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Chapter 10 - 1 Fe 3 C (cementite) 1600 1400 1200 1000 800 600 400 0 1 2 3 4 5 6 6.7 L γ (austenite) γ +L γ +Fe 3 C α +Fe 3 C L+Fe 3 C δ (Fe) C o , wt%C 1148°C T(°C) a Chapter 10: Phase Transformations

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Chapter 10 - 1

Fe3C

(cem

entit

e)

1600

1400

1200

1000

800

600

400 0 1 2 3 4 5 6 6.7

L γ

(austenite) γ +L

γ +Fe3C

α +Fe3C

L+Fe3C

δ

(Fe) Co , wt%C

1148°C

T(°C)

a

Chapter 10: Phase Transformations

Chapter 10 - 2

• Transforming one phase into another takes time.

• How does the rate of transformation depend on time and T ? • How can we slow down the transformation so that we can engineer non-equilibrium structures? • Are the mechanical properties of non-equilibrium structures better?

Fe γ

(Austenite) Eutectoid

transformation C FCC

Fe3C (cementite)

α (ferrite)

+ (BCC)

Chapter 10: Phase Transformations

Chapter 10 -

t=time! T=temperature!

ΔT= temperature difference from phase transition temperature

3

Chapter 10 - 4

Phase Transformations Nucleation

–  nuclei (seeds) act as template to grow crystals Driving force to nucleate increases as we increase ΔT In general, rate of nucleation higher with higher Δ T

–  supercooling (eutectic, eutectoid)

Small supercooling few nuclei - large crystals Large supercooling rapid nucleation - many nuclei,

small crystals

Growth Growth rate increases with T (thermally activated)

dG/dt= C exp (-Q/kT)

Chapter 10 -

Chapter 10 - 6

Rate of Phase Transformations

Kinetics - measure approach to equilibrium vs. time

•  Hold temperature constant & measure conversion vs. time

sound waves – one sample electrical conductivity – follow one sample X-ray diffraction – have to do many samples

How is conversion measured?

Chapter 10 - 7

Rate of Phase Transformation

Avrami rate equation => y = 1- exp (-ktn)

–  k & n fit for specific sample

All out of material - done

log t Frac

tion

trans

form

ed, y

Fixed T

fraction transformed

time

0.5

By convention r = 1 / t0.5

Adapted from Fig. 10.10, Callister 7e.

maximum rate reached – now amount unconverted decreases so rate slows

t0.5 rate increases as surface area increases & nuclei grow

Chapter 10 - 8

Eutectoid Transformation Rate

Course pearlite formed at higher T - softer Fine pearlite formed at low T - harder

Diffusive flow of C needed

α

α γ γ

α

• Growth of pearlite from austenite:

Adapted from Fig. 9.15, Callister 7e.

γ α α α α

α

α

pearlite growth direction

Austenite (γ) grain boundary

cementite (Fe3C) Ferrite (α)

γ

• Recrystallization rate increases with ΔT. Adapted from

Fig. 10.12, Callister 7e.

675°C (ΔT smaller)

0

50

y (%

pea

rlite

) 600°C

(ΔT larger) 650°C

100

Chapter 10 - 9

Fe3C

(cem

entit

e)

1600

1400

1200

1000

800

600

400 0 1 2 3 4 5 6 6.7

L γ

(austenite) γ +L

γ +Fe3C

α +Fe3C

L+Fe3C

δ

(Fe) Co , wt%C

1148°C

T(°C)

a

Chapter 10 - 10

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

• Examples:

Adapted from Fig. 10.10, Callister 7e.

Nucleation and Growth

% Pearlite

0

50

100

Nucleation regime

Growth regime

log (time) t 0.5

Nucleation rate increases with ΔT Growth rate increases with T

T just below TE Nucleation rate low Growth rate high

γ pearlite colony

T moderately below TE

γ

Nucleation rate med . Growth rate med.

Nucleation rate high T way below TE

γ

Growth rate low

Chapter 10 - 11

Isothermal Transformation Diagrams • Fe-C system, Co = 0.76 wt% C • Transformation at T = 675°C.

100

50

0 1 10 2 10 4

T = 675°C

y,

% tr

ansf

orm

ed

time (s)

400 500

600 700

1 10 10 2 10 3 10 4 10 5

Austenite (stable) TE (727°C) Austenite (unstable)

Pearlite

T(°C)

time (s)

isothermal transformation at 675°C

Chapter 10 - 12

• Eutectoid composition, Co = 0.76 wt% C • Begin at T > 727°C • Rapidly cool to 625°C and hold isothermally.

Adapted from Fig. 10.14,Callister 7e. (Fig. 10.14 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.)

Effect of Cooling History in Fe-C System

400

500

600

700 Austenite (stable) TE (727°C)

Austenite (unstable)

Pearlite

T(°C)

1 10 10 2 10 3 10 4 10 5 time (s)

γ γ γ

γ γ γ

Chapter 10 - 13

Non-Equilibrium Transformation Products: Fe-C

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

Adapted from Fig. 10.18, Callister 7e. (Fig. 10.18 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.)

