POSCO Lectures on Bainite Microstructure Mechanism Properties Superbainite

POSCO Lectures on Bainite • Microstructure • Mechanism • Properties • Superbainite Graduate Institute of Ferrous Technology

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 (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

Summary • Strength produced by deformation limits shape: wires, sheets. . . • Strength in small particles relies on perfection. Doomed as size increases.

Smallest size possible in polycrystalline substance?

Yokota & Bhadeshia, 2004

Summary Thermomechanical processing limited by recalescence 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

Fe-2 Si-3 Mn-C wt% Temperature / K 800 BS 600 400 MS 200 0 0 0. 2 0. 4 0. 6 0. 8 1 Carbon / wt% 1. 2 1. 4

Fe-2 Si-3 Mn-C wt% 1. E+08 1 year Time / s 1 month 1. E+04 1. E+00 0 0. 5 Carbon / wt% 1 1. 5

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

Homogenisation Austenitisation Temperature 1200 o. C 2 days Isothermal transformation 1000 o. C 15 min Air cooling slow cooling 125 o C-325 o. C hours-months Quench Time

700 Temperature/ o. C 600 500 400 BS ~ 350 o. C 300 200 MS = 120 o. C 100 0 1. E+02 1. E+04 Time / s 1. E+06 1. E+08

Percentage of phase 100 80 retained austenite X-ray diffraction results 60 40 bainitic ferrite 20 0 200 250 300 Temperature/o. C 325

a g g a g 50 nm

g g a a a Caballero, Mateo, Bhadeshia 200 Å

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

Stress / GPa Velocity km s-1 Hammond and Cross, 2004

“more serious battlefield threats”

ballistic mass efficiency consider unit area of armour

Peet, Bhadeshia, 2004

Very strong Huge uniform ductility g g a No deformation No rapid cooling a No residual stresses Cheap a Uniform in very large sections 200 Å

Faster Transformation Cobalt (1. 5 wt%) and aluminium (1 wt%) increase the stability of ferrite relative to austenite Refine austenite grain size

200 o. C 250 o. C 300 o. C

original

Co

Co+Al

Need to improve mechanical stability of austenite

Hard Bainite 180 Fracture toughness / MPa m 1/2 160 18 wt%Ni maraging steel 140 120 100 80 60 40 20 1000 QT 1500 2000 Ultimate tensile strength / 2500 MPa

700 650 H V 600 550 500 450 400 30 min 60 min 24 h 300 350 400 450 500 550 600 650 o Temperature / C

Fe-0. 34 C-5. 08 Cr-1. 43 Mo-0. 92 V-0. 4 Mn-1. 07 Si wt%

450 o C 1 h 670 HV

600 o C 1 h 530 HV

600 o C 24 h 370 HV

excess carbon in solid solution in ferrite !

Peet, Babu, Miller, Bhadeshia, 2004

30 Tesla field, 485 HV 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.

Fe-2 Si-3 Mn-C wt% Temperature / K 800 BS 600 400 MS 200 0 0 0. 2 0. 4 0. 6 0. 8 1 Carbon / wt% 1. 2 1. 4

2104 Fe-1. 75 C-Si-Mn wt% Chatterjee & Bhadeshia, 2004