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The iron-iron carbide (Fe-Fe3C) phase diagram
Ferrite-α -BCC, low C solubility(0.022%wt), magneticAustenite-γ-FCC, high C solubility(2.14%wt), nonmagneticFerrite-δ-BCCCementite (Fe3C)Eutectic, peritectic, eutectoidIron, ferrite (C<0.008wt%)Stainless steel, α+Fe3C (0.008-2.14wt%)
Microstructures of iron
α- ferrite austenite
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Interstitial sites of FCC
Interstitial sites of BCC
3
Microstructure in iron-carbon alloys
Eutectic--pearlite
Hypoeutectoid alloys
Hypoeutectoid steel: has a carbon concentration less than the eutectoid
4
Hypereutectoid alloys
Hypereutectoid steel: has a carbon content greater than the eutectoid
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Example: Phase Equilibria
For a 99.6 wt% Fe-0.40 wt% C at a temperature just below the eutectoid, determine the following
a) composition of Fe3C and ferrite (α)b) the amount of carbide (cementite) in grams that
forms per 100 g of steelc) the amount of pearlite and proeutectoid ferrite (α)
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Phase Equilibria
Solution:
g 3.94g 5.7 CFe
g7.5100 022.07.6022.04.0
100xCFe
CFe
3
CFe3
3
3
=α
=
=−−
=
−−
=α+ α
α
x
CCCCo
b) the amount of carbide (cementite) in grams that forms per 100 g of steel
a) composition of Fe3C and ferrite (α)CO = 0.40 wt% CCα = 0.022 wt% CCFe C = 6.70 wt% C
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Fe3C
(cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
γ (austenite)
γ+L
γ + Fe3C
α + Fe3C
L+Fe3C
δ
Co, wt% C
1148°C
T(°C)
727°C
CO
R S
CFe C3Cα
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Phase Equilibria
c. the amount of pearlite and proeutectoid ferrite (α) note: amount of pearlite = amount of γ just above TE
Co = 0.40 wt% CCα = 0.022 wt% CCpearlite = Cγ = 0.76 wt% C
γγ + α
=Co −CαCγ −Cα
x 100 = 51.2 g
pearlite = 51.2 gproeutectoid α = 48.8 g
Fe3C
(cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
γ (austenite)
γ+L
γ + Fe3C
α + Fe3C
L+Fe3C
δ
Co, wt% C
1148°C
T(°C)
727°C
CO
R S
CγCα
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The influence of other alloying elements
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• Teutectoid changes: • Ceutectoid changes:
T Eut
ecto
id(°
C)
wt. % of alloying elements
Ti
Ni
Mo SiW
Cr
Mn
wt. % of alloying elements
Ceu
tect
oid
(wt%
C)
Ni
Ti
Cr
SiMnWMo
Review Fe-C phase diagram
• 2 important points
-Eutectoid (B):γ ⇒ α +Fe3C
-Eutectic (A):L ⇒ γ +Fe3C
Fe3C
(cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
γ (austenite)
γ+L
γ+Fe3C
α+Fe3C
α+γ
L+Fe3C
δ
(Fe) Co, wt% C
1148°C
T(°C)
α 727°C = Teutectoid
ASR
4.30Result: Pearlite = alternating layers of α and Fe3C phases
120 μm
γ γγγ
R S
0.76
Ceu
tect
oid
B
Fe3C (cementite-hard)α (ferrite-soft)
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• Phase diagrams are useful tools 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 fora range of microstructures.
Summary
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