1

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

2

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

8

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)

5

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

3

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

10

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

6

The influence of other alloying elements

11

• 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)

7

13

• 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