Hard Bainite

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Hard Bainite
Francisca Garcia Caballero Carlos Garcia
Mateo Mathew Peet Pippa Swanell
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Minimum temperature at which bainite can be
obtained? Mechanism of the nucleation and growth
of bainite Low temperature bainite Hard
bainite Future
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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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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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Swallow and Bhadeshia, 1996
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Growth is diffusionless.
There is a strain energy which must be accounted
for.
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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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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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700
600
500
Vickers Hardness
400
T 200C
300
1.E04
1.E05
1.E06
1.E07
Time/ s
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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
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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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Conclusions
Low temperature transformation 0.25 T/Tm Fine
microstructure 20-40 nm thick plates
Carbide-free Designed using theory alone Typical
mechanical properties
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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
10 Denier
Scifer is 9 Denier
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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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original
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Co
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CoAl
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Need to improve the mechanical stability of the
austenite
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Hard Bainite
m1/2
18 wtNi maraging steel

MPa
Fracture toughness /
QT
Ultimate tensile strength /
MPa
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700
650
600
550
H V
500
450
30 min
400
60 min
24 h
300
350
400
450
500
550
600
650
o
Temperature /
C
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