(Adapted from Fig. 10.17, Callister, 7e. (Fig. 10.17 from Metals Handbook, 8th ed., Vol. 8, Metallography, Structures, and Phase Diagrams, American Society for Metals, Materials Park, OH, 1973.)

Fe3C (cementite)

5 µm

α (ferrite)

10 10 3 10 5 time (s)

10 -1

400

600

800 T(°C)

Austenite (stable)

200

P

B

TE

pearlite/bainite boundary A

A 100% bainite

100% pearlite

Chapter 10 - 14

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

Adapted from Fig. 10.22, Callister 7e.

(Adapted from Fig. 10.21, Callister, 7e. (Fig. 10.21 courtesy United States Steel Corporation.)

• Isothermal Transf. Diagram

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

(Adapted from Fig. 10.20, Callister, 7e.

Martensite: Fe-C System

Martensite needles Austenite

60 µ

m

10 10 3 10 5 time (s) 10 -1

400

600

800 T(°C)

Austenite (stable)

200

P

B

TE A

A

M + A M + A

M + A 0% 50% 90%

x x x x

x

x potential C atom sites

Fe atom sites

(involves single atom jumps)

Chapter 10 - 15

γ (FCC) α (BCC) + Fe3C

Martensite Formation

slow cooling

tempering

quench

M (BCT)

M = martensite is body centered tetragonal (BCT)

Diffusionless transformation BCT if C > 0.15 wt% BCT few slip planes hard, brittle

Chapter 10 - 16

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

Spheroidite: Fe-C System

(Adapted from Fig. 10.19, Callister, 7e. (Fig. 10.19 copyright United States Steel Corporation, 1971.)

60 µm

α (ferrite)

(cementite) Fe3C

Chapter 10 - 17

Mechanical Prop: Fe-C System (1)

Adapted from Fig. 10.29, Callister 7e. (Fig. 10.29 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. 9.30,Callister 7e. (Fig. 9.30 courtesy Republic Steel Corporation.)

Adapted from Fig. 9.33,Callister 7e. (Fig. 9.33 copyright 1971 by United States Steel Corporation.)

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

• Effect of wt% C

Co < 0.76 wt% C Hypoeutectoid

Pearlite (med) ferrite (soft)

Co > 0.76 wt% C Hypereutectoid

Pearlite (med) C ementite

(hard)

300 500 700 900

1100 YS(MPa) TS(MPa)

wt% C 0 0.5 1

hardness

0.76

Hypo Hyper

wt% C 0 0.5 1 0

50

100 %EL

Impa

ct e

nerg

y (Iz

od, f

t-lb)

0

40

80

0.76

Hypo Hyper

Chapter 10 - 18

Mechanical Prop: Fe-C System (2)

Adapted from Fig. 10.30, Callister 7e. (Fig. 10.30 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.)

• Fine vs coarse pearlite vs spheroidite

• Hardness: • %RA:

fine > coarse > spheroidite fine < coarse < spheroidite

80

160

240

320

wt%C 0 0.5 1

Brin

ell h

ardn

ess fine

pearlite coarse pearlite spheroidite

Hypo Hyper

0

30

60

90

wt%C D

uctil

ity (%

AR

)

fine pearlite

coarse pearlite

spheroidite

Hypo Hyper

0 0.5 1

Chapter 10 - 19

Mechanical Prop: Fe-C System (3)

• Fine Pearlite vs Martensite:

• Hardness: fine pearlite << martensite.

Adapted from Fig. 10.32, Callister 7e. (Fig. 10.32 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.)

0

200

wt% C 0 0.5 1

400

600

Brin

ell h

ardn

ess martensite

fine pearlite

Hypo Hyper

Chapter 10 - 20

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

Adapted from Fig. 10.33, Callister 7e. (Fig. 10.33 copyright by United States Steel Corporation, 1971.)

• decreases TS, YS but increases %RA • produces extremely small Fe3C particles surrounded by α.

Adapted from Fig. 10.34, Callister 7e. (Fig. 10.34 adapted from Fig. furnished courtesy of Republic Steel Corporation.)

9 µ

m

YS(MPa) TS(MPa)

800 1000 1200 1400 1600 1800

30 40 50 60

200 400 600 Tempering T (°C)

%RA

TS YS

%RA

Chapter 10 - 21

Summary: Processing Options Adapted from Fig. 10.36, Callister 7e.

Austenite (γ)

Bainite (α + Fe3C plates/needles)

Pearlite (α + Fe3C layers + a proeutectoid phase)

Martensite (BCT phase diffusionless

transformation)

Tempered Martensite (α + very fine Fe3C particles)

slow cool

moderate cool

rapid quench

reheat

Stre

ngth

Duc

tility

Martensite

T Martensite bainite

fine pearlite coarse pearlite

spheroidite General Trends