phase transformation - kasetsart...
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Kasetsart University
Dr.Peerapong Triyacharoen Department of Materials Engineering
213211: Phase Transformation
240
Phase TransformationISSUES TO ADDRESS...• Transforming one phase into another takes time.
• How does the rate of transformation depend ontime and T?
• How can we slow down the transformation so thatwe can engineering non-equilibrium structures?
• Are the mechanical properties of non-equilibriumstructures better?
Fe
γ (Austenite)
Eutectoid transformation
C FCC
Fe3C (cementite)
α (ferrite)
+(BCC)
Kasetsart University
Dr.Peerapong Triyacharoen Department of Materials Engineering
213211: Phase Transformation
241
Fraction of Transformation• Fraction transformed depends on time.
fraction transformed time
y = 1− e−ktn
Avrami Eqn.
• Transformation rate depends on T.
1 10 10 2 10 40
50
10 0 135°
C
119°
C11
3°C
102°
C88
°C
43°Cy (%)
log (t) min
Ex: recrystallization of Cu
r = 1t
0.5
= Ae−Q /RT
activation energy
• r often small: equilibrium is not possible!
y
log (t)
Fixed T
0
0.5
1
t0.5
Kasetsart University
Dr.Peerapong Triyacharoen Department of Materials Engineering
213211: Phase Transformation
242
Transformation & Undercooling• Can make it occur at:
...727ºC (cool it slowly) ...below 727ºC (“undercool” it!)
• Eutectoid transf. (Fe-C System):
α ferrite
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6
.7
L
γ austenite
γ+L
γ+Fe 3C
Fe 3C
cementiteα+Fe 3C
α+γ
L+Fe 3C
(Fe) Co, wt% C
Eutectoid:
0.7
7
727°C
T(°C)
∆T
0.0
22
γ ⇒ α + Fe 3 C0.77wt%C
0.022wt%C6.7 wt%C
Undercooling by ∆T: T transf. < 727ºC
Equil. cooling: T transf. = 727ºC
Kasetsart University
Dr.Peerapong Triyacharoen Department of Materials Engineering
213211: Phase Transformation
243
Eutectoid Transformation Rate ~ ∆T
γαααα
α
α
pearlite growth direction
Austenite ( γ) grain boundary
cementite (Fe 3C)
ferrite (α)
γ
Diffusive flow of C needed
α
αγ γ
α
• Growth of pearlite from austenite:
• Reaction rateincreases with∆T. 675°C
(∆ T smaller)
1 10 10 2 10 3time (s)
0
50
10 0
y (%
pe
arl
ite
) 0
50
10 0
600°C (∆ T larger)
650°C
% a
ust
en
ite
Kasetsart University
Dr.Peerapong Triyacharoen Department of Materials Engineering
213211: Phase Transformation
244
Isothermal Transformation Diagram• Fe-C system, Co = 0.77wt%C Transformation at T = 675°C.
400
5 00
6 00
7 00
1 10 10 2 10 3 10 4 10 5
0%pearlite
100%
50%
Austenite (stable) T E (727°C)Austenite (unstable)
Pearlite
T(°C)
1 00
5 0
01 10 2 10 4
T=675°C
y,
% t
ran
sfo
rme
d
time (s)
time (s)
isothermal transformation at 675°C
Kasetsart University
Dr.Peerapong Triyacharoen Department of Materials Engineering
213211: Phase Transformation
245
Ex: Cooling in Fe-C System• Eutectoid composition, Co = 0.77wt%C• Begin at T > 727°C• Rapidly cool to 625°C and hold isothermally.
Kasetsart University
Dr.Peerapong Triyacharoen Department of Materials Engineering
213211: Phase Transformation
246
Pearlite Morphology
- Smaller ∆T: colonies are larger
- Larger ∆T: colonies are smaller
• Ttransf just below TE--Larger T: diffusion is faster--Pearlite is coarser.
Two cases:• Ttransf well below TE
--Smaller T: diffusion is slower--Pearlite is finer.
