Post on 15-Dec-2020
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Numerical Alloy Design for Case Hardening Steels with
DP microstructure H. Farivar, U. Prahl
Thermo-Calc User Meeting 11. - 12. September 2014
ACCESS e.V. Materials & Processes
Intzestr. 5, D-52072 Aachen
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Part 1:
Idea of the project
Part 2:
• How can we benefit from Thermo-Calc Software?
Numerical Alloy Design for Case Hardening Steels with
DP microstructure H. Farivar, U. Prahl
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Overall purpose (Part 1):
Applying DP concept in the gear steels in order to reduce distortion and imporve mechanical properties.
Purpose of this work (Part 2):
• Using numerical thermodynamic based approaches by means of ThermoCalc, DICTRA an alloy
concept for DP steel.
• The design approach concentrates on critical transformation temperatures and kinetics of phase
transformation as function of (local) chemical composition being predicted from forming simulation and
carburising simulation.
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Effects of micro-alloying elements:
Transformation temperatures
Grain growth control
Suprresing low temperature tempering
Advantages of DP steel:
Using the exisiting manufacturing processes and lines
Decreasing in rejection rate and tooth profile correction
Fatigue strength
Effects of Aluminum nitrides (ALN)
Numerical Alloy Design for Case Hardening Steels with DP microstructure
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Chemical Composition (mass%)
Steel C Si Mn Cr Mo Nb
Dual Phase Steel 0.21 1.43 0.43 0.61 0.78 0.021
Conventional Steel 0.21 0.21 0.76 1.10 0.18 ---
Ide
a
Ac3
Acm
0.18 0.80
Carburization Temp.
Quenching Temp.
0.18 0.80
Ac3
Acm
Carburization Temp.
Quenching Temp.
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Fatigue strength
Heat treatment induced distortion
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Quenching Temperature
Distortion
Influences of Quenching Temperature on final microstructure
• Martensite Volume Fraction
• Martensite Distribution
• Retained Austenite
• …
Martensite Volume Fraction
Coordinate Measurment Method (CMM)
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Although the amount of distortion is influenced mainly by shape of a part, other significant factors are :
Chemical composition of the steel
Quenching temperature
Residual stresses in the parts prior to heat treatment
Methods of stacking and fixturing parts during heating and quenching
Growth of surfaces during carburizing
Severity of the quench
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Carbon 0.18 % , 0.80 %
Silicon 0.20 % - 0.70 %
Manganese 0.40 % - 1.50 %
Chromium 0.30 % - 1.00 %
Molybdenum 0.20 % - 0.50 %
C Si Mo Mn Cr X X X X
2 6 8 11 6 = 6336
Total number of calculations done in
Thermo-Calc
X X X X
Increment = 0.1
Defining ranges for variation of element contents
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Batch.sh (Shell Script)
Linux Shell (Bash) Executable, run
General Scheme of the codes:
AlloyDesign.TCM
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Collecting all resulted chemical composition sets and their critical temperatures:
C % Si % Mn % Cr % Mo % Ac3 (ºC) Acm (ºC)
0.18 0.20 0.40 0.30 0.20 839 779
0.18 0.30 0.40 0.50 0.25 845 785
0.18 0.40 0.40 1.00 0.30 839 792
. . . . . . .
. . . . . . .
. . . . . . .
How should we
sort these chemical sets in a wise way?
Criteria:
The more ∆T, the more flexibility
The lowest possible Acm is preferable
The more Ideal Diameter, the more Hardenable material ! (The most hardenable material should be preferred?)
Reasonable time for reaching a specified volume fraction of ferrite in the core
Reasonable time for enriching the carbon content of the case to a specified amount
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C % Si % Mn % Cr % Mo % Ac3 (ºC) Acm (ºC)
∆T (ºC)
0.18 0.70 0.40 0.30 0.50 869 793 76
0.18 0.70 0.40 0.30 0.45 868 792 75
0.18 0.40 0.40 0.80 0.40 867 791 74
. . . . . . . . 0.18 0.80
Ac3
Acm
Carburization Temp.
Quenching Temp. ∆T
If ∆T (ºC) < 50 ºC
exclude from the list
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ASTM Specification A255-02
(Standard Test Method for Determining Hardenability of Steel)
Database for Calculating the DI (ideal diameter):
• Quantitative measure of a steel’s hardenability
is expressed by its DI, or ideal diameter, value.
• DI values are an excellent means of comparing
the relative hardenability of two materials as
well as determining if it is possible to harden a
particular cross section (or ruling section) of a
given steel.
DI = DI Jominy × fMn × fSi × fNi × fCr × fMo
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DI
(ideal diameter)
C % Si % Mn % Cr % Mo % Ac3 (ºC) Acm (ºC)
∆T (ºC) DI
(mm)
0.18 0.70 1.00 0.30 0.50 847 794 53 101
0.18 0.60 1.00 0.30 0.50 842 791 50 96
0.18 0.70 0.80 0.40 0.50 852 800 52 95
. . . . . . . .
If ideal diameter < 80 mm
exclude from the list
Database
At this stage we are left with 25 sets .
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α + ɣ
ɣ
ɣ+cementite
Criteria:
The more ∆T, the more flexibility
The lowest possible Acm is preferable
The more Ideal Diameter, the more Hardenable material ! (The most hardenable material should be preferred?)
Reasonable time for reaching a specified volume fraction of ferrite in the core
Reasonable time for enriching the carbon content of the case to a specified amount
C % Si % Mn % Cr % Mo % Ac3 (ºC) Acm (ºC)
∆T (ºC) DI
(mm)
0.18 0.40 0.90 0.30 0.50 840 786 51 80
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α ɣ
Dictra Calculations (future work):
• Time needed for ferrite formation matters. The
shorter holding time at the quenching temperature
is preferable.
• Considering the effect of all alloying elements on
diffusion process
v Equi. State
DICTRA
C % Si % Mn % Cr % Mo %
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Dictra Calculations (future work):
• Time needed for enriching the surface matters.
The shorter holding time in the furnace is
preferable.
• Considering the effect of all alloying elements on
carburization and carbon diffusion process
Carburization Process Carbon Diffusion Process
DICTRA