Phase transformations

Fe 3

C (

cem

entit

e)

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

(austenite)

+L

+ Fe3C

+ Fe3C

L+Fe3C

Co , wt% C

1148°C

T(°C)

727°C

Diffusive flow of C needed

• Growth of pearlite from austenite:

pearlite growth direction

Austenite ()grain boundary

cementite (Fe3C)

Ferrite ()

675°C

(T smaller)

0

50

y (%

pea

rlite

)

600°C

(T larger)650°C

100

Transformations & Supercooling

+ Fe3C

0.76 wt% C0.022 wt% C

6.7 wt% C

Fe 3

C (

cem

entit

e)

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

(austenite)

+L

+Fe3C

+Fe3C

L+Fe3C

(Fe) Co , wt%C

1148°C

T(°C)

ferrite

727°C

Eutectoid:

T

0.7

6

0.0

22

Isothermal Transformation Diagrams100

50

01 10 2 10 4

T = 675°Cy,

% tr

ansf

orm

ed

time (s)

400

500

600

700

1 10 10 2 10 3 10 4 10 50%

pearlite100%

50%

Austenite (stable) TE (727C)Austenite

(unstable)

Pearlite

T(°C)

time (s)

• Eutectoid composition, Co = 0.76 wt% C.

Effect of Cooling History in Fe-C System

400

500

600

700

0%pearlite

100%

50%

Austenite (stable)TE (727C)

Austenite

(unstable)

Pearlite

T(°C)

1 10 102 103 104 105

time (s)

Transformations with Proeutectoid Materials

CO = 1.13 wt% C

TE (727°C)

T(°C)

time (s)

A

A

A+

C

P

1 10 102 103 104

500

700

900

600

800

A+

P

Fe 3

C (

cem

entit

e)

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

(austenite)

+L

+Fe3C

+Fe3C

L+Fe3C

(Fe) Co , wt%C

T(°C)

727°CT

0.7

6

0.0

22

1.13

Non-Equilibrium Transformation Products: Fe-C

• Bainite: -- strips with long

needles of Fe3C --diffusion controlled.• Isothermal Transf. Diagram

FeFe33CC

(cementite)(cementite)

5 m

(ferrite)

10 103 105

time (s)10-1

400

600

800

T(°C)Austenite (stable)

200

P

B

TE

0%

100%

50%

pearlite/bainite boundary

A

A

Spheroidite: Fe-C System

(ferrite)

60 m

(cementite)

Fe3C

• Martensite: --(FCC) to Martensite (BCT)

• Isothermal Transf. Diagram

Martensite: Fe-C System

Martensite needlesAustenite

60

m

10 103 105 t/s10-1

400

600

800

T(°C)Austenite (stable)

200

P

B

TE

0%

100%50%

A

A

M + AM + A

M + A

0%50%90%

xx x

xx

xpotential C atom sites

Fe atom sites

(involves single atom jumps)

Phase Transformations of Alloys

Dynamic Phase Transformations

On the isothermal transformation diagram for 0.45 wt% C Fe-C alloy, sketch and label the time-temperature paths to produce the following microstructures:a) 42% proeutectoid ferrite and 58% coarse pearlite

b) 50% fine pearlite and 50% bainite

c) 100% martensite

d) 50% martensite and 50% austenite

Example Problem for Co = 0.45 wt%

A + B

A + P

A + A

BP

A50%

0

200

400

600

800

0.1 10 103 105

time (s)

M (start)

M (50%)

M (90%)

T (°C)

Example Problem for Co = 0.45 wt%

A + B

A + P

A + A

BP

A50%

0

200

400

600

800

0.1 10 103 105

time (s)

M (start)

M (50%)

M (90%)

T (°C)

Example Problem for Co = 0.45 wt%

A + B

A + P

A + A

BP

A50%

0

200

400

600

800

0.1 10 103 105

time (s)

M (start)

M (50%)

M (90%)

T (°C)

Mechanical Prop: Fe-C System (1)

• Effect of wt% C

Co < 0.76 wt% C

Hypoeutectoid

Pearlite (med)ferrite (soft)

Co > 0.76 wt% C

Hypereutectoid

Pearlite (med)Cementite

(hard)

300

500

700

900

1100

YS

TS(MPa)

wt% C0 0.5 1

hardness

0.7

6

Hypo Hyper

wt% C0 0.5 1

0

50

100%EL

Imp

act

en

erg

y (I

zod

, ft

-lb

)

0

40

80

0.7

6

Hypo Hyper

TS

EL

Izod

Mechanical Prop: Fe-C System (2)

• Fine vs coarse pearlite vs spheroidite

80

160

240

320

wt%C0 0.5 1

Bri

ne

ll h

ard

ne

ss

fine pearlite

coarse pearlitespheroidite

Hypo Hyper

0

30

60

90

wt%CD

uct

ility

(%

AR

)

fine pearlite

coarse pearlite

spheroidite

Hypo Hyper

0 0.5 1

Mechanical Prop: Fe-C System (3)• Fine Pearlite vs Martensite:

0

200

wt% C0 0.5 1

400

600

Bri

ne

ll h

ard

ne

ss martensite

fine pearlite

Hypo Hyper

Tempering Martensite

9 m

MPa

800

1000

1200

1400

1600

1800

30

40

50

60

200 400 600

Tempering T (°C)

%RA

TS

YS

%RA

Summary: Processing Options

Austenite ()

Bainite( + Fe3C plates/needles)

Pearlite( + Fe3C layers + a proeutectoid phase)

Martensite(BCT phase diffusionless

transformation)

Tempered Martensite ( + very fine Fe3C particles)

slow cool

moderate cool

rapid quench

reheat

Str

engt

h

Duc

tility

Martensite T Martensite

bainite fine pearlite

coarse pearlite spheroidite

General Trends