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Posco Bainite 1
 

Posco Bainite 1

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Slide shows on bainite lectures by Prof. H. K. D. H. Bhadeshia

Slide shows on bainite lectures by Prof. H. K. D. H. Bhadeshia

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    Posco Bainite 1 Posco Bainite 1 Presentation Transcript

    • 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
    • Bundy (1965)
    • Allotropes of iron in three dimensions
    • Cementite Fe 3 C
    • Steel: Fe-C ferrite austenite
    • Fe 3 C
    • Diffusionless Diffusion
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    • Allotriomorpic Idiomorphic ferrite
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    • Pearlite
    • 20 µm
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    • Jaramillo et al., 2004
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    • Martensite
    • Ferrite Austenite
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    • transformation twins (Wayman)
    • 50 m m
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    • austenite
    • Chang and Bhadeshia
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    • Bainite
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    • 1 µm upper bainite
    • lower bainite
    • Surface 1 Surface 2 Srinivasan & Wayman, 1968 50 µm
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    • 50 µm
    • Bhadeshia and Waugh, 1981
    • Takahashi and Bhadeshia
    • Temperature / °C Decarburisation time / s Fe-0.4C wt% 500 400 300 0 1 2
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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.
    • Takahashi and Bhadeshia
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    • 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
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    • DISPLACIVE RECONSTRUCTIVE
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    • Watson and McDougall, 1973
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    • Mechanism displacive but carbon must partition during growth. Pairs of plates grow together to minimise strain. Summary
    • Nucleation of martensite, bainite, Widmanstätten ferrite
    •   x Gibbs free energy Carbon Thermodynamics  G m x  m   G
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    • T h G N
    • 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
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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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