POSCO Bainite 3

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POSCO Lectures on Bainite
Graduate Institute of Ferrous Technology
Microstructure
Mechanism
Properties
Superbainite
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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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Strength produced by deformation limits shape: wires, sheets...
Strength in small particles relies on perfection. Doomed as size increases. 
Summary
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Smallest size possible in polycrystalline substance?
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Yokota & Bhadeshia, 2004
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Thermomechanical processing limited by recalescence
Summary
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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0
200
400
600
800
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Carbon / wt%
Temperature / K
Fe-2Si-3Mn-C wt%
B
S
M
S
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1.E+00
1.E+04
1.E+08
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0.5
1
1.5
Carbon / wt%
Time / s
Fe-2Si-3Mn-C wt%
1 month
1 year
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wt%
Low transformation temperature
Bainitic hardenability
Reasonable transformation time
Elimination of cementite
Austenite grain size control
Avoidance of temper embrittlement
		C
		Si
		Mn
		Mo
		Cr
		V
		P
		0.98
		1.46
		1.89
		0.26
		1.26
		0.09
		< 0.002
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Temperature
Time
1200 
o
C
2 days
1000 
o
C
15 min
Isothermal 
transformation
125
o
C
-
325
o
C
hours
-
months
slow 
cooling
Air 
cooling
Quench
Austenitisation
Homogenisation
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0
100
200
300
400
500
600
700
1.E+00
1.E+02
1.E+04
1.E+06
1.E+08
Time / s
Temperature/ oC
B
S
 ~ 350
o
C
M
S
 = 120
o
C
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X-ray diffraction results
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20
40
60
80
100
200
250
300
325
Temperature/ 
o
C
 Percentage of phase
bainitic ferrite
retained austenite
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50 nm 
a
g
g
g
a
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200 Å 
g
g
a
a
a
Caballero, Mateo, Bhadeshia
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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
Designed using theory alone
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Hammond and Cross, 2004
Velocity km s-1
Stress / GPa
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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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200 Å 
g
g
a
a
a
Very strong
Huge uniform ductility
No deformation
No rapid cooling
No residual stresses
Cheap
Uniform in very large sections
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Cobalt (1.5 wt%) and aluminium (1 wt%) increase the stability of ferrite relative to austenite 
Refine austenite grain size
Faster Transformation
		C
		Si
		Mn
		Mo
		Cr
		V
		P
		0.98
		1.46
		1.89
		0.26
		1.26
		0.09
		< 0.002
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200oC
250oC
300oC
		Original
		5h
		3/4d
		63
		550
		Co
		4h
		11h
		77
		640
		Co + Al
		1h
		8h
		76
		640
		Steel
		Beginning
		End
		% Bainite
		HV
		Original
		4d
		9d
		69
		618
		Co
		2d
		5d
		79
		690
		Co+ Al
		16h
		3d
		78
		690
		Original
		2.5h
		1/2d
		55
		420
		Co
		1h
		5h
		66
		490
		Co + Al
		0.5h
		4h
		66
		490
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original
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Co
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Co+Al
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Need to improve mechanical stability of austenite
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Hard Bainite
Ultimate tensile strength /
 MPa
Fracture toughness /
 MPa
 
m1/2
18 wt%Ni
maraging steel
QT
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400
450
500
550
600
650
700
300
350
400
450
500
550
600
650
Temperature /
o
C
H V
30 min
60 min
24 h
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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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		C
		Si
		Mn
		Mo
		Cr
		V
		P
		0.98
		1.46
		1.89
		0.26
		1.26
		0.09
		< 0.002
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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.
30 Tesla field, 485 HV
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200
400
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800
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0.2
0.4
0.6
0.8
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1.2
1.4
Carbon / wt%
Temperature / K
Fe-2Si-3Mn-C wt%
B
S
M
S
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Chatterjee & Bhadeshia, 2004
Fe-1.75C-Si-Mn wt%
2104