Material Technology and Testing

(MNF 222)

CHAPTER 5

The Iron Carbon System

1Material Technology and Testing Dr. Gamal Abdou

Fe-Carbon Diagram

Cooling curve for pure iron

Definition of structures

Various phases that appear on the

Iron-Carbon equilibrium phase diagram

are as under:

•Austenite

•Ferrite

•Pearlite

•Cementite

•Martensite*

•Ledeburite

Unit Cells of Various Metals

• FIGURE - The unit cell for (a) austentite, (b) ferrite, and (c) martensite. The

effect of the percentage of carbon (by weight) on the lattice dimensions for

martensite is shown in (d). Note the interstitial position of the carbon atoms and

the increase in dimension c with increasing carbon content. Thus, the unit cell of

martensite is in the shape of a rectangular prism.

IRON CARBON CONSTITUTIONAL DIAGRAM-II

phases in the transformation, occurring with iron-carbon alloys:

L - Liquid solution of carbon in iron;

δ-ferrite – Solid solution of carbon in iron. Maximum

concentration of carbon in δ-ferrite is 0.09% at 2719 ºF (1493ºC)

– temperature of the peritectic transformation. The crystal

structure of δ-ferrite is BCC (cubic body centered).

Austenite – interstitial solid solution of carbon in γ-iron.

Austenite has FCC (cubic face centered) crystal structure,

permitting high solubility of carbon – up to 2.06% at 2097 ºF

(1147 ºC). Austenite does not exist below 1333 ºF (723ºC) and

maximum carbon concentration at this temperature is 0.83%.

α-ferrite – solid solution of carbon in α-iron. α-ferrite has BCC

crystal structure and low solubility of carbon – up to 0.25% at

1333 ºF (723ºC). α-ferrite exists at room temperature.

Cementite – iron carbide, intermetallic compound, having fixed

composition Fe3C.

Upper critical temperature (point) A3 is the temperature, below

which ferrite starts to form as a result of ejection from austenite in

the hypoeutectoid alloys.

Upper critical temperature (point) ACM is the temperature, below

which cementite starts to form as a result of ejection from

austenite in the hypereutectoid alloys.

Lower critical temperature (point) A1 is the temperature of the

austenite-to-pearlite eutectoid transformation. Below this

temperature austenite does not exist.

Magnetic transformation temperature A2 is the temperature

below which α-ferrite is ferromagnetic.

CRITICAL TEMPERATURE

Various Features of Fe-C diagram

Peritectic L + d = g

Eutectic L = g + Fe3C

Eutectoid g = a + Fe3C

Phases present

L

Reactions

dBCC structure

Paramagnetic

g austenite

FCC structure

Non-magnetic

ductile

a ferrite

BCC structure

Ferromagnetic

Fairly ductile

Fe3C cementite

Orthorhombic

Hard

brittle

Max. solubility of C in ferrite=0.022%

Max. solubility of C in austenite=2.11%

Three Phase Reactions

• Peritectic, at 1490 deg.C, with low wt% C

alloys (almost no engineering importance).

• Eutectic, at 1130 deg.C, with 4.3wt% C, alloys

called cast irons.

• Eutectoid, at 723 deg.C with eutectoid

composition of 0.8wt% C, two-phase mixture

(ferrite & cementite). They are steels.

Hypoeutectoid steels (carbon content from 0 to 0.83%)

consist of primary proeutectoid) ferrite (according to the

curve A3) and pearlite.

Eutectoid steel (carbon content 0.83%) entirely consists of

pearlite.

Hypereutectoid steels (carbon content from 0.83 to 2.06%)

consist of primary (proeutectoid) cementite (according to the

curve ACM) and pearlite.

Cast irons (carbon content from 2.06% to 4.3%) consist of

proeutectoid cementite C2 ejected from austenite according

to the curve ACM , pearlite and transformed ledeburite

(ledeburite in which austenite transformed to pearlite.

PHASE COMPOSITIONS AT ROOM TEMP.

