Institut für Eisenhüttenkunde

Department of Ferrous Metallurgy

Multiscale phase field approach to

bainite transformation

Wenwen Song Ulrich Prahl Wolfgang Bleck

Thermocalc User Meeting, Sep. 11-12, 2014, ACCESS, Aachen

Introduction and motivation

Mechanical properties

• High tensile strength (> 2 GPa)

• High hardness (> 700HV)

• Fatigue strength

• Wear resistance

• Ductility

• Toughness

• ……

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C Mn Si Cr Mo P S Ni Al Cu Co

wt.% 0.967 0.232 0.303 1.38 0.0172 0.0027 <0.001 0.0724 0.0263 0.0471 0.0124

at.% 4.325 0.227 0.58 1.426 0.0096 0.0047 0.0017 0.0663 0.0524 0.0398 0.0113

Chemical composition

of the investigated steel 100Cr6

Research approaches

TEM Characterization

Phase field simulation

Atom Probe Tomography

Ab initio calculation

Experimental

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Heat treatment cycle of the investigated

steel 100Cr6. (ITT stands for Isothermal

Transformation Temperature)

• Specimens are φ3 mm×10

mm.

• TEM foils were prepared

from the heat-treated

specimen with a twin-jet

electro polishing device.

• APT specimens were cut

from the heat-treated

material and electro-

polished with standard

electro-polishing methods.

Spheroidized carbide & cementite in bainite

C

Cr

Mn

Si (Fe,Cr)3C / Fe3C

boundary

Fe3C/αB

boundary

Z zone

(Fe,Cr)3C + Fe3C αB

(Fe,Cr)3C + Fe3C

~12nm Cr atoms map

αB

αB

αB (Fe,Cr)3C Fe3C

(Fe,Cr)3C Fe3C

(Fe,Cr)3C

Fe3C

αB

(Fe,Cr)3C

Fe3C

αB

1-Dimensional concentration profile showing the

distribution of C, Si, Cr, Mn in undissolved spheroidized

carbide (Fe,Cr)3C, newly formed cementite at 500 °C and

bainitic ferrite matrix.

Cr exhibits a gradual

chemical gradient from

surface to the core in

spheroidized carbides.

Si exhibits a large

enrichment at the growth

front of cementite, which

hinders the coarsening of

cementite particles.

The spheroidized carbide

may exist as a nucleation site

for the precipitation of

cementite within bainite.

This microstructral feature

might be benefit for wear

resistance and fatigue

properties of the material.

Spheroidized carbide & cementite in bainite

Carbide precipitation within bainite structure in 100Cr6

Gibbs free reactions energies between ε Fe2.4C and

cementite (Fe3C, θ) as a function of temperature in a

ferritic and austenitic matrix.

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ε and θ carbides in ferrite have almost identical thermodynamic stability. In austenite, however,

cementite formation is clearly preferred. This indicates that ε carbide is more prone to

precipitation from lower bainite than from upper bainite.

θ and ε carbide precipitation in lower bainite in 100Cr6

isothermally heat-treated at 260 °C for 2500 s.

Atom Probe Tomography Ab initio calculation

Simulation of bainite transformation in 100Cr6

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Phase field simulation of isothermal bainitic transformation in 100Cr6

Simulated microstructure evolution and carbon concentration

evolution v.s. SEM experimental observation.

Simulated transformation kinetics

v.s.Dilotometry and SEM experimental data

Atom probe data input

Simulation of not only

bainitic ferrite formation

but also nano-sized carbide

precipitation.

Microstructure evolution

Fig.. Predicted microstructure evolution in 100Cr6 during

isothermal bainite transformation at 260 °C. Red represents

austenite (γ), yellow bainitic ferrite (αB), and white cementite

(θ). Interfaces are colourized in blue. The simulation reveals that

finely dispersed cementite forms inside the lower bainite.

Fig. TEM micrographs of investigated 100Cr6

steel, heat treated at 850°C austenitization for

300 s followed by isothermal bainite heat

treatment at 260 °C for 500 s and a subsequent

rapid cooling to room temperature.

PF simu v.s. TEM

Phase transformation kinetics

Simulated bainite transformation kinetics v.s experimentally observed

bainite transformation kinetics measured by dilatometry for 100Cr6 steel.

Carbon partitioning

Carbon concentration evolution along

a-b line showing the migrating

interfaces and carbon redistribution

behavior during bainitic ferrite

thickening during bainite formation at

260 °C in 100Cr6; ① indicates the γ/αB

interface; ② and ③ indicate the αB/αB

interfaces.

Predicted microstructure and carbon

concentration in the 100Cr6 steel during

isothermal bainite formation at 260 °C for

400 s.

Conclusions and Outlooks

With the aid of atom probe detected concentration data input

and para-equilibrium phase diagram calculation using Thermo-

Calc software, microstructure evolution and phase

transformation kinetics during isothermal bainite formation is

predicted by means of phase field (PF) simulation.

Phase field simulation is a helpful and promising tool to solve

the diffusion problem in bainite transformation simulation.

In the future, to achieve a full image of bainite transformation

simulation, the plasticity part will be included; the simulation

of two-step bainite transformation and continuous bainite

transformation will also be taken into consideration.

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Acknowledgments

This work has been performed within the project “Diffusion

controled bainitic phase transformation“ in Interdisciplinary Centre

for Advanced Materials Simulation (ICAMS) at Ruhr University

Bochum.

The authors gratefully acknowledge the Atom Probe experimental

support from Max-Planck Institut für Eisenforschung GmbH (MPIE)

and ab initio calculation support from Dr.Von Appen in IAC institute

in RWTH Aachen University .

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Thank you

for your attention!