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
- Slides: 60