About the Bainite Property Model

The Bainite Property Model, available with the Property Model Calculator and the Steel Model Library, describes the thermodynamics and isothermal kinetics of bainite transformation from austenite. In this Model, bainite is modeled as ferrite or a ferrite-cementite mixture.

For a given austenite composition, the bainite start temperature, WBs, is calculated following Leach et al. [2018Lea] (see Eqs. 8 and 9 and Table 1 in the paper). At WBs, the driving force for ferrite to precipitate out of austenite under the “para” condition (i.e. no partitioning of substitutional elements, carbon in equilibrium) is balanced by a barrier, which is modeled as a function of composition.

At a temperature below WBs, there is a net driving force (over the barrier) for the growth of bainitic ferrite. The steady-state lengthening rate of ferrite plate is calculated based on the method from Leach et al. [2019Lea] with modifications. In the Bainite Model, unlike in [2019Lea], local equilibrium of carbon at the ferrite/austenite is sought, and a finite interfacial friction term is added to the dissipation terms. Otherwise the two methods are both based on a driving-force–dissipation balance and the Zener-Hillert equation, with a unique lengthening rate determined by maximizing it with respect to radius of curvature at the plate tip.

The Bainite Model also considers the case where cementite (under the “para” condition) can precipitate on the ferrite/austenite interface and grow into ferrite, forming a ferrite-cementite mixture. The formation of para-cementite usually accelerates lengthening, by enlarging the driving force of transformation while reducing carbon partitioning from bainite to austenite. Carbon partitioning to austenite vanishes if para-cementite forms in bainite to its maximum fraction allowed by mass balance.

Thickening of bainitic plates is usually much slower than lengthening. In the Bainite Model, it is assumed that whenever there is a positive driving force of precipitating para-cementite on the ferrite/austenite interface, the final state of the bainite transformation should be a mixture of ferrite and cementite, with austenite being completely transformed. Otherwise, the final state is a ferrite-austenite mixture with no cementite but a finite fraction of retained austenite (incomplete transformation).

The overall kinetics of bainite transformation is calculated in the extended-volume approach devised by Cahn [1956Cah] and generalized for ellipsoids. Both grain boundary (GB) nucleation and volume nucleation are considered, the former being very temperature-sensitive, the latter temperature-insensitive. This is used to model the kinks at about 350 °C in the TTT diagrams of some medium- and high-carbon steels—At higher temperatures GB nucleation is predominant, whereas at lower temperatures it is effectively suppressed, and volume nucleation becomes significant.

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 Bainite Property Model Settings for details.

For an example, see PM_Fe_05: Fe-C-Mn-Si-Ni-Cr-Mo Bainite.

[1956Cah] J. W. Cahn, “The kinetics of grain boundary nucleated reactions”, Acta Metallurgica 4, 449–459 (1956).

[2018Lea] L. Leach, P. Kolmskog, L. Höglund, M. Hillert, and A. Borgenstam, “Critical driving forces for formation of bainite”, Metallurgical and Materials Transactions A 49, 4509–4520 (2018).

[2019Lea] L. Leach, J. Ågren, L. Höglund, and A. Borgenstam, “Diffusion-controlled lengthening rates of bainitic ferrite a part of the Steel Genome”, Metallurgical and Materials Transactions A 50, 2613–2618 (2019)