Bainitic phase transformation - Bainitic phase transformation according to the diffusive mechanism [

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Transcript of Bainitic phase transformation - Bainitic phase transformation according to the diffusive mechanism [

  • Department für Ferrous Metallurgy

    RWTH Aachen University

    Bainitic phase transformation

    W. Song, H.H. Dickert, C. Keul,

    K. Mukherjee, U. Prahl, W. Bleck

    Department for Ferrous Metallurgy

    RWTH Aachen University

  • Outline

    • Bainite description

    • The debate of bainite

    • Short history of approaches

    • Approaches available at IEHK

    • Bhadeshia‟s model and its application

    • Quidort & Brechet‟s model and its application

    • Azuma‟s model and its application

    • Phase-field modeling – how to incorporate bainitic

    transformation in MICRESS®

    • Experimental evaluation

    • Summary and outlook

    2

  • Old-fashioned Bainite description

    Upper Bainite (steel with 0,1%C)

    Lower Bainite (steel with 0,6%C) [ Source: Bhadeshia, H. K. D. H.: Bainite in Steels IOM Communications Ltd.,

    2nd. Ed., Cambridge University Press (2001) ]

    Schematic presentation of the development of upper and

    lower bainite and its growth

    3

  • Classification system for microstructure

    description of bainite at IEHK

    Form

    Polygonal1

    Quasi-Polygonal1

    Granular

    Widmanstätten

    Acicular

    Lath-like2

    Basic structure (LOM) Sub structures (≤LOM)

    2nd phase form

    Round1

    Elongated2

    Lath-like2

    Film like2

    Clustered

    Crystal

    structure

    bcc

    Location

    Boundary

    Intragranular

    2nd phase

    Fe3C-Carbides

    ε -carbide

    Martensite

    Austenite

    None

    Defined using: 1Roundness (Diff. of enclosing/enclosed ellipse) 2Aspect ratio (Length/width)

    [ Source: F.Gerdemann, RWTH Achen University , PhD thesis in preparation,2010 ]

    4

  • B-L,S-B/Fe3C-E Lath-like ferrite &

    boundary cementite

    B-L,S-B/Fe3C-E Lath-like ferrite &

    boundary cementite

    B-L,S-I/Fe3C-E Lath-like ferrite &

    intragranular cementite

    B-L,S-I/Fe3C-E Lath-like ferrite &

    intragranular cementite

    B-L,S-B/A-L Lath-like ferrite &

    boundary austenite

    B-L,S-B/A-L Lath-like ferrite &

    boundary austenite

    5

    [ Source: F.Gerdemann, RWTH Achen University , PhD thesis in preparation,2010 ]

    Classification system for microstructure

    description of bainite at IEHK

  • Debate on Bainite transformation

    mechanism

    6

    H.K.D.H.Bhadeshia (1982) Bainite: overall transformation kinetics

    M.Hillert (1960), L. Kaufman

    & H. I. Aaronson (1962) Explain the growth rates as controlled by

    carbon diffusion.

    D. Quidort & Y. J. M. Brechet (2001) Diffusion controlled phase transformation model

    C. Zener (1946) Bainite forms in a manner

    similar to martensite

    Diffusive Mechanism Displacive Mechanism

    Discovery of Bainite

    (1930, Bain)

    A. Hultgren (1947) Bainite forms following ledgewise growth

    mechanism (based on microstructure

    observations)

    T. Ko & A. H. Cottrell (1952) Surface relief in lower Bainite

    → similar to martensite

    R. F. Hehemann (1972) “ It„s difficult to argue against

    these diffusion controlled

    models. ”

    R. F. Hehemann, K. R.

    Kinsman, and H. I. Aaronson,

    Trans. AIME. 3, 1077 (1972).

    T im

    e

    A. P. Miodownik (1956) Surface relief in Widmanstätten ferrite

    W.-Z Zhang and G. C. Weatherly, Acta Mater.

    46, 1837 (1998).

    J. P. Hirth, G. Spanos, M. G. Hall, and H. I.

    Aaronson, Acta Mater. 48, 1047 (1997).

    R. C. Pond, P. Shang, T. T. Cheng, and M.

    Aindow, Acta Mater. 48, 1047 (2000).

