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POSCO Lectures on Bainite
Graduate Institute of Ferrous Technology
•Microstructure
•Mechanism
•Properties
•Superbainite
Claimed strength of carbon nanotube is 130 GPaEdwards, Acta Astronautica, 2000
Claimed modulus is 1.2 TPaTerrones et al., Phil. Trans. Roy. Soc., 2004
•Strength produced by deformation limits shape: wires, sheets...
•Strength in small particles relies on perfection. Doomed as size increases.
Summary
Thermomechanical processing limited by recalescence
Summary
Need to store the heat
Reduce rate
Transform at low temperature
Courtesy of Tsuji, Ito, Saito, Minamino, Scripta Mater. 47 (2002) 893.
Howe, Materials Science and Technology 16 (2000) 1264.
Fine crystals by transformation
Introduce work-hardening capacity
Need to store the heat
Reduce rate
Transform at low temperature
C Si Mn Mo Cr V P
0.98 1.46 1.89 0.26 1.26 0.09 < 0.002
wt%
Low transformation temperature
Bainitic hardenability
Reasonable transformation time
Elimination of cementite
Austenite grain size control
Avoidance of temper embrittlement
Temperature
Time
1200 oC2 days
1000 oC15 min
Isothermal transformation
125 oC-325 oChours-monthsslow
cooling
Air cooling
Quench
AustenitisationHomogenisation
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
BS ~ 350oC
MS = 120oC
X-ray diffraction results
0
20
40
60
80
100
200 250 300 325
Temperature/ oC
Percentage of phase
bainitic ferrite
retained austenite
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
200 Å
Very strong
Huge uniform ductility
No deformation
No rapid cooling
No residual stresses
Cheap
Uniform in very large sections
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
Original 5h 3/ 4d 63 550
Co 4h 11h 77 640Co + Al 1h 8h 76 640
200oC
250oC
300oC
Steel Beginning End % Bainite HV
Original 4d 9d 69 618Co 2d 5d 79 690
Co+ Al 16h 3d 78 690
Original 2.5h 1/ 2d 55 420Co 1h 5h 66 490
Co + Al 0.5h 4h 66 490
Hard Bainite
250020001500100020
40
60
80
100
120
140
160
180
Ultimate tensile strength /
MPa
Fracture toughness / MPa
m1/2
18 wt%Nimaraging steel
QT
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