Chapter 10: Phase transformations in metalsChapter 10: Phase transformations in metals

Simple diffusion-dependentNo change in either the number or composition of phases present, e.g., solidification of pure metal, allotropic transformation, recrystallization and grain growth.

Other diffusion dependent transformationsSome changes in phase composition and the number of phases present, e.g., eutectoid reaction.

Diffusion-less transformationsMetastable phase is produced, e.g., martensitic transformation.

33

• Can make it occur at: ...727ºC (cool it slowly) ...below 727ºC (“undercool” it!)

• Eutectoid transf. (Fe-C System):

ferrite

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6

.7

L

austenite

+L

+Fe3C

Fe3C cementite+Fe3C

+

L+Fe3C

(Fe) Co, wt% C

Eutectoid:

0.7

7

727°C

T(°C)

T

0.0

22

Fe3C0.77wt%C

0.022wt%C6.7wt%C

Undercooling by T: Ttransf. < 727ºC

Equil. cooling: Ttransf. = 727ºC

Adapted from Fig. 9.21,Callister 6e. (Fig. 9.21 adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.)

TRANSFORMATIONS & UNDERCOOLINGTRANSFORMATIONS & UNDERCOOLING

4

pearlite growth direction

Austenite () grain boundary

cementite (Fe3C)

ferrite ()

Diffusive flow of C needed

• Growth of pearlite from austenite:

• Reaction rate increases with T.

Adapted from Fig. 9.13, Callister 6e.

Adapted from Fig. 10.3, Callister 6e.

EUTECTOID TRANSFORMATION RATEEUTECTOID TRANSFORMATION RATE

675°C (T smaller)

1 10 102 103time (s)

0

50

100

y (

% p

earl

ite)

0

50

100

600°C (T larger)

650°C

% a

ust

enit

e

5

• Reaction rate is a result of nucleation and growth of crystals.

• Examples:

% Pearlite

0

50

100

Nucleation regime

Growth regime

log (time)t50

Nucleation rate increases w/ T

Growth rate increases w/ T

Nucleation rate high

T just below TE T moderately below TE T way below TENucleation rate low

Growth rate high

pearlite colony

Nucleation rate med Growth rate med. Growth rate low

Adapted fromFig. 10.1, Callister 6e.

NUCLEATION AND GROWTHNUCLEATION AND GROWTH

8

10m

- Smaller T: colonies are larger

- Larger T: colonies are smaller

• Ttransf just below TE --Larger T: diffusion is faster --Pearlite is coarser.

Two cases:• Ttransf well below TE --Smaller T: diffusion is slower --Pearlite is finer.

Adapted from Fig. 10.6 (a) and (b),Callister 6e. (Fig. 10.6 from R.M. Ralls et al., An Introduction to Materials Science and Engineering, p. 361, John Wiley and Sons, Inc., New York, 1976.)

PEARLITE MORPHOLOGYPEARLITE MORPHOLOGY

Iron-Carbon alloy of other compositionIron-Carbon alloy of other composition

9

Bainite reaction rate:

rbainitee Q /RT

• Bainite: -- lathes (strips) with long rods of Fe3C --diffusion controlled.• Isothermal Transf. Diagram

Adapted from Fig. 10.9,Callister 6e.(Fig. 10.9 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.)

(Adapted from Fig. 10.8, Callister, 6e. (Fig. 10.8 from Metals Handbook, 8th ed.,Vol. 8, Metallography, Structures, and Phase Diagrams, American Society for Metals, Materials Park, OH, 1973.)

Fe3C

(cementite)

5 m

(ferrite)

NON-EQUIL TRANSFORMATION PRODUCTS: Fe-CNON-EQUIL TRANSFORMATION PRODUCTS: Fe-C

10 103 105

time (s)10-1

400

600

800

T(°C)Austenite (stable)

200

P

B

TE

0%

100%

50%

100% bainite

pearlite/bainite boundary100% pearlite

A

A

Bainite

I-T Diagram

(Isothermal Cooling Diagram)

or also called

T-T-T- Diagram

(Time-Temperature-Transformation Diagram)

10

60 m

(ferrite)

Fe3C

(cementite)

• Spheroidite: -- crystals with spherical Fe3C --diffusion dependent. --heat bainite or pearlite for long times --reduces interfacial area (driving force)• Isothermal Transf. Diagram

Adapted from Fig. 10.9,Callister 6e.(Fig. 10.9 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.)

(Adapted from Fig. 10.10, Callister, 6e. (Fig. 10.10 copyright United States Steel Corporation, 1971.)

OTHER PRODUCTS: Fe-C SYSTEM (1)OTHER PRODUCTS: Fe-C SYSTEM (1)

10 103 105time (s)10-1

400

600

800

T(°C)Austenite (stable)

200

P

B

TE

0%

100%

50%

A

A

Spheroidite100% spheroidite

100% spheroidite

Spheroidite

11

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

Adapted from Fig. 10.13, Callister 6e.

(Adapted from Fig. 10.12, Callister, 6e. (Fig. 10.12 courtesy United States Steel Corporation.)

• Isothermal Transf. Diagram

xx x

xx

xpotential C atom sites

Fe atom sites

(involves single atom jumps)

time (s)10 103 10510-1

400

600

800

T(°C)Austenite (stable)

200

P

B

TE

0%

100%50%

A

A

S

M + AM + A

M + A

0%50%90%

Martentite needlesAustenite

60

m

• to M transformation.. -- is rapid! -- % transf. depends on T only.

(Adapted from Fig. 10.11, Callister, 6e.

