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![Page 1: fe-c diagram](https://reader036.fdocuments.net/reader036/viewer/2022081418/558cf14bd8b42a95318b4780/html5/thumbnails/1.jpg)
The Iron–Carbon Phase Diagram
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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
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Phases in Fe–C Phase Diagram• α-ferrite - solid solution of C in BCC Fe
• Stable form of iron at room temperature.• The maximum solubility of C is 0.025 % at eutectoid temperature• Transforms to FCC γ-austenite at 910°C
• γ-austenite - solid solution of C in FCC Fe• The maximum solubility of C is 2.14 % at eutectic temperature• Transforms to BCC δ-ferrite at 1395 °C• Is not stable below the eutectoid temperature(727 ° C) unless cooled rapidly
• δ-ferrite solid solution of C in BCC Fe• The same structure as α-ferrite• Stable only at high Temp., above 1394 °C• Melts at 1538 °C
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Phases in Fe–C Phase Diagram
• Fe3C (iron carbide or cementite)
It is an intermetallic compound between iron and carbon. It contains 6.67 % carbon and is hard and brittle.This intermetallic compound is a metastablephase and it remains as a compound indefinitely at room temperature.
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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) - BCC has relatively small interstitial positions
• Maximum solubility in FCC austenite is 2.14 % at 1147 °C - 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, how ferrite and cementite are distributed.
• Magnetic properties: α -ferrite is magnetic below 768 °C, austenite is non-magnetic
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Classification
• Three types of ferrous alloy• Iron: less than 0.008% C in α−ferrite at room temp.• Steels: 0.008 - 2.14% C (usually < 1%) α-ferrite + Fe3C at room temp.
• Cast iron: 2.14 - 6.7% (usually < 4.5%)
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Eutectic and eutectoid reactions in Fe–C
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
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• Eutectic and eutectoid reactions are very important in heat treatment of steels
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Development of Microstructure in Iron - Carbon alloys
• Microstructure depends on composition (carbon content) and heat treatment. In the discussion below we consider slow cooling in which equilibrium is maintained.
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12
Iron-Carbon (Fe-C) Phase Diagram
• 2 points
- Eutectoid (B):
g Þ a +Fe3C
- 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
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Microstructure of eutectoid steel
• When alloy of eutectoid composition (0.76 wt % C) is cooled slowly it forms perlite, 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 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 eutectoid steelIn the micrograph, the dark areas areFe3C layers, the light phase is α-ferrite
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Microstructure of hypoeutectoid steel
Compositions to the left of eutectoid (0.022 - 0.76 wt % C) is hypoeutectoid (less than eutectoid -Greek) alloys.
γ → α + γ → α + Fe3C
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17
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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18
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 hypoeutectoid steelHypoeutectoid alloys contain proeutectoid ferrite (formedabove the eutectoid temperature) plus the eutectoid perlitethat contain eutectoid ferrite and cementite.
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Microstructure of hypereutectoid steel
Compositions to the right of eutectoid (0.76 - 2.14 wt % C) is hypereutectoid (more than eutectoid -Greek) alloys.
γ → γ + Fe3C → α + Fe3C
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Microstructure of hypereutectoid steel
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22
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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23
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
Hypereutectoid Steel
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Relative amounts of proeutectoidphase (α or Fe3C) and pearlite?
Application of the lever rule with tie line that extends from the eutectoid composition (0.76 wt% C) to α – (α + Fe3C) boundary (0.022 wt% C) for hypoeutectoid alloys and to (α + Fe3C) – Fe3C boundary (6.7 wt% C) for hypereutectoid alloys.
Fraction of 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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26
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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27
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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28
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
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29
• Phase (T vs c) diagrams are 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 system.
• Alloying to produce a solid solution usually
- increases the tensile strength (TS) - decreases the ductility.
• Binary eutectics and binary eutectoids allow for a range of microstructures.
- Alloy composition and annealing temperature and time determine possible microstructure(s). Slow vs. Fast.- In 2-phase regions, use “lever rule” to get wt% of phases.- Varying microstructure affects mechanical properties.
SummaryPhase Diagrams and Microstructures
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30
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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