About the Ferrite Property Model

The Ferrite Property Model, available with the Property Model Calculator and the Steel Model Library, calculates the thermodynamics and isothermal kinetics of proeutectoid ferrite forming from austenite in hypoeutectoid steels.

This model is for the form of proeutectoid ferrite which forms without distinct crystallographic preferences at austenite grain boundaries. This type of ferrite is referred to as grain boundary allotriomorph, or polygonal ferrite. Another type, plate-shaped ferrite (Widmanstätten ferrite), is treated in the Bainite Property Model as a special case of bainite without cementite.

About the Bainite Property Model

Growth Models

Growth is modeled based on a simplified diffusion-control theory with the ferrite/austenite interfacial composition determined by:

  • Orthoequilibrium (OE): The ferrite and austenite compositions at the interface are fixed to the final equilibrium compositions
  • Paraequilibrium (PE): Only carbon is allowed to partition, reaching PE at the interface
  • Negligible-partitioning local equilibrium (NPLE)
  • Orthoprecipitate (OP): Austenite interfacial composition being the bulk composition, ferrite interfacial composition from the maximum driving force condition (parallel tangent construction in Gibbs energy-composition diagram)
  • Paraprecipitate (PP): Only carbon is allowed to partition, otherwise the same as OP

The growth is parabolic if only diffusion-controlled. The parabolic rate constant can be modified by a phase addition because the driving force and interfacial compositions are changed. With a finite interfacial mobility and solute drag effects, the growth is then mixed-controlled. The effect is handled in the overall transformation kinetics but not reflected in the parabolic rate constant.

Nucleation Rate and Interfacial Mobility

Nucleation rate of ferrite allotriomorph on austenite grain boundaries is calculated from classical nucleation theory assuming a pillbox shape of the ferrite nucleus [1988Lan].

When you are setting up the model, parameters for nucleation rate (site density, edge energy, and epsilon, see [1988Lan] for definition) and interfacial mobility (pre-exponential factor and activation energy) can be customized. Otherwise, default values assessed by Thermo-Calc are used. The overall kinetics of isothermal transformation is calculated in the extended-volume framework by numerical integration over time. Austenite composition changes over time due to partitioning of elements, and its effects on growth and nucleation are handled.

To run calculations with the Steel Models requires a valid maintenance license plus licenses for both the TCFE (version 9 and newer) and MOBFE (version 4 and newer) databases. Also see our website to learn more about the Steel Model Library and other related examples.

The input parameters are entered on the Configuration window for the Property Model Calculator. There are also settings on the Plot Renderer where you can choose from the available and relevant axis variables.

See Ferrite Property Model Settings for details.

For an example, see PM_Fe_09: Fe-C-Ni Ferrite.

[1988Lan] W. F. Lange, M. Enomoto, H. I. Aaronson, The kinetics of ferrite nucleation at austenite grain boundaries in Fe-C alloys. Metall. Trans. A. 19, 427–440 (1988).