The Iron-Iron Carbide Diagram

The diagram shows three horizontal lines which indicate

isothermal reactions (on cooling / heating):

• First horizontal line is at 1490°C, where peritectic

reaction takes place:

Liquid + d ↔ austenite

• Second horizontal line is at 1130°C, where eutectic

reaction takes place:

liquid ↔ austenite + cementite

• Third horizontal line is at 723°C, where eutectoid

reaction takes place:

austenite ↔ pearlite (mixture of ferrite &

cementite)

Delta region of Fe-Fe carbide diagram

Liquid + d ↔ austenite

1538

1493

Ferrite region of

Fe-Fe Carbide

diagram

Simplified Iron-Carbon phase diagram

austenite ↔ pearlite (mixture of ferrite & cementite)

The Austenite to ferrite / cementite transformation in relation to Fe-C diagram

MICROSTRUCTURE OF AUSTENITE

MICROSTRUCTURE OF PEARLITE

Photomicrographs of (a) coarse pearlite (b) fine pearlite. 3000X

MICROSTRUCTURE OF MARTENSITE

20

IRON-CARBON (Fe-C) PHASE DIAGRAM

(EXAMPLE 1)• 2 important

points

- Eutectoid (B):g a +Fe3C

- Eutectic (A):L g +Fe3C

Adapted from Fig. 9.24,

Callister & Rethwisch 8e.

Fe

3C

(ce

me

ntite

)

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

g

(austenite)

g+L

g+Fe3C

a+Fe3C

d

(Fe) C, wt% C

1148ºC

T(ºC)

a727ºC = Teutectoid

4.30

Result: Pearlite = alternating layers of a and Fe3C phases

120 mm

(Adapted from Fig. 9.27,

Callister & Rethwisch 8e.)

0.76

B

g g

gg

A L+Fe3C

Fe3C (cementite-hard)

a (ferrite-soft)

21

EXAMPLE 1

• An alloy of eutectoid composition (0.76 wt% C) as it is

cooled down from a temperature within the g-phase

region (e.g., at 800 ºC).

• Initially the alloy is composed entirely of the austenitic

phase having a composition of 0.76 wt% C

• As the alloy is cooled, no changes will occur until the

eutectoid temperature (727 ºC).

• Upon crossing this temperature to point B, the austenite

transforms according to:

Eutectoid (B):

g (0.76 wt% C) a (0.022 wt% C) + Fe3C (6.7 wt% C)

22

EXAMPLE 1 (cont.)

• The microstructure for this eutectoid steel is slowly

cooled through the eutectoid temperature consists of

alternating layers or lamellae of the two phases (a andFe3C) that form simultaneously during the

transformation.

• Point B is called pearlite.

• Mechanically, pearlite has properties intermediate

between the soft, ductile ferrite and the hard, brittle

cementite.

23

EXAMPLE 1 (cont.)

• The alternating a and Fe3C layers in pearlite form as

such for the same reason that the eutectic structure

forms because the composition of austenite (0.76 %wt

C) is different from either of ferrite (0.022 wt% C) and

cementite (6.70 wt% C), and the phase transformation

requires that there be a redistribution of the carbon by

diffusion.

• Subsequent cooling of the pearlite from point B will

produce relatively insignificant microstructural changes.

24

Fe

3C

(ce

me

ntite

)

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

1148ºC

T(ºC)

a727ºC

(Fe-C

System)

C0

0.7

6

Hypoeutectoid Steel (EXAMPLE 2)

Adapted from Figs. 9.24

and 9.29,Callister &

Rethwisch 8e.

(Fig. 9.24 adapted from

Binary Alloy Phase

Diagrams, 2nd ed., Vol.

1, T.B. Massalski (Ed.-in-

Chief), ASM International,

Materials Park, OH,

1990.)

Adapted from Fig. 9.30, Callister & Rethwisch 8e.

proeutectoid ferritepearlite

100 mmHypoeutectoid

steel

a

pearlite

g

g g

ga

aa

ggg g

g g

gg

25

EXAMPLE 2 (cont.)

• Within the a + g region, most of the a particles will form

along the original g grain boundaries.

• The particles will grow larger just above the eutectoidline. As the temperature is lowered below T

e, all the g

phase will transform to pearlite according to:

• There will be virtually no change in the a phase thatexisted just above the T

e.

• This a that is formed above Te

is called proeutectoid

(pro=pre=before eutectoid) ferrite.

g a +Fe3C

26

EXAMPLE 2 (cont.)

