1

Eutectoid transformation rate

Diffusive flow of C needed

α

α

γγ

α

• Growth of pearlite from austenite:

Adapted fromFig. 9.15,Callister 7e.

γαααα

α

α

pearlite growth direction

Austenite (γ)grain boundary

cementite (Fe3C)Ferrite (α)

γ

• Recrystallization rate increases with ΔT.

Adapted fromFig. 10.12,Callister 7e.

675°C (ΔT smaller)

0

50

y (%

pea

rlite

)

600°C (ΔT larger)

650°C

100

Coarse pearlite formed at higher T - softerFine pearlite formed at low T - harder

2

Transformations and undercooling

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

• Eutectoid transformation (Fe-C System): γ ⇒ α + Fe3C0.76 wt% C

0.022 wt% C6.7 wt% C

Fe3C

(cem

entit

e)

1600

1400

1200

1000

800

600

4000 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)

αferrite

727°C

Eutectoid:Equil. Cooling: Ttransf. = 727ºCΔT

Undercooling by ΔTtransf. < 727°C

0.76

0.02

2

Adapted from Fig.9.24,Callister 7e. (Fig. 9.24adapted from Binary AlloyPhase Diagrams, 2nd ed.,Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International,Materials Park, OH, 1990.)

3

Isothermal transformation diagrams

Adapted from Fig. 10.13,Callister 7e.(Fig. 10.13 adapted from H. Boyer (Ed.)Atlas of Isothermal Transformation andCooling Transformation Diagrams,American Society for Metals, 1977, p.369.)

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

100

50

01 102 104

T = 675°C

y,

% tr

ansf

orm

ed

time (s)

400

500

600

700

1 10 102 103 104 105

0%pearlite

100%

50%

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

Pearlite

T(°C)

time (s)

isothermal transformation at 675°C

4

Effect of cooling history

• 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 fromH. Boyer (Ed.) Atlas ofIsothermal Transformationand CoolingTransformation Diagrams,American Society forMetals, 1997, p. 28.)

400

500

600

700

0%pearlite

100%

50%

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

Pearlite

T(°C)

1 10 102 103 104 105

time (s)

γ γ

γ

γ γ

γ

5

Transformations with proeutectoid materials

Hypereutectoid composition – proeutectoid cementite

α

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

Adapted from Fig. 10.16,Callister 7e.

Adapted from Fig. 9.24,Callister 7e.

Fe3C

(cem

entit

e)

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

γ (austenite)

γ+L

γ +Fe3C

α +Fe3C

L+Fe3C

δ

(Fe) Co , wt%C

T(°C)

727°CΔT

0.76

0.02

2

1.13

6

Nonequilibrium transformation products

• 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 CoolingTransformation 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 PhaseDiagrams, American Society for Metals,Materials Park, OH, 1973.)

Fe3C(cementite)

5 µm

α (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

A100% bainite

100% pearlite

7

Spheroidite

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

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

60 µm

α(ferrite)

(cementite)Fe3C

8

Martensite

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

Adapted fromFig. 10.22,Callister 7e.

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

• Isothermal Transf. Diagram

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

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

Martensite needlesAustenite

60 µ

m

10 103 105 time (s)10-1

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

x potential C atom sites

Fe atom sites

(involves single atom jumps)

9

Cooling curve

Adapted fromFig. 10.25,Callister 7e.

plot temp vs. time

10

Mechanical properties 1

Adapted from Fig.10.29, Callister 7e.(Fig. 10.29 based ondata from MetalsHandbook: HeatTreating, Vol. 4, 9thed., V. Masseria(Managing Ed.),American Society forMetals, 1981, p. 9.)

Adapted from Fig. 9.30,Callister7e. (Fig. 9.30 courtesy RepublicSteel Corporation.)

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

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

• Effect of wt% C

Co < 0.76 wt% CHypoeutectoid

Pearlite (med)ferrite (soft)

Co > 0.76 wt% CHypereutectoid

Pearlite (med)Cementite

(hard)

300

500

700

900

1100YS(MPa)TS(MPa)

wt% C0 0.5 1

hardness

0.76

Hypo Hyper

wt% C0 0.5 10

50

100%EL

Impa

ct e

nerg

y (Iz

od, f

t-lb)

0

40

80

0.76

Hypo Hyper

11

Mechanical properties 2

Adapted from Fig. 10.30, Callister 7e.(Fig. 10.30 based on data from MetalsHandbook: Heat Treating, Vol. 4, 9thed., V. Masseria (Managing Ed.),American Society for Metals, 1981, pp.9 and 17.)

• Fine vs coarse pearlite vs spheroidite

• Hardness:• %RA:

fine > coarse > spheroiditefine < coarse < spheroidite

80

160

240

320

wt%C0 0.5 1

Brin

ell h

ardn

ess

fine pearlite

coarse pearlitespheroidite

Hypo Hyper

0

30

60

90

wt%CD

uctil

ity (%

AR)

fine pearlite

coarse pearlite

spheroidite

Hypo Hyper

0 0.5 1

12

Mechanical properties 3

• Fine Pearlite vs Martensite:

• Hardness: fine pearlite << martensite.

Adapted from Fig. 10.32,Callister 7e. (Fig. 10.32 adaptedfrom Edgar C. Bain, Functions ofthe 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% C0 0.5 1

400

600

Brin

ell h

ardn

ess martensite

fine pearlite

Hypo Hyper

13

Tempered martensite

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

Adapted fromFig. 10.33,Callister 7e.(Fig. 10.33copyright byUnited StatesSteelCorporation,1971.)

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

Adapted fromFig. 10.34,Callister 7e.(Fig. 10.34adapted fromFig. furnishedcourtesy ofRepublic SteelCorporation.)

9 µ

m

YS(MPa)TS(MPa)

800

1000

1200

1400

1600

1800

30405060

200 400 600Tempering T (°C)

%RA

TS

YS

%RA

14

Summary

Adapted fromFig. 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