POSCO Lectures on Bainite

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POSCO Lectures on Bainite

• Microstructure
• Mechanism
• Properties
• Superbainite

Graduate Institute of Ferrous Technology
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http//cml.postech.ac.kr/
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To understand bainite, we must consider all other
microstructures in steels
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Bundy (1965)
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Allotropes of iron in three dimensions
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Cementite Fe3C
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Steel Fe-C
ferrite
austenite
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Fe3C
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Diffusionless
Diffusion
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AllotriomorpicIdiomorphicferrite
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Pearlite
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20 µm
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Jaramillo et al., 2004
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Martensite
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Ferrite
Austenite
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transformation twins (Wayman)
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austenite
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Chang and Bhadeshia
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Bainite
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upper bainite
1 µm
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lower bainite
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Surface 1
Surface 2
50 µm
Srinivasan Wayman, 1968
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50 µm
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Bhadeshia and Waugh, 1981
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Takahashi and Bhadeshia
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Fe-0.4C wt
Decarburisation time / s
Temperature / C
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Bhadeshia Edmonds, 1979
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TRIP-assisted steel Fe-0.29C-1.41Si-1.42Mn wt
(Jacques, 2001)
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Growth is diffusionless.
Strain energy must be accounted for.
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Takahashi and Bhadeshia
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Oka and Okamoto
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Ohmori and Honeycombe
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Summary
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
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Think of bainite as martensite which tempers
during transformation
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DISPLACIVE
RECONSTRUCTIVE
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Watson and McDougall, 1973
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Summary
Mechanism displacive but carbon must partition
during growth. Pairs of plates grow together to
minimise strain.
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Nucleation of martensite, bainite, Widmanstätten
ferrite
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Thermodynamics
g
a
Gibbs free energy
x
Carbon
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Each point represents a different steel
Bhadeshia, 1981
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The nucleation of bainite must involve the
partitioning of carbon
Why does the required free energy vary linearly
with T?
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hexagonal close-packed
cubic close-packed
Christian, 1951
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Brooks, Loretto and Smallman, 1979
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Olson Cohen, 1976
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Nucleation of bainite must involve the
partitioning of carbon.
Mechanism of nucleation is otherwise identical to
that of martensite.
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