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

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 (10 20 ) 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. Strength produced by deformation limits shape: wires, sheets... Strength in small particles relies on perfection. Doomed as size increases. Summary

11. Smallest size possible in polycrystalline substance?

13. Yokota & Bhadeshia, 2004

14. Thermomechanical processing limited by recalescence Summary 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. 0 200 400 600 800 00.20.40.60.811.21.4 Carbon / wt% Temperature / K Fe-2Si-3Mn-C wt% B S M S

19. 1.E+00 1.E+04 1.E+08 00.511.5 Carbon / wt% Time / s Fe-2Si-3Mn-C wt% 1 month 1 year

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

21. 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

22. 0 100 200 300 400 500 600 700 1.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

24. X-ray diffraction results 0 20 40 60 80 100 200250300325 Temperature/ o C Percentage of phase bainitic ferrite retained austenite

26. 50 nm     

27. 200 Å      Caballero, Mateo, Bhadeshia

28. Low temperature transformation: 0.25 T/T m Fine microstructure: 20-40 nm thick plates Harder than most martensites (710 HV) Carbide-free Designed using theory alone

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

30. “more serious battlefield threats”

31. ballistic mass efficiency consider unit area of armour

32. Peet, Bhadeshia, 2004

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

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

35. 200 o C 250 o C 300 o C

36. original

37. Co

38. Co+Al

44. Need to improve mechanical stability of austenite

45. Hard Bainite 2500200015001000 20 40 60 80 100 120 140 160 180 Ultimate tensile strength / MPa Fracture toughness / MPa m 1/2 18 wt%Ni maraging steel QT

46. 400 450 500 550 600 650 700 300350400450500550600650 Temperature / o C H V 30 min 60 min 24 h

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

48. 450 C 1 h 670 HV o

49. 600 C 1 h 530 HV o

50. 600 C 24 h 370 HV o

51. excess carbon in solid solution in ferrite !

53. Peet, Babu, Miller, Bhadeshia, 2004

58. 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. 30 Tesla field, 485 HV

59. 0 200 400 600 800 00.20.40.60.811.21.4 Carbon / wt% Temperature / K Fe-2Si-3Mn-C wt% B S M S

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