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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 Δ T range close to Tm, 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 -
Chapter 10 - 16
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 - 17
• 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 - 18
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 - 19
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 - 20
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 - 21
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 - 22
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