1. Fundamentals and Applications of Bainitic Steels
Thermodynamics
Simulation of kinetics
“Superbainite”
Mechanical properties
Future

4. upper bainite
1 µm

5. lower bainite

6. Surface Surface 2
50 µm
Srinivasan & Wayman, 1968

7. s d c r 1

11. 50 µm

13. Carbon supersaturated plate
Carbon diffusion into
Carbon diffusion into
austenite
austenite and carbide
precipitation in ferrite
Carbide precipitation
from austenite
UPPER BAINITE
LOWER BAINITE
(High Temperature)
(Low Temperature)

14. 1 2
Fe-0.4C wt%
Decarburisation
time / s
500
400
300
Temperature / °C

16. Temperature
Ae3' T' o x
Carbon in austenite

18. Growth is diffusionless.
Strain energy must be accounted for.

19. Takahashi and Bhadeshia
Carbon supersaturated plate
Carbon diffusion into
Carbon diffusion into
austenite
austenite and carbide
precipitation in ferrite
Carbide precipitation
from austenite
UPPER BAINITE
LOWER BAINITE
(High Temperature)
(Low Temperature)
Takahashi and Bhadeshia

21. Oka and Okamoto

22. Ohmori and Honeycombe

24. T h G N

25. Each point represents a different steel
Nucleation function G N
Each point represents a different steel
Bhadeshia, 1981

26. The nucleation of bainite must involve the partitioning of carbon
Why does the required free energy vary linearly with T?

27. hexagonal close-packed cubic close-packed
Christian, 1951

28. Brooks, Loretto and Smallman, 1979

29. Olson & Cohen, 1976

33. Nucleation of bainite must involve the partitioning of carbon.
Mechanism of nucleation is otherwise identical to that of martensite.

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

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

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

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

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

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

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

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

43. “more serious battlefield threats”

44. ballistic mass efficiency
consider unit area of armour

48. Caballero, Mateo, Bhadeshia

49. Caballero, Mateo, Bhadeshia

51. Sherif, 2005, Ph.D. thesis, Cambridge

52. Below percolation threshold
Above percolation threshold

53. Geometrical percolation threshold of overlapping ellipsoids

55. 0.4 C
2 Si
3 Mn
wt%
1 µm

56. Very poor toughness!

57. 50 µm

58. stress transfer length
Fe-1C-1.5Si…… wt%
periodic cracking
stress transfer length
Chatterjee & Bhadeshia, 2005

60. Carbide-free alloys
wt %

62. Impact Energy Charpy impact / J Temperature / °C Test temperature / °C10 20 30 40 50 60 70
0.4C-3Mn-2Si
0.4C-4Ni -2Si
0.2C-3Mn-2Si
Charpy impact / J
200
100
-100
-200
Temperature / °C
Test temperature / °C

64. kilocycles to Crack Initiation
Pearlite
Martensite
Bainite
1200
1000
800
600
400
200
kilocycles to Crack Initiation
Yates, Jerath

65. Yates, Jerath

66. U.K.