 0.76 wt% C
V X
T(°C)
C0 C
C C
0.40 0.022
0.76 0.022
L
1400
1200
+L
L+Fe3C
1148°C
(austenite)
1000
+ Fe3C
0.512
800
727°C
VX
Amount of pearlite in 100 g
= (100 g)Wpearlite
= (100 g)(0.512) = 51.2 g
600
400
0
+ Fe3C
1
C C0 C
2
3
4
5
6
Fe C (cementite)
Wpearlite
V
1600
6.7
C, wt% C
35

36. Summary
Fe – C Diagram
• Fe – C diagram is useful to determine:
- the number and types of phases,
- the wt% of each phase,
- and the composition of each phase
for a given T and composition of the steel or cast iron.
36

37. Alloying Steel With More Elements
Ti
Mo
• Ceutectoid changes:
Si
W
Cr
Mn
Ni
wt. % of alloying elements
Adapted from Fig. 10.38,Callister & Rethwisch 3e.
(Fig. 10.38 from Edgar C. Bain, Functions of the
Alloying Elements in Steel, American Society for
Metals, 1939, p. 127.)
Ceutectoid (wt% C)
T Eutectoid (°C)
• Teutectoid changes:
Ni
Cr
Si
Ti Mo
W
Mn
wt. % of alloying elements
Adapted from Fig. 10.39,Callister & Rethwisch 3e.
(Fig. 10.39 from Edgar C. Bain, Functions of the
Alloying Elements in Steel, American Society for
Metals, 1939, p. 127.)
37

38. THANKS