Developments in Martensitic and Bainitic Steels:

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Developments in Martensitic and Bainitic Steels
Role of Shape Deformation www.msm.cam.ac.uk/phase
-trans
s
d
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Watson and McDougall
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Swallow Bhadeshia, 1996
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Lower bainite, transformed with and without
stress
Chang et al., 1996
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Jones Alberry, 1977
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Ohta et al., 2000-2003
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Ohta et al., 2000-2003
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Lower bainite, transformed with and without
stress
Chang et al., 1996
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Ohta et al., 2000-2003
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Ohta et al., 2000-2003
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s
d
c
r
1
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Singh Bhadeshia, 1998
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What is the minimum temperature at which bainite
can be obtained?
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The nucleation of bainite must involve the
partitioning of carbon.
The mechanism of nucleation is otherwise
identical to that of martensite.
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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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Bainite-start temperature
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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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700
600
500
400
o
B
350
C
Temperature/ oC
S
300
200
o
M
120
C
S
100
0
1.E00
1.E02
1.E04
1.E06
1.E08
Time / s
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100
retained austenite
80
X-ray diffraction results
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Percentage of phase
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bainitic ferrite
20
0
200
250
300
325
o
Temperature/
C
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a
g
a
g
g
50 nm
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g
g
a
a
a
20 nm
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Faster Transformation
Cobalt (1.5 wt) and aluminium (1 wt) increase
the stability of ferrite relative to austenite
Refine austenite grain size
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200oC
250oC
300oC
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Scifer, 5.5 GPa with ductility!
Kobe Steel
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1 Denier weight in grams, of 9 km of fibre
50-10 Denier
Scifer is 9 Denier
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Ueji et al., 2002
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Ueji et al., 2002
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• Can completely eliminate tensile residual
  stresses in welded joints.
• Extremely fine, strong and tough steel in bulk
  form.

s
d
1
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Tempered 400C 30 min
Tempered 400C 30 min 500 MPa stress
Stewart et al., 1994
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Martensite, low and high stress
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Mateo et al., 2003