OTHER PRODUCTS: Fe-C SYSTEM (2)OTHER PRODUCTS: Fe-C SYSTEM (2)

12

Adapted from Fig. 10.15, Callister 6e.

COOLING EX: Fe-C SYSTEM (1)COOLING EX: Fe-C SYSTEM (1)• Co = Ceutectoid• Three histories...

time (s)10 103 10510-1

400

600

800

T(°C)Austenite (stable)

200

P

B

0%

100%50%

A

S

M + AM + AM + A

0%50%90%

100% Bainite

A

100%A 100%B

Case I

Rapid cool to:

350°C

250°C

650°C

Hold for:

104s

102s

20s

Rapid cool to:

Troom

Troom

400°C

Hold for:

104s

102s

103s

Rapid cool to:

Troom

Troom

Troom

• Co = Ceutectoid• Three histories...

time (s)10 103 10510-1

400

600

800

T(°C)Austenite (stable)

200

P

B

0% 100%50%

A

S

M + AM + AM + A

0%50%90%

M + trace of A

A

100%A

Case II

Rapid cool to:

350°C

250°C

650°C

Hold for:

104s

102s

20s

Rapid cool to:

Troom

Troom

400°C

Hold for:

104s

102s

103s

Rapid cool to:

Troom

Troom

Troom

13

Adapted from Fig. 10.15, Callister 6e.

COOLING EX: Fe-C SYSTEM (2)COOLING EX: Fe-C SYSTEM (2)

14

Adapted from Fig. 10.15, Callister 6e.

COOLING EX: Fe-C SYSTEM (3)COOLING EX: Fe-C SYSTEM (3)Rapid cool to:

350°C

250°C

650°C

Hold for:

104s

102s

20s

Rapid cool to:

Troom

Troom

400°C

Hold for:

104s

102s

103s

Rapid cool to:

Troom

Troom

Troom

• Co = Ceutectoid• Three histories...

time (s)10 103 10510-1

400

600

800T(°C)

Austenite (stable)

200

P

B

0%

100%50%

A

S

M + AM + AM + A

0%50%90%

50%P, 50%B

A

50%P, 50%A

50%P, 50%A

100%A

50%P, 50%B

Case III

Isothermal transformation diagram for 4340 steelIsothermal transformation diagram for 4340 steel

15

Adapted from Fig. 10.20, Callister 6e. (Fig. 10.20 based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, p. 9.)

Adapted from Fig. 9.27,Callister6e. (Fig. 9.27 courtesy Republic Steel Corporation.)

Adapted from Fig. 9.30,Callister 6e. (Fig. 9.30 copyright 1971 by United States Steel Corporation.)

MECHANICAL PROP: Fe-C SYSTEM (1)MECHANICAL PROP: Fe-C SYSTEM (1)

• Effect of wt%C

• More wt%C: TS and YS increase, %EL decreases.wt%C

0 0.5 10

50

100%EL

Impact

energ

y (

Izod,

ft-lb)

0

40

80

300

500

700

900

1100YS(MPa)TS(MPa)

wt%C0 0.5 1

hardness

0.7

7

0.7

7

Co>0.77wt%C Hypereutectoid

Co<0.77wt%C Hypoeutectoid

Pearlite (med)ferrite (soft)

Pearlite (med)Cementite

Hypo HyperHypo Hyper

(hard)

16

Adapted from Fig. 10.21, Callister 6e. (Fig. 10.21 based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, pp. 9 and 17.)

MECHANICAL PROP: Fe-C SYSTEM (2)MECHANICAL PROP: Fe-C SYSTEM (2)

• Fine vs coarse pearlite vs spheroidite

• Hardness: fine > coarse > spheroidite • %AR: fine < coarse < spheroidite

80

160

240

320

wt%C0 0.5 1

Bri

nell

hard

ness

fine pearlite

coarse pearlitespheroidite

0

30

60

90

wt%C0 0.5 1

Duct

ility

(%

AR

)

fine pearlite

coarse pearlite

spheroidite

Hypo Hyper Hypo Hyper

17

• Fine Pearlite vs Martensite:

• Hardness: fine pearlite << martensite.

Adapted from Fig. 10.23, Callister 6e. (Fig. 10.23 adapted from Edgar C. Bain, Functions of the Alloying Elements in Steel, American Society for Metals, 1939, p. 36; and R.A. Grange, C.R. Hribal, and L.F. Porter, Metall. Trans. A, Vol. 8A, p. 1776.)

MECHANICAL PROP: Fe-C SYSTEM (3)MECHANICAL PROP: Fe-C SYSTEM (3)

0

200

wt%C0 0.5 1

400

600

Bri

nell

hard

ness

martensite

fine pearlite

Hypo Hyper

18

• reduces brittleness of martensite,• reduces internal stress caused by quenching.

Adapted from Fig. 10.24, Callister 6e. (Fig. 10.24 copyright by United States Steel Corporation, 1971.)

Adapted from Fig. 10.25, Callister 6e. (Fig. 10.25 adapted from Fig. furnished courtesy of Republic Steel Corporation.)

TEMPERING MARTENSITETEMPERING MARTENSITE

• decreases TS, YS but increases %AR

YS(MPa)TS(MPa)

800

1000

1200

1400

1600

1800

304050

60

200 400 600Tempering T (°C)

%AR

TS

YS

%AR

9

m

• produces extremely small Fe3C particles surrounded by

19

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

ength

Duct

ilit

yMartensite

T Martensite bainite

fine pearlite coarse pearlite

spheroidite

General Trends

Adapted from Fig. 10.27, Callister 6e.

SUMMARY: PROCESSING OPTIONSSUMMARY: PROCESSING OPTIONS