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1600. d. L. 1400. T (°C). g. + L. g. 1200. L +Fe 3 C. 1148°C. (austenite). 1000. g. + Fe 3 C. 800. Fe 3 C (cementite). 727°C. 600. a. + Fe 3 C. 400. 0. 1. 2. 3. 4. 5. 6. 6.7. C o , wt% C. Phase transformations. • Growth of pearlite from austenite:. - PowerPoint PPT Presentation
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Phase transformations
Fe 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
(austenite)
+L
+ Fe3C
+ Fe3C
L+Fe3C
Co , wt% C
1148°C
T(°C)
727°C
Diffusive flow of C needed
• Growth of pearlite from austenite:
pearlite growth direction
Austenite ()grain boundary
cementite (Fe3C)
Ferrite ()
675°C
(T smaller)
0
50
y (%
pea
rlite
)
600°C
(T larger)650°C
100
Transformations & Supercooling
+ Fe3C
0.76 wt% C0.022 wt% C
6.7 wt% C
Fe 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
(austenite)
+L
+Fe3C
+Fe3C
L+Fe3C
(Fe) Co , wt%C
1148°C
T(°C)
ferrite
727°C
Eutectoid:
T
0.7
6
0.0
22
Isothermal Transformation Diagrams100
50
01 10 2 10 4
T = 675°Cy,
% tr
ansf
orm
ed
time (s)
400
500
600
700
1 10 10 2 10 3 10 4 10 50%
pearlite100%
50%
Austenite (stable) TE (727C)Austenite
(unstable)
Pearlite
T(°C)
time (s)
• Eutectoid composition, Co = 0.76 wt% C.
Effect of Cooling History in Fe-C System
400
500
600
700
0%pearlite
100%
50%
Austenite (stable)TE (727C)
Austenite
(unstable)
Pearlite
T(°C)
1 10 102 103 104 105
time (s)
Transformations with Proeutectoid Materials
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
Fe 3
C (
cem
entit
e)
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
(austenite)
+L
+Fe3C
+Fe3C
L+Fe3C
(Fe) Co , wt%C
T(°C)
727°CT
0.7
6
0.0
22
1.13
Non-Equilibrium Transformation Products: Fe-C
• Bainite: -- strips with long
needles of Fe3C --diffusion controlled.• Isothermal Transf. Diagram
FeFe33CC
(cementite)(cementite)
5 m
(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
A
• Martensite: --(FCC) to Martensite (BCT)
• Isothermal Transf. Diagram
Martensite: Fe-C System
Martensite needlesAustenite
60
m
10 103 105 t/s10-1
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
xpotential C atom sites
Fe atom sites
(involves single atom jumps)
Dynamic Phase Transformations
On the isothermal transformation diagram for 0.45 wt% C Fe-C alloy, sketch and label the time-temperature paths to produce the following microstructures:a) 42% proeutectoid ferrite and 58% coarse pearlite
b) 50% fine pearlite and 50% bainite
c) 100% martensite
d) 50% martensite and 50% austenite
Example Problem for Co = 0.45 wt%
A + B
A + P
A + A
BP
A50%
0
200
400
600
800
0.1 10 103 105
time (s)
M (start)
M (50%)
M (90%)
T (°C)
Example Problem for Co = 0.45 wt%
A + B
A + P
A + A
BP
A50%
0
200
400
600
800
0.1 10 103 105
time (s)
M (start)
M (50%)
M (90%)
T (°C)
Example Problem for Co = 0.45 wt%
A + B
A + P
A + A
BP
A50%
0
200
400
600
800
0.1 10 103 105
time (s)
M (start)
M (50%)
M (90%)
T (°C)
Mechanical Prop: Fe-C System (1)
• Effect of wt% C
Co < 0.76 wt% C
Hypoeutectoid
Pearlite (med)ferrite (soft)
Co > 0.76 wt% C
Hypereutectoid
Pearlite (med)Cementite
(hard)
300
500
700
900
1100
YS
TS(MPa)
wt% C0 0.5 1
hardness
0.7
6
Hypo Hyper
wt% C0 0.5 1
0
50
100%EL
Imp
act
en
erg
y (I
zod
, ft
-lb
)
0
40
80
0.7
6
Hypo Hyper
TS
EL
Izod
Mechanical Prop: Fe-C System (2)
• Fine vs coarse pearlite vs spheroidite
80
160
240
320
wt%C0 0.5 1
Bri
ne
ll h
ard
ne
ss
fine pearlite
coarse pearlitespheroidite
Hypo Hyper
0
30
60
90
wt%CD
uct
ility
(%
AR
)
fine pearlite
coarse pearlite
spheroidite
Hypo Hyper
0 0.5 1
Mechanical Prop: Fe-C System (3)• Fine Pearlite vs Martensite:
0
200
wt% C0 0.5 1
400
600
Bri
ne
ll h
ard
ne
ss martensite
fine pearlite
Hypo Hyper
Tempering Martensite
9 m
MPa
800
1000
1200
1400
1600
1800
30
40
50
60
200 400 600
Tempering T (°C)
%RA
TS
YS
%RA
Summary: Processing Options
Austenite ()
Bainite( + Fe3C plates/needles)
Pearlite( + Fe3C layers + a proeutectoid phase)
Martensite(BCT phase diffusionless
transformation)
Tempered Martensite ( + very fine Fe3C particles)
slow cool
moderate cool
rapid quench
reheat
Str
engt
h
Duc
tility
Martensite T Martensite
bainite fine pearlite
coarse pearlite spheroidite
General Trends