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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