Fe-C diagram

The Iron–Carbon Phase Diagram

Prof. H. K. KhairaProfessor in MSME Deptt.

MANIT, Bhopal

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.

Fe – C Equilibrium Diagram

©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).

Phases Observed in Fe-C Diagram -

• Phases1. Ferrite

2. Austenite

3. Cementite

4. δ-ferrite

• And phase mixtures

1. Pearlite

2. Ledeburite

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.

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%.

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

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

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.

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

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)

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)

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.

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.

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

Pearlite Formation

Growth direction

Pearlite nucleates at austenite grain boundaries and grows into the grain

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.

Development of Microstructure in Iron - Carbon alloys

20

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

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)

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.

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

24

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

Microstructure of Hypoeutectoid SteelHypoeutectoid steels contain proeutectoid ferrite (formedabove the eutectoid temperature) plus the pearlite that contains eutectoid ferrite and cementite.

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.

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

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

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

30

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

Microstructure of hypereutectoid steel

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

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

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

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

36

• 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

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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