Mechanism of the Bainite Transformation in Steels

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Mechanism of the Bainite Transformation in Steels
www.msm.cam.ac.uk/phase-trans
IIT Kharagpur
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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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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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austenite
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0.2C 1.25Si 1.55Mn 0.5Cr 0.15V 0.15Mo wt
15 mm thick plate, without Mo 550C for 5 h
Chang and Bhadeshia
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U.K.
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18 wt Ni
maraging steel
Fracture toughness / MPa vm
QT
Ultimate tensile strength / MPa
Caballero Bhadeshia, 2001
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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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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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excess carbon in solid solution in ferrite !
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Peet, Babu, Miller, Bhadeshia, 2004
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Peet, Babu, Miller, Bhadeshia, 2004
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Peet, Babu, Miller, Bhadeshia, 2004