POSCO Lectures on Bainite

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

2. Problem: to design a bulk nanocrystalline steel which is very strong, tough, cheap ….

3. Brenner, 1956

4. Morinobu Endo, 2004

5. 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

6. Equilibrium number of defects (1020) Strength of a nanotube rope 2 mm long is less than 2000 MPa

7. Scifer, 5.5 GPa and ductile Kobe Steel

9. 1 Denier: weight in grams, of 9 km of fibre 50-10 Denier Scifer is 9 Denier

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

11. Smallest size possible in polycrystalline substance?

13. Yokota & Bhadeshia, 2004

14. Summary Thermomechanical processing limited by recalescence Need to store the heat Reduce rate Transform at low temperature

15. Courtesy of Tsuji, Ito, Saito, Minamino, Scripta Mater. 47 (2002) 893. Howe, Materials Science and Technology 16 (2000) 1264.

16. Fine crystals by transformation Introduce work-hardening capacity Need to store the heat Reduce rate Transform at low temperature

18. Fe-2Si-3Mn-C wt% 800 B S 600 Temperature / K 400 M S 200 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Carbon / wt%

19. Fe-2Si-3Mn-C wt% 1.E+08 1 year 1 month Time / s 1.E+04 1.E+00 0 0.5 1 1.5 Carbon / wt%

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

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

22. 700 600 500 400 o Temperature/ oC B ~ 350 C S 300 200 o M = 120 C S 100 0 1.E+00 1.E+02 1.E+04 1.E+06 1.E+08 Time / s

24. 100 retained austenite 80 X-ray diffraction results 60 Percentage of phase 40 bainitic ferrite 20 0 200 250 300 325 o Temperature/ C

26. a g a g g 50 nm

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

28. 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

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

30. “more serious battlefield threats”

31. ballistic mass efficiency consider unit area of armour

32. Peet, Bhadeshia, 2004

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

34. Faster Transformation Cobalt (1.5 wt%) and aluminium (1 wt%) increase the stability of ferrite relative to austenite Refine austenite grain size

35. 200oC 250oC 300oC

36. original

37. Co

38. Co+Al

44. Need to improve mechanical stability of austenite

45. 180 160 140 120 100 80 60 40 20 1000 1500 2000 2500 Hard Bainite m1/2 18 wt%Ni maraging steel MPa Fracture toughness / QT Ultimate tensile strength / MPa

46. 700 650 600 550 H V 500 450 30 min 400 60 min 24 h 300 350 400 450 500 550 600 650 o Temperature / C

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

48. o 450 C 1 h 670 HV

49. o 600 C 1 h 530 HV

50. o 600 C 24 h 370 HV