    Surface relief ≠> Martensitic type of growth

  • Short history of approaches

    •Nucleation controlled

    •Taking carbides precipitation

    into consideration

    •Diffusion controlled

    •No consideration of carbides

    •Nucleation controlled

    •Si + Al > 1%

    •No consideration of carbides

    •Nucleation controlled

    •Si + Al > 1%

    •No consideration of carbides

    •Nucleation controlled

    •Si + Al > 1%

    •No consideration of carbides

    •Nucleation controlled

    •Si + Al > 1%

    •No consideration of carbides

    Phase Field Method

    Calphad Method Dictra

    ThermoCalc

    MICRESS®

    7

  • Approaches available at IEHK

    Bhadeshia‟s Model and its application in 25MnMoV steel

    Quidort and Brechet‟s Model and its application in 100Cr6 steel

    Azuma‟s Model and its application in TRIP steel

    Phase-field Method and its application in 100Cr6 steel

    8

    Experimental evaluation

  • Bainite formation by displacive

    mechanism

    • In the first step, the bainitic laths are

    formed by displacive mechanism,

    which is similar to the martensite

    formation.

    • In the second step, a redistribution

    of carbon from the supersaturated

    bainitic ferrite into the austenite

    occurs.

    Different stages of the development of the

    bainitic microstructure

    [ Source: Bhadeshia, H. K. D. H.: Bainite in Steels IOM Communications Ltd., 2nd. Ed., Cambridge University Press (2001) ]

    9

  • Model for the bainite formation by

    displacive mechanism (Bhadeshia)

    Thermodynamical criteria for the stability of bainite

    where:

      molJTGN /254015,273637,3  ThermoCalcGm : 0)(

    00

    Nmmm GGGG  

    Nm GG  molJG /400 

    Analytical solution:

               

     

     

      

     

    rRT

    GK

    RT

    K uK

    CBA t

    m

    0

    22 1

    22

    exp

    exp11ln1ln 

    Equation of the time dependent volume fraction at different

    temperatures:

      

      

     

     

      

      

     2

    0

    21 exp)1)(1( r

    G

    RT

    KuK

    dt

    d m

    where:  

    rRT

    GGK Nm  0

    2 2

    10

    ( Martenstie-like) nucleation: Growth:

    expresses the minimum free energy required to obtain bainite is the maximum possible free energy for paraequilibruim nucleation

    mG NG

  • Fit parameter in Bhadeshia’s model

    Application of Bhadeshia’s model

    450 ℃ 475 ℃ 500 ℃

    Material: 25MnMoV

    [ Source: C.Keul, RWTH Achen University , Diploma thesis,2006 ]

    Comparison between experimental and calculated results of bainite fraction at different isothermal

    holding temperatures

    Process:

    11

  • Approaches available at IEHK

    Bhadeshia‟s Model and its application in 25MnMoV steel

    Quidort and Brechet‟s Model and its application in 100Cr6 steel

    Azuma‟s Model and its application in TRIP steel

    Phase-field Method and its application in 100Cr6 steel

    12

    Experimental evaluation

  • Bainite formation by diffusive

    mechanism

    • In the first step, bainite forms

    with the same mechanism as

    Widmanstätten ferrite, there is

    no supersaturation of carbon

    in the bainitic ferrite.

    • Afterwords, a mixture of ferrite

    and cementite forms between

    the bainitic laths.

    Bainitic phase transformation according to

    the diffusive mechanism

    [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions of the American Society for Metals, 1947, Vol. 39,

    pp. 915-1005 ]

  • Model for the diffusion controlled bainite

    formation (Brechet & Quidort)

    Nucleation (Classical nucleation theory):

     

      

      

      

     

    

    RT

    G

    RT

    Q KN C expexp1

    Isothermal formation kinetics:

          

      

     

     

      

    2

    00

    2

    1 expexp1 tt RT

    QK KtX C 

    Growth of bainite (schematic)

    Growth:

    3

    *0 256

    27 

    C

    D

     

      

    

    

    x

    x

    C dxTxD xx

    D , 1

    ]/)(exp[)(),( 0 RTxQxDTxD CC  

    183,4)105.5109.138300( 255  xxQC 

    14

    N

    where:

    is nucleation rate