POSCO Lectures on Bainite

1
POSCO Lectures on Bainite

• Microstructure
• Mechanism
• Properties
• Superbainite

Graduate Institute of Ferrous Technology
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Problem to design a bulk nanocrystalline steel
which is very strong, tough, cheap .
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Brenner, 1956
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Claimed strength of carbon nanotube is 130
GPa Edwards, Acta Astronautica, 2000
Claimed modulus is 1.2 TPa Terrones et al., Phil.
Trans. Roy. Soc., 2004
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Equilibrium number of defects (1020) Strength of
a nanotube rope 2 mm long is less than 2000 MPa
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Scifer, 5.5 GPa and ductile
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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Summary

• Strength produced by deformation limits shape
  wires, sheets...
• Strength in small particles relies on perfection.
  Doomed as size increases.

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Smallest size possible in polycrystalline
substance?
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Yokota Bhadeshia, 2004
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Summary
Thermomechanical processing limited by
recalescence
Need to store the heat Reduce rate Transform at
low temperature
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Courtesy of Tsuji, Ito, Saito, Minamino, Scripta
Mater. 47 (2002) 893.
Howe, Materials Science and Technology 16 (2000)
1264.
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Fine crystals by transformation
Introduce work-hardening capacity Need to store
the heat Reduce rate Transform at low
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
20
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
400
o
Temperature/ oC
B
350
C
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
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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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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Peet, Bhadeshia, 2004
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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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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 mechanical stability of 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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Fe-0.34C-5.08Cr-1.43Mo-0.92V-0.4Mn-1.07Si wt
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excess carbon in solid solution in ferrite !
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Peet, Babu, Miller, Bhadeshia, 2004
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30 Tesla field, 485 HV
R. A. Jaramillo, S. S. Babu, G. M. Ludtka, R. A.
Kisner, J. B. Wilgen, G. Makiewicz-Ludtka, D. M.
Nicholson, S. M. Kelly, M. Murugananth and H. K.
D. H. Bhadeshia Scripta Materialia, 52 (2004)
461-466.
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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
60
2104
Fe-1.75C-Si-Mn wt
Chatterjee Bhadeshia, 2004