institut für eisenhüttenkunde department of ferrous...
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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!