• The ferrite that is present in the pearlite is called

eutectoid ferrite.

• As a result, two microconstituents are present in thelast micrograph (the one below T

e): proeutectoid ferrite

and pearlite

27

Fe

3C

(ce

me

ntite

)

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

1148ºC

T(ºC)

a727ºC

(Fe-C

System)

C0

0.7

6

EXAMPLE 2

g

g 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 Figs. 9.24

and 9.29,Callister &

Rethwisch 8e.

(Fig. 9.24 adapted from

Binary Alloy Phase

Diagrams, 2nd ed., Vol.

1, T.B. Massalski (Ed.-in-

Chief), ASM International,

Materials Park, OH,

1990.)

Adapted from Fig. 9.30, Callister & Rethwisch 8e.

proeutectoid ferritepearlite

100 mmHypoeutectoid

steel

28

HYPEREUTECTOID STEEL (EXAMPLE 3)

Fe

3C

(ce

me

ntite

)

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

1148ºC

T(ºC)

a

Adapted from Figs. 9.24

and 9.32,Callister &

Rethwisch 8e. (Fig. 9.24

adapted from Binary Alloy

Phase Diagrams, 2nd

ed., Vol. 1, T.B. Massalski

(Ed.-in-Chief), ASM

International, Materials

Park, OH, 1990.)

(Fe-C

System)

0.7

6 C0

Fe3C

gg

g g

ggg g

ggg g

Adapted from Fig. 9.33, Callister & Rethwisch 8e.

proeutectoid Fe3C

60 mmHypereutectoid steel

pearlite

pearlite

29

Fe

3C

(ce

me

ntite

)

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

1148ºC

T(ºC)

a

EXAMPLE 3 (cont.)

(Fe-C

System)

0.7

6 C0

pearlite

Fe3C

gg

g 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. 9.33, Callister & Rethwisch 8e.

proeutectoid Fe3C

60 mmHypereutectoid steel

pearlite

Adapted from Figs. 9.24

and 9.32,Callister &

Rethwisch 8e. (Fig. 9.24

adapted from Binary Alloy

Phase Diagrams, 2nd

ed., Vol. 1, T.B. Massalski

(Ed.-in-Chief), ASM

International, Materials

Park, OH, 1990.)

30

PROBLEM

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 (a).

b) The amount of cementite (in grams) that forms in 100 g

of steel.

c) The amounts of pearlite and proeutectoid ferrite (a) in

the 100 gr.

31

SOLUTION TO PROBLEM

WFe3C R

R + S

C0 Ca

CFe3C Ca

0.40 0.022

6.70 0.022 0.057

b) Using the lever rule with

the tie line shown

a) Using the RS tie line just below the eutectoid

Ca = 0.022 wt% C

CFe3C = 6.70 wt% C

Fe

3C

(ce

me

ntite

)

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

1148ºC

T(ºC)

727ºC

C0

R S

CFe C3Ca

Amount of Fe3C in 100 g

= (100 g)WFe3C

= (100 g)(0.057) = 5.7 g

32

c) Using the VX tie line just above the eutectoid and realizing

that

C0 = 0.40 wt% C

Ca = 0.022 wt% C

Cpearlite = Cg = 0.76 wt% C

Fe

3C

(ce

me

ntite

)

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

1148ºC

T(ºC)

727ºC

C0

V X

CgCa

Wpearlite V

V + X

C0 Ca

Cg Ca

0.40 0.022

0.76 0.022 0.512

Amount of pearlite in 100 g

= (100 g)Wpearlite

= (100 g)(0.512) = 51.2 g

SOLUTION TO PROBLEM (CONT.)

33

ALLOYING WITH OTHER ELEMENTS

• Teutectoid changes:

Adapted from Fig. 9.34,Callister & Rethwisch 8e.

(Fig. 9.34 from Edgar C. Bain, Functions of the

Alloying Elements in Steel, American Society for

Metals, 1939, p. 127.)

TE

ute

cto

id(º

C)

wt. % of alloying elements

Ti

Ni

MoSi

W

Cr

Mn

• Ceutectoid changes:

Adapted from Fig. 9.35,Callister & Rethwisch 8e.

(Fig. 9.35 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(w

t% C

)

Ni

Ti

Cr

SiMn

WMo