Carbide-Free Bainite: design

principleshttp://cml.postech.ac.kr

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

tough, cheap ….

Brenner, 1956

Scifer, 5.5 GPa and ductile

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

50-10 Denier

Scifer is 9 Denier

Carbon nanotube with iron particle in the middle

Kinloch, 2002

blobby iron particle

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 of whiskers decreases with size

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

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

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

Summary

Smallest size that can be achieved in a 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

Swallow and Bhadeshia, 1996

1 µm

Cementite suppressed using silicon

0

200

400

600

800

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Carbon / wt%

Tem

pera

ture

/ KFe-2Si-3Mn-C wt%

BS

MS

1.E+00

1.E+04

1.E+08

0 0.5 1 1.5Carbon / wt%

Tim

e /

sFe-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

Tem

pera

ture

Time

1200 oC2 days

1000 oC15 min

Isothermal transformation

125oC-325oChours-monthsslow

cooling

Air cooling

Quench

AustenitisationHomogenisation

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

Caballero, Mateo, Bhadeshia

Above percolation threshold

Below percolation threshold

Geometrical percolation threshold of overlapping ellipsoids

Hammond and Cross, 2004

Velocity km s-

1

Str

ess

/ G

Pa

“more serious battlefield threats”

ballistic mass efficiency

consider unit area of armour

Very strong

Huge uniform ductility

No deformation

No rapid cooling

No residual stresses

Cheap

Uniform in very large sections