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• 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)
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