Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive...

28
Bainitic phase transformation W. Song, H.H. Dickert, C. Keul, K. Mukherjee, U. Prahl, W. Bleck Department for Ferrous Metallurgy RWTH Aachen University

Transcript of Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive...

Page 1: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 2: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 3: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 4: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 5: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 6: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 7: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 8: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 9: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 10: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 11: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 12: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 13: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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 ]

Page 14: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 15: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 16: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 17: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 18: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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℃.

Page 19: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 20: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

Phase-field method

[ Source : MICRESS Manual]

20

Page 21: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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:

Page 22: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 23: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

[ Source: H.-S. Fang, 2002 ]

Schematics of ledgewise growth theory

Integrating Bainitic phase

transformation model in MICRESS®

23

Page 24: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 25: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 26: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 27: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

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

Page 28: Bainitic phase transformation - ICAMS · Bainitic phase transformation according to the diffusive mechanism [ Source: Hultgren, B.: Isothermal transformation of austenite Transactions

Thank you

for your attention!!!