1

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

• Eutectoid transformation (Fe-C System): γ ⇒ α + Fe3C 0.76 wt% C

0.022 wt% C 6.7 wt% C

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)

α ferrite

727°C Eutectoid: Equil. Cooling: Ttransf. = 727ºC ΔT

Undercooling by ΔTtransf. < 727°C

0.76

0.

022

Adapted from Fig. 9.24,Callister 7e. (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.)

2

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

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

3

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

• 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

4

• 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.)

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

5

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 800 600 400 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

T(°C)

727°C ΔT

0.76

0.

022

1.13

6

• 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

7

• 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 States Steel Corporation, 1971.)

60 µm

α (ferrite)

(cementite) Fe3C

8

• 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 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)

9

Adapted from Fig. 10.25, Callister 7e.

plot temp vs. time

10

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

11

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

12

• 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

13

• 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

14

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