TTT Diagram Property Model Settings

Select an option from the Austenite composition from list:

• Nominal composition uses the system composition as austenite composition.
• Equilibrium composition at austenizing temperature. Select to enter an Austenitizing temperature. The austenite composition is determined from an equilibrium calculation at the austenitizing temperature (if austenite is stable). This option is useful when austenite coexists with other phase(s), e.g. primary carbide, before the transformation(s) being calculated starts. But the Model only considers the effect of other phase(s) on austenite composition.

This setting is available when Equilibrium composition at austenitizing temperature is selected from the Austenite composition from list.

Enter an Austentizing temperature in the field. The unit is the same as that selected under Condition Definitions.

Enter the Grain size of the austenite in micrometers (μm). The default is 100 μm.

Quick is used for a relatively quick and rough calculation to reveal the general feature of a TTT diagram. Interpolation is used when necessary in order to avoid calculating the growth rates and nucleation rates of pearlite and bainite at every time step.

• For simplicity, Ferrite mode is PE (paraequilibrium), and Pearlite mode is Para-pearlite.
• Recommended temperature interval: 50-100 K.

Each temperature can take several minutes.

Accurate evaluates the growth rates and nucleation rates at every time step accurately (without interpolation). The time step control is also tighter than when using the Quick setting.

• Ferrite mode is faster start, meaning the mode between OE and PE which gives a faster initial transformation.
• Pearlite mode is Optimal pearlite, but its rate calculation becomes slower if austenite contains many alloying elements.
• Recommended temperature interval: 20-50 K or even lower.

Calculating the whole TTT diagram may take several hours.

When you choose Quick or Accurate as the calculation setting, the following is what is automatically set in the background for each of the sections. These sections are visible to you and also can be edited when Custom is chosen:

By default the Ferrite selected, Pearlite selected, Bainite selected, and Martensite selected checkboxes are selected. For ferrite and pearlite, additional settings are then available.

Click to deselect as required to exclude one or more from the calculation.

This setting is available when the Pearlite selected checkbox is selected.

Select an option from the Criterion list to determine the pearlite growth condition. This list is called Pearlite criterion for the CCT Diagram and TTT Diagram Property Models.

• For Maximize growth rate, pearlite growth rate is maximized with respect to lamellar spacing (and for optimal pearlite, partition coefficients of substitutional alloying elements between ferrite and cementite). The calculation gives maximal growth rate and minimal lamellar spacing. This is the default criterion.
• For Maximize Gibbs energy dissipation rate, Gibbs energy dissipation rate is maximized with respect to lamellar spacing (and for optimal pearlite, partition coefficients of substitutional alloying elements between ferrite and cementite). The calculation gives a smaller growth rate than the maximal, and a larger lamellar spacing than the minimal, which may better represent the average growth rate and lamellar spacing of all pearlite colonies in the material.

From the Carbide in pearlite list, select the carbide to be present in pearlite: CEMENTITE (default), M7C3, or M23C6. For M23C6, this usually depends on Cr content.

When M23C6 is selected, parameters optimized for stainless steels are invoked.

Recommended Composition Ranges for Steel Models

From the Carbide in bainite list, select the carbide to be present in pearlite: CEMENTITE (default) or M7C3. For M7C3, this usually depends on Cr content.

This setting is available when the Pearlite selected checkbox is selected. It is used to determine how substitutional alloying elements partition between ferrite and cementite in pearlite. Select an option from the Pearlite mode list.

• For Optimal pearlite, the partitioning of substitutional alloying elements between ferrite and cementite is optimized according to the Criterion while keeping carbon equilibrium between the two phases. This is the default mode. This mode realizes a smooth transition between ortho-pearlite and para-pearlite.
• For Para-pearlite, ferrite and cementite are in para-equilibrium where substitutional alloying elements do not partition and carbon reaches equilibrium.
• For Ortho-pearlite, ferrite and cementite are in ortho-equilibrium.

If the Use interpolation when necessary checkbox is selected, and depending on alloy composition and temperature, if the code determines that an interpolation is necessary and possible to implement, then a composition grid (TTT diagram) or composition-temperature grid (CCT diagram) is generated and refined as needed.

The growth rates and nucleation rates of pearlite and bainite, as functions of composition (TTT diagram) or composition and temperature (CCT diagram), are determined by interpolation (if sufficiently accurate). This is intended to avoid direct calculations of pearlite and bainite rates for every time step, which is usually time-consuming.

This setting is available when the Use interpolation when necessary checkbox is selected.

The growth and nucleation rates of pearlite and bainite, if determined from interpolation, are accepted only when the relative interpolation error estimated is below the value entered here. A large number implies few grid points necessary (shorter calculation time) and lower accuracy, and a small number implies the opposite.

The Maximum phase fraction change (absolute) in a time step and Maximum phase fraction change (relative) in a time step settings are used to control time step size during time integration.

The phase fraction (X) of ferrite, pearlite, and bainite cannot change more than during a time step, where and are the absolute and relative maximum change, respectively. If the change is too large, the new time step is rejected, and time integration is retried with a smaller time step.

The Error tolerance for austenite fraction setting is used to control the accuracy for austenite fraction during time integration. Finite time steps cause an error in austenite volume fraction. When the error is larger than the tolerance, time integration is backed up and retried with a smaller time step.