Recommended Composition Ranges for Steel Models
The Steel Property Models (a.k.a. Steel Models or just Models) included with the Steel Model Library in Thermo-Calc calculate the process of austenite decomposition into ferrite, pearlite, bainite, and martensite. These Models are developed to satisfy the needs for a wide range of steels and are built on the thermodynamic and kinetic descriptions in the TCS Steel and Fe-alloys Database (TCFE) and TCS Steels/Fe-Alloys Mobility Database (MOBFE) databases, which are widely validated. However, the Steel Models also include their own modeling approach and parameters, which may limit the applicability to certain ranges of compositions and temperature, often due to some underlying assumptions of the modeling approaches and/or availability of calibration data.
In general, the Models for ferrite, bainite, and pearlite are for low-alloy, low-to-medium carbon steels involving austenite (Fe-rich disordered FCC_A1) transforming into ferrite (Fe-rich disordered BCC_A2) and cementite. For steels with higher Cr contents, it is also possible to select M7C3 and M23C6 to be the carbide in pearlite, and M7C3 for bainite. Otherwise, the Models are not designed for other phase relations, for example:
- Austenite transforming into a mixture of ferrite and another type of carbide (e.g. kappa carbide for high-Mn high-Al alloys).
- Precipitation (e.g. FCC-Cu, FCC-carbides due to microalloying elements Ti, V, Nb) taking place concurrently while austenite is transforming into ferrite (and possibly cementite).
If some other phases coexist with austenite prior to its transformation, the Steel Models assume these do not participate in or affect the transformations by any means (e.g. by providing extra nucleation sites). The only effect of these phases is to determine the austenite composition (which is now different from the nominal composition). In this case, the setting Equilibrium composition at austenitizing temperature is selected from the Austenite composition from list on the Configuration window.
For an explanation of Austenite composition from see the individual configuration settings for the different Steel Models (Ferrite, Bainite, and Pearlite). Links to these are listed in About the Steel Model Library Property Models.
Ferrite and bainite calculations rely on carbon partitioning from ferrite to austenite. Therefore, the Models are not suitable for the ferrite transformed from austenite in a "massive" manner (composition invariant), in some "interstitial-free" ferrous alloys with very low carbon contents.
Pearlite is assumed to be steady-state, i.e. growth rate and lamellar spacing are constant over time under isothermal conditions. This is reasonable if composition and transformation temperature lie in the ferrite-cementite two-phase region of a phase diagram. The Pearlite Model does not cover the so-called "divergent pearlite", or "partitioned pearlite", which usually forms in the austenite-ferrite-cementite three-phase region with growth rate and lamellar spacing changing over time. Also, pearlite nucleation is modeled to be on austenite grain boundaries only. In practice, however, pearlite nucleation at grain edges and corners may also be considerable if the undercooling is small.
Parameters within the Steel Models are calibrated against experimental data on nucleation, growth, and overall transformation kinetics. However, the composition space covered by the calibration data, on which the Steel Models calculations are checked, is limited. As a result, together with the limitations from modeling approach discussed above, some recommended composition ranges are listed below. The Models may not give reasonable results when the composition in question is far outside the recommended ranges.
| Element | Ferrite | Bainite | Pearlite3 | |
|---|---|---|---|---|
| Bainite Start Temperature [2018Lea]1 | Bainite Growth, Nucleation, Transformation Kinetics2 | |||
| C | Max. 0.7 | Max. 0.7 | 0.2-1.5 | 0.6-1 |
| Mn | Max. 3 | Max. 5.5 | Max. 0.6 | Max. 1.8 |
| Ni | Max. 5 | Max. 9 | Max. 5.3 | Max. 3 |
| Si | Max. 2 | Max. 2 | Max. 1 | Max. 2 |
| Al | Max. 1.4 |
– |
– |
(unknown, assuming similar to Si) |
| Co | Max. 3.2 |
– |
– |
Max. 2.2 |
| Cr | Max. 3 | Max. 5.7 | Max. 5 | Max. 13[4] |
| Mo | Max. 2 | Max. 4 | (unknown) | Max. 0.6 |
| W |
– |
– |
– |
Max. 0.37 |
|
1 The model is from [2018Lea]. The supporting bainite start data is compiled and published in [2019Lea]. 2 Parameters are not assessed to be composition dependent. The listed composition ranges come from the data of growth considered in a “universal” fitting. 3 References in [2020Yan]. [4] Requires calculation setting Custom for type of carbide. Estimated boundaries: CEMENTITE for <5 wt.% Cr, M7C3 for 5~10 wt. % Cr, M23C6 for >10 wt.% Cr. |
||||
References
[2018Lea] L. Leach, P. Kolmskog, L. Höglund, M. Hillert, A. Borgenstam, Critical Driving Forces for Formation of Bainite. Metall. Mater. Trans. A. 49, 4509–4520 (2018).
[2019Lea] L. Leach, P. Kolmskog, L. Höglund, M. Hillert, A. Borgenstam, Use of Fe-C Information as Reference for Alloying Effects on BS. Metall. Mater. Trans. A. 50, 4531–4540 (2019).
[2020Yan] J.-Y. Yan, J. Ågren, J. Jeppsson, Pearlite in Multicomponent Steels: Phenomenological Steady-State Modeling. Metall. Mater. Trans. A. 51, 1978–2001 (2020).