fe-c diagram
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![Page 1: Fe-C diagram](https://reader034.vdocuments.site/reader034/viewer/2022050708/554a4240b4c90582328b52a3/html5/thumbnails/1.jpg)
The Iron–Carbon Phase Diagram
Prof. H. K. KhairaProfessor in MSME Deptt.
MANIT, Bhopal
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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.
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Fe – C Equilibrium Diagram
![Page 4: Fe-C diagram](https://reader034.vdocuments.site/reader034/viewer/2022050708/554a4240b4c90582328b52a3/html5/thumbnails/4.jpg)
©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).
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Phases Observed in Fe-C Diagram -
• Phases1. Ferrite
2. Austenite
3. Cementite
4. δ-ferrite
• And phase mixtures
1. Pearlite
2. Ledeburite
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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 non-magnetic 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.
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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. LedeburiteLedeburite is an eutetcic mixture of austenite and cementite in the form of alternate layers. The average carbon content in ledeburite is 4.3%.
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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
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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
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Fe-C Alloys
• Fe-C alloys can be of two types.1. SteelsSteels 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.
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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
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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)
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Important Reactions in Fe–C System
Eutectic: 4.30 wt% C, 1147 °C L (4.30% C) ↔ γ (2.14% C) + Fe3C
Eutectoid: 0.76 wt%C, 727 °Cγ(0.76% C) ↔ α (0.022% C) + Fe3C
Peritectic: 0.16% C, 14930 Cδ(0.11% C) + L(0.51)%C ↔ γ (0.16%C)
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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.
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Eutectoid Reaction
• Eutectoid reaction: at 0.76 %C and 727 °Cγ(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.
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Microstructure of Eutectoid SteelIn the micrograph, the dark areas areFe3C layers, the light phase is α- ferrite
Pearlite nucleates at austenite grain boundaries and grows into the grain
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Pearlite Formation
Growth direction
Pearlite nucleates at austenite grain boundaries and grows into the grain
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Peritectic Reaction
• Peritectic reaction: at 0.16% C and 14930 Cδ(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.
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Development of Microstructure in Iron - Carbon alloys
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Iron-Carbon (Fe-C) Phase Diagram
• 2 important points
2. Eutectoid (B):
g Þ a +Fe3C
1. Eutectic (A):
L Þ g + Fe3C
Result: Pearlite is alternating layers of and Fe3C phases a
120 mm
A
Fe 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
g(austenite)
g+L
g+Fe3C
a+Fe3C
a+g
d
(Fe) C, wt% C
1148°C
T(°C)
a 727°C = Teutectoid
Adapted from Fig. 10.28,Callister & Rethwisch 3e.
4.30
g ggg
AL+Fe3C
Fe3C (cementite-hard)a (ferrite-soft)
0.76
B
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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)
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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.
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Microstructure of Hypoeutectoid SteelCompositions to the left of eutectoid (0.022 - 0.76 wt % C) is hypoeutectoid (less than eutectoid) alloys. Microstructure change is
γ → α + γ → α + P1. First ferrite is formed when temperature comes down below Ae3 temperature.
γ → α + γ2. 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
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Microstructure of Hypoeutectoid Steel
Adapted from Fig. 10.34, Callister & Rethwisch 3e.
proeutectoid ferritepearlite
100 mm Hypoeutectoidsteel
Fe 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
g(austenite)
g+L
g + Fe3C
a + Fe3C
L+Fe3C
d
(Fe) C, wt% C
1148°C
T(°C)
a727°C
(Fe-C System)
C0
0.76
a
pearlite
gg g
ga
aa
ggg g
g ggg
Adapted from Figs. 10.28 and 10.33
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Microstructure of Hypoeutectoid SteelHypoeutectoid steels contain proeutectoid ferrite (formedabove the eutectoid temperature) plus the pearlite that contains eutectoid ferrite and cementite.
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Relative amounts of proeutectoidphase (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.
