Mechanism of The Bainite Transformation in Steels

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Mechanism of The Bainite Transformation in Steels
Harry Bhadeshia
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Bundy (1965)
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body-centred cubic
cubic close-packed
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DISPLACIVE
RECONSTRUCTIVE
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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)
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Oka and Okamoto
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Ohmori and Honeycombe
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Thermodynamics
g
a
Gibbs free energy
x
Carbon
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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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g
g
a
a
a
20 nm
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Summary
The mechanism of transformation is
displacive. Transformation temperature higher
than martensite. Bainite grows without
diffusion. But carbon then escapes into the
residual austenite. Shape deformation plastically
accommodated. Sub-unit mechanism of
growth. Nucleation must involve carbon
partitioning.
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Watson and McDougall, 1973
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Summary
Mechanism displacive but carbon must partition
during growth. Pairs of plates grow together to
minimise strain.