Fundamentals and Applications of Bainitic Steels

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Fundamentals and Applications of Bainitic Steels
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upper bainite
1 µm
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lower bainite
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Surface 1
Surface 2
50 µm
Srinivasan Wayman, 1968
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s
d
c
r
1
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50 µm
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Carbon supersaturated plate
Carbon diffusion into
Carbon diffusion into
austenite and carbide
austenite
precipitation in ferrite
Carbide precipitation
from austenite
LOWER BAINITE
UPPER BAINITE
(Low Temperature)
(High Temperature)
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Fe-0.4C wt
Decarburisation time / s
Temperature / C
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Temperature
Ae3'
T'
o
x
Carbon in austenite
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Growth is diffusionless.
Strain energy must be accounted for.
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Carbon supersaturated plate
Carbon diffusion into
Carbon diffusion into
austenite and carbide
austenite
precipitation in ferrite
Carbide precipitation
from austenite
LOWER BAINITE
UPPER BAINITE
(Low Temperature)
(High Temperature)
Takahashi and Bhadeshia
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Oka and Okamoto
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Ohmori and Honeycombe
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Each point represents a different steel
Bhadeshia, 1981
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The nucleation of bainite must involve the
partitioning of carbon
Why does the required free energy vary linearly
with T?
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hexagonal close-packed
cubic close-packed
Christian, 1951
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Brooks, Loretto and Smallman, 1979
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Olson Cohen, 1976
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Nucleation of bainite must involve the
partitioning of carbon.
Mechanism of nucleation is otherwise identical to
that of martensite.
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Fe-2Si-3Mn-C wt
800
B
S
600
Temperature / K
400
M
S
200
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Carbon / wt
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Fe-2Si-3Mn-C wt
1.E08
1 year
1 month
Time / s
1.E04
1.E00
0
0.5
1
1.5
Carbon / wt
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Low transformation temperature Bainitic
hardenability Reasonable transformation
time Elimination of cementite Austenite grain
size control Avoidance of temper embrittlement
wt
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Isothermal
Austenitisation
Homogenisation
transformation
1200
C
o
2 days
1000
o
C
15 min
Temperature
125
o
C
-
325
o
C
Air
slow
hours
-
months
cooling
cooling
Quench
Time
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g
g
a
a
a
Caballero, Mateo, Bhadeshia
200 Å
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Low temperature transformation 0.25 T/Tm Fine
microstructure 20-40 nm thick plates Harder
than most martensites (710 HV) Carbide-free Design
ed using theory alone
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Very strong Huge uniform ductility
g
g
a
No deformation No rapid cooling No residual
stresses
a
Cheap Uniform in very large sections
a
200 Å
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Stress / GPa
Velocity km s-1
Hammond and Cross, 2004
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more serious battlefield threats
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ballistic mass efficiency consider unit area of
armour
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Caballero, Mateo, Bhadeshia
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Caballero, Mateo, Bhadeshia
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Sherif, 2005, Ph.D. thesis, Cambridge
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Geometrical percolation threshold of overlapping
ellipsoids
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0.4 C 2 Si 3 Mn wt
1 µm
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Very poor toughness!
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50 µm
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Fe-1C-1.5Si wt periodic cracking stress
transfer length
Chatterjee Bhadeshia, 2005
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Carbide-free alloys
wt
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Impact Energy
Charpy impact / J
Temperature / C
Test temperature / C
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kilocycles to Crack Initiation
Yates, Jerath
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Yates, Jerath
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U.K.