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-ELath-like ferrite &
boundary cementite
B-L,S-B/Fe3C-ELath-like ferrite &
boundary cementite
B-L,S-I/Fe3C-ELath-like ferrite &
intragranular cementite
B-L,S-I/Fe3C-ELath-like ferrite &
intragranular cementite
B-L,S-B/A-LLath-like ferrite &
boundary austenite
B-L,S-B/A-LLath-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).
Tim
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
KuK
CBAt
m
0
221
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
22
10
( Martenstie-like) nucleation: Growth:
expresses the minimum free energy required to obtain bainite
is the maximum possible free energy for paraequilibruim nucleation
mGNG
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
QKN C expexp1
Isothermal formation kinetics:
2
00
2
1 expexp1 ttRT
QKKtX C
Growth of bainite (schematic)
Growth:
3
*0256
27
C
D
x
x
C dxTxDxx
D ,1
]/)(exp[)(),( 0 RTxQxDTxD CC
183,4)105.5109.138300( 255 xxQC
14
N
where:
is nucleation rate
is growth rate
is carbon diffusivity in austenite
is composition dependent carbon
diffusivity in austenite
is activation energy for bulk
diffusion of carbon in austenite
0
D),( TxDC
CQ
Material: 1.01%C – 0.27 %Si – 0.30%Mn – 1.36%Cr (100Cr6)
Application of Quidort and Brechet’s model
Comparison between experimental and calculated results
of bainite fraction at different isothermal holding temperatures
Input parameters in Quidort and Brechet’s model
[ Source: N.V. Luzginova et al., Materials Science and Engineering,2007 ]
Process:
15
Approaches available at IEHK
Bhadeshia‟s Model and its application in forging steel
Azuma‟s Model and its application in TRIP steel
Phase-field Method and its application in 100Cr6 steel
16
Quidort and Brechet‟s Model and its application in 100Cr6 steel
Bhadeshia‟s Model and its application in 25MnMoV steel
Experimental evaluation
Complete kinetic model (Azuma)
1) Without any diffusion: Transformation of γ
to supersaturated with carbon αB
2) Diffusion of the carbon out of the
supersaturated αB in γ
3) M3C precipitations in αB
4) M3C precipitations in carbon-enriched
residual austenite
Complete kinetic model which considers the nucleation and the growth of
bainitic Ferrite and carbides.
The model describes the following processes:
[ Source: Azuma et al., ISIJ Intern., 45 (2005), p. 221 ]
Schematric illustration of upper and lower bainite
microstructure development
17
Application of Azuma’s model
Material:0.60 %C – 1.50 %Si – 1.50 %Mn
[ Source: Azuma et al., ISIJ Intern., 45 (2005), p. 221 ]
Process:
Volume fraction changes of each phase
during isothermal holding at 300℃.
18
Volume fraction changes of each phase
during isothermal holding at 450℃.
Approaches available at IEHK
Bhadeshia‟s Model and its application in forging steel
Azuma‟s Model and its application in TRIP steel
Phase-field Method and its application in bearing steel 100Cr6
19
Quidort and Brechet‟s Model and its application in 100Cr6 steel
Bhadeshia‟s Model and its application in 25MnMoV steel
Experimental evaluation
Phase-field Method and its application in bearing steel 100Cr6
Phase-field method
[ Source : MICRESS Manual]
20
21
Material: 1.00%C – 0.27 %Si – 0.30%Mn – 1.43%Cr (100Cr6)
Integrating Bainitic phase
transformation model in MICRESS®
Experimental results of bainite fraction at
different isothermal holding temperatures
Process:
Experimental
Predicted
Time[s]
Fra
cti
on
Integrating Bainitic phase
transformation model in MICRESS®
Para equilibrium diagram of
100Cr6 calculated by ThermoCalc
Comparison between experimental and
calculated results of bainite fraction in
at different isothermal holding temperatures
2.5μm
30μm
Bainite fraction: 10% → 20% → 30% → 40% → 60%
22
Initial Austenite grain size: 60μm (average)
Faceted growth
[ Source: H.-S. Fang, 2002 ]
Schematics of ledgewise growth theory
Integrating Bainitic phase
transformation model in MICRESS®
23
Integrating Bainitic phase
transformation model in MICRESS®
Carbon concentration profile
Carbon diffusion during Bainite growth at
isothermal holding temperature of 260℃ in
100Cr6 simulated by Micress®
24
Equilibrium diagram of 100Cr6
calculated by ThermoCalc
Approaches available at IEHK
Bhadeshia‟s Model and its application in forging steel
Azuma‟s Model and its application in TRIP steel
Phase-field Method and its application in bearing steel 100Cr6
25
Quidort and Brechet‟s Model and its application in 100Cr6 steel
Bhadeshia‟s Model and its application in 25MnMoV steel
Phase-field Method and its application in bearing steel 100Cr6
Experimental evaluation
Experimental approaches at IEHK
New dilatometry
06/2010
Dilatometry
1994/2010
Thermomechanical Treatment Simulator
10/2009
Complex process simulation
from liquid state over the rolling
and controlled cooling process
Thermomechanical treatment
and its correlation with
mechanical properties
Contactless dilatometry
State of the art of deformation
dilatometry
Phase transformation kinetics
description
Two dimensional way of
measurement for phase
transformation description
—— anisotropic effects
measurement
New mathematical
approach developed by
WIAS for the phase
transformation evaluation
New dilatometry
Summary and outlook
- Current research topics on bainite in IEHK
- Experimental evaluation (dilatometry, TTS, SEM, EBSD)
- Comparison of the different models
Efforts vs. Accuracy
Applicability to industrial problems
Applicability to different steels
27
Thank you
for your attention!!!
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