Kasetsart University
Dr.Peerapong Triyacharoen Department of Materials Engineering
213211: Phase Transformation
247
Non-Equil. Transformation Products: Fe-C
Bainite reaction rate:
rbainite = e−Q /RT
• Bainite:--α lathes (strips) with long
rods of Fe3C--diffusion controlled.
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%
100% bainite
pearlite/bainite boundary100% pearlite
A
A
Kasetsart University
Dr.Peerapong Triyacharoen Department of Materials Engineering
213211: Phase Transformation
248
Other Products: Fe-C System (I)
60 µm
α (ferrite)
Fe3C
(cementite)
• Spheroidite:--α crystals with spherical Fe3C--diffusion dependent.--heat bainite or pearlite for long times--reduces interfacial area (driving force)
10 103 105time (s)10-1
400
600
800
T(°C)Austenite (stable)
200
P
B
TE
0%
100%
50%
A
A
Spheroidite100% spheroidite
100% spheroidite
Kasetsart University
Dr.Peerapong Triyacharoen Department of Materials Engineering
213211: Phase Transformation
249
Other Products: Fe-C System (II)• Martensite: γ (FCC) to Martensite (BCT)
xx x
xx
xpotential C atom sites
Fe atom sites
(involves single atom jumps)
time (s)10 103 10510-1
400
600
800
T(°C)Austenite (stable)
200
P
B
TE
0%
100%50%
A
A
S
M + AM + A
M + A
0%50%90%
Martentite needlesAustenite
60
µm
• γ to M transformation..-- is rapid!-- % transf. depends on T only.
Kasetsart University
Dr.Peerapong Triyacharoen Department of Materials Engineering
213211: Phase Transformation
250
• Effect of quenching medium:Medium
airoil
water
Severity of Quenchsmall
moderatelarge
Hardnesssmall
moderatelarge
• Effect of geometry:When surface-to-volume ratio increases:
--cooling rate increases--hardness increases
Positioncentersurface
Cooling ratesmalllarge
Hardnesssmalllarge
Quenching Medium & Geometry
Kasetsart University
Dr.Peerapong Triyacharoen Department of Materials Engineering
213211: Phase Transformation
251
Cooling Ex.(a) Rapidly cool to 350°C, hold
for 104s, quench to Troom
(b) Rapidly cool to 250°C, hold for 100s, quench to Troom
(c) Rapidly cool to 650°C, hold for 20s, rapidly cool to
400°C, hold for 103s, quench to Troom
(a)(b)
(c)
(c)
Kasetsart University
Dr.Peerapong Triyacharoen Department of Materials Engineering
213211: Phase Transformation
252
Mechanical Prop: Fe-C System (I)Hypoeutectoid Hypereutectoid
Pearlite (med)
Ferrite (soft) Pearlite (med)
Cementite (hard)
Kasetsart University
Dr.Peerapong Triyacharoen Department of Materials Engineering
213211: Phase Transformation
253
Mechanical Prop: Fe-C System (II)
Kasetsart University
Dr.Peerapong Triyacharoen Department of Materials Engineering
213211: Phase Transformation
254
Mechanical Prop: Fe-C System (III)
Kasetsart University
Dr.Peerapong Triyacharoen Department of Materials Engineering
213211: Phase Transformation
255
Summary: Processing OptionsAustenite ( γ)
Bainite (α + Fe 3C plates/needles)
Pearlite (α + Fe 3C layers + a
proeutectoid phase)
Martensite (BCT phase diffusionless
transformation)
Tempered Martensite (α + very fine
Fe 3C particles)
slow cool
moderate cool
rapid quench
reheat
Str
en
gth
Du
cti
lity
Martensite T Martensite
bainite fine pearlite
coarse pearlite spheroidite
General Trends
Kasetsart University
Dr.Peerapong Triyacharoen Department of Materials Engineering
213211: Phase Transformation
256
Problem 10.16