POSCO Lectures on Bainite

Graduate Institute of Ferrous Technology

•Microstructure

•Mechanism

•Properties

•Superbainite

Problem: to design a bulk nanocrystalline steel which is very strong,

tough, cheap ….

Brenner, 1956

Morinobu Endo, 2004

Claimed strength of carbon nanotube is 130 GPaEdwards, Acta Astronautica, 2000

Claimed modulus is 1.2 TPaTerrones et al., Phil. Trans. Roy. Soc., 2004

Equilibrium number of defects (1020)

Strength of a nanotube rope 2 mm long is less than 2000 MPa

Scifer, 5.5 GPa and ductile

Kobe Steel

1 Denier: weight in grams, of 9 km of fibre

50-10 Denier

Scifer is 9 Denier

•Strength produced by deformation limits shape: wires, sheets...

•Strength in small particles relies on perfection. Doomed as size increases.

Summary

Smallest size possible in polycrystalline substance?

Yokota & Bhadeshia, 2004

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

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%

BS

MS

1.E+00

1.E+04

1.E+08

0 0.5 1 1.5Carbon / wt%

Time / s

Fe-2Si-3Mn-C wt%

1 month1 year

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

50 nm

200 Å

Caballero, Mateo, Bhadeshia

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

Hammond and Cross, 2004

Velocity km s-1

Stress /

GPa

“more serious battlefield threats”

ballistic mass efficiency

consider unit area of armour

Peet, Bhadeshia, 2004

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

original

Co

Co+Al

Need to improve mechanical stability of austenite

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

400

450

500

550

600

650

700

300 350 400 450 500 550 600 650Temperature /oC

H V

30 min60 min24 h

Fe-0.34C-5.08Cr-1.43Mo-0.92V-0.4Mn-1.07Si wt%

450 C

1 h

670 HV

o

600 C

1 h

530 HV

o

600 C

24 h

370 HV

o

excess carbon in solid solution in ferrite !

Peet, Babu, Miller, Bhadeshia, 2004

C Si Mn Mo Cr V P

0.98 1.46 1.89 0.26 1.26 0.09 < 0.002

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

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%

BS

MS

Chatterjee & Bhadeshia, 2004

Fe-1.75C-Si-Mn wt%

2104