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Slide shows on bainite lectures by Prof. H. K. D. H. Bhadeshia
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POSCO Lectures on Bainite
Graduate Institute of Ferrous Technology
•Microstructure
•Mechanism
•Properties
•Superbainite
http://cml.postech.ac.kr/
To understand bainite, we must consider all other microstructures
in steels
0 50 100 150 200
Pressure / kbar
800
400
0
γ
α ε
1200
1600δ
0 50 100 150 200
Pressure / kbar
800
400
0
γ
α ε
1200
1600δ Bundy
(1965)
Allotropes of iron in three dimensions
Cementite Fe3C
Steel: Fe-C
ferriteaustenite
Fe3C
Diffusionless
Diffusion
Diffusion of all atoms during nucleation and growth.Sluggish below about 850 K.
Invariant-plane strain shape deformation with large shear component.No iron or substitutional solute diffusion.Thin plate shape.
Cooperative growth of ferrite & cementite.
No change in bulk composition.
Diffusionless nucleation & growth.
Carbon diffusion during paraequilibrium nucleation. No diffusion during growth.
Carbon diffusion during paraequilibrium nucleation & growth.
ALLOTRIOMORPHIC FERRITE
IDIOMORPHIC FERRITE
MASSIVE FERRITE
PEARLITE
WIDMANSTATTEN FERRITE
..
BAINITE & ACICULAR FERRITE
MARTENSITE
RECONSTRUCTIVE DISPLACIVE
log{time}
Ms
ferrite
pearlite
bainite
martensite
Ae3
upper bainite
lowerbainite
Widmanstatten ferriteBs
Ws
Allotriomorpic
Idiomorphicferrite
Pearlite
log{time}
Ms
ferrite
pearlite
bainite
martensite
Ae3
upper bainite
lowerbainite
Widmanstatten ferriteBs
Ws
20 µm
Jaramillo et al., 2004
Martensite
log{time}
Ms
ferrite
pearlite
bainite
martensite
Ae3
upper bainite
lowerbainite
Widmanstatten ferriteBs
Ws
Ferrite
Austenite
transformation twins (Wayman)
50 mm
austenite
Chang and Bhadeshia
Bainite
1 µm
upper bainite
lower bainite
Surface 1 Surface 2
Srinivasan & Wayman, 1968
50 µm
50 µm
Bhadeshia and Waugh, 1981
Takahashi and Bhadeshia
5004003000
1
2
Temperature / °C
Decarburisation
time / s
Fe-0.4C wt%
α γ
γα
TAe3Ae1
o
Carbon Concentration
T1
T1
Bhadeshia & Edmonds, 1979
TRIP-assisted steel
Fe-0.29C-1.41Si-1.42Mn wt%
(Jacques, 2001)
Growth is diffusionless.
Strain energy must be accounted for.
Takahashi and Bhadeshia
Oka and Okamoto
Ohmori and Honeycombe
The mechanism of transformation is displacive.
Transformation temperature higher than martensite.
Bainite grows without diffusion.
But carbon then escapes into the residual austenite.
Shape deformation plastically accommodated.
Sub-unit mechanism of growth
Summary
Think of bainite as martensite which tempers during transformation
DISPLACIVE
RECONSTRUCTIVE
Watson and McDougall, 1973
Mechanism displacive but carbon must partition during growth.
Pairs of plates grow together to minimise strain.
Summary
Nucleation of martensite, bainite,
Widmanstätten ferrite
α
γ
x
Gibbs free energy
Carbon
G m
xαm
γαG
Thermodynamics
Th
GN
Bhadeshia, 1981
Each point represents a different steel
Nucleation function G
N
The nucleation of bainite must involve the partitioning of carbon
Why does the required free energy vary linearly with T?
hexagonal close-packed
cubic close-packed
Christian, 1951
Brooks, Loretto and Smallman, 1979
Olson & Cohen, 1976
Nucleation of bainite must involve the partitioning of carbon.
Mechanism of nucleation is otherwise identical to that of martensite.
Diffusion of all atoms during nucleation and growth.Sluggish below about 850 K.
Invariant-plane strain shape deformation with large shear component.No iron or substitutional solute diffusion.Thin plate shape.
Cooperative growth of ferrite & cementite.
No change in bulk composition.
Diffusionless nucleation & growth.
Carbon diffusion during paraequilibrium nucleation. No diffusion during growth.
Carbon diffusion during paraequilibrium nucleation & growth.
ALLOTRIOMORPHIC FERRITE
IDIOMORPHIC FERRITE
MASSIVE FERRITE
PEARLITE
WIDMANSTATTEN FERRITE
..
BAINITE & ACICULAR FERRITE
MARTENSITE
RECONSTRUCTIVE DISPLACIVE