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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)
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Amount of Phases in Hypoeutectoid Steel
Fe 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
g(austenite)
g+L
g + Fe3C
a + Fe3C
L+Fe3C
d
(Fe) C, wt% C
1148°C
T(°C)
a727°C
(Fe-C System)
C0
0.76
gg g
ga
aa
srWa = s/(r + s)
Wg =(1 - Wa)R S
a
pearlite
Wpearlite = Wg
Wa’ = S/(R + S)
W =(1 – Wa’)Fe3C
Adapted from Fig. 10.34, Callister & Rethwisch 3e.
proeutectoid ferritepearlite
100 mm Hypoeutectoidsteel
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Microstructure of Hypereutectoid SteelCompositions to the right of eutectoid (0.76 - 2.14 wt % C) is hypereutectoid (more than eutectoid) alloys.
γ → γ + Fe3C → P + Fe3C1. First cementite is formed when temperature comes down below Acm temperature.
γ → γ + Fe3C2. 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
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Microstructure of Hypereutectoid Steel
Fe 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
g(austenite)
g+L
g +Fe3C
a +Fe3C
L+Fe3C
d
(Fe) C, wt%C
1148°C
T(°C)
a
0.7
6 C0
Fe3C
ggg g
ggg g
ggg g
Adapted from Fig. 10.37, Callister & Rethwisch 3e.
proeutectoid Fe3C
60 mmHypereutectoid steel
pearlite
pearlite
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Microstructure of hypereutectoid steel
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32
Fe 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
g(austenite)
g+L
g +Fe3C
a +Fe3C
L+Fe3C
d
(Fe) C, wt%C
1148°C
T(°C)
a
0.7
6 C0
pearlite
Fe3C
ggg g
xv
V X
Wpearlite = Wg
Wa = X/(V + X)
W =(1 - Wa)Fe3C’
W =(1-Wg)
Wg =x/(v + x)
Fe3C
Adapted from Fig. 10.37, Callister & Rethwisch 3e.
proeutectoid Fe3C
60 mmHypereutectoid steel
pearlite
Amounts of Phases Hypereutectoid Steel
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33
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.
Example Problem Steel
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34
Solution to Problem
WFe3C R
R S
C0 C
CFe3C C
0.40 0.0226.70 0.022
0.057
b) Use lever rule with the tie line shown
a) Use RS tie line just below Eutectoid
Ca = 0.022 wt% C
CFe3C = 6.70 wt% C
Amount of Fe3C in 100 g
= (100 g)WFe3C
= (100 g)(0.057) = 5.7 g
Fe 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
g(austenite)
g+L
g + Fe3C
a + Fe3C
L+Fe3C
d
C , wt% C
1148°C
T(°C)
727°C
C0
R S
CFe C3C
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35
Solution to Problem
c) Using the VX tie line just above the eutectoid and realizing that
C0 = 0.40 wt% C
Ca = 0.022 wt% C
Cpearlite = C = 0.76 wt% C
Wpearlite V
V X
C0 C
C C
0.40 0.0220.76 0.022
0.512
Amount of pearlite in 100 g = (100 g)Wpearlite = (100 g)(0.512) = 51.2 g
Fe 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
g(austenite)
g+L
g + Fe3C
a + Fe3C
L+Fe3C
d
C, wt% C
1148°C
T(°C)
727°C
C0
V X
CC
![Page 36: Fe-C diagram](https://reader034.vdocuments.site/reader034/viewer/2022050708/554a4240b4c90582328b52a3/html5/thumbnails/36.jpg)
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• 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.
SummaryFe – C Diagram
![Page 37: Fe-C diagram](https://reader034.vdocuments.site/reader034/viewer/2022050708/554a4240b4c90582328b52a3/html5/thumbnails/37.jpg)
37
Alloying Steel With More Elements
• Teutectoid changes:
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.)
TE
ute
cto
id (
°C)
wt. % of alloying elements
Ti
Ni
Mo SiW
Cr
Mn
• Ceutectoid changes:
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.)
wt. % of alloying elements
Ce
ute
cto
id (
wt%
C)
Ni
Ti
Cr
SiMn
WMo
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