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 GPa Edwards, Acta Astronautica, 2000 Claimed modulus is 1.2 TPa Terrones et al., Phil. Trans. Roy. Soc., 2004

Equilibrium number of defects (10 20 ) 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 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

Carbon / wt% Temperature / K Fe-2Si-3Mn-C wt% B S M S

1.E+00 1.E+04 1.E Carbon / wt% Time / s Fe-2Si-3Mn-C wt% 1 month 1 year

wt% Low transformation temperature Bainitic hardenability Reasonable transformation time Elimination of cementite Austenite grain size control Avoidance of temper embrittlement

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 AustenitisationHomogenisation

E+001.E+021.E+041.E+061.E+08 Time / s Temperature/ o C B S ~ 350 o C M S = 120 o C

X-ray diffraction results Temperature/ o C Percentage of phase bainitic ferrite retained austenite

50 nm     

200 Å      Caballero, Mateo, Bhadeshia

Low temperature transformation: 0.25 T/T m Fine microstructure: 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

200 o C 250 o C 300 o C

original

Co

Co+Al

Need to improve mechanical stability of austenite

Hard Bainite Ultimate tensile strength / MPa Fracture toughness / MPa m 1/2 18 wt%Ni maraging steel QT

Temperature / o C H V 30 min 60 min 24 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

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) Tesla field, 485 HV

Carbon / wt% Temperature / K Fe-2Si-3Mn-C wt% B S M S

Chatterjee & Bhadeshia, 2004 Fe-1.75C-Si-Mn wt% 2104