Quantitative Assessment of the Time to End Bainitic Transformation
Abstract
:1. Introduction
2. Materials and Methods
- A first stage, R0, called the incubation period, during which the transformation has not yet started, or it is not yet detectable, and the microstructure remains fully austenitic;
- The transformation period, where nucleation and growth occur, and the microstructure tends to evolve from γ to αb + γ+ (R1 & R2). Differences between R1 and R2 become clear later in the text;
- The last stage in which the signal reaches a steady-state, and transformation does not proceed any further, R3. This region is not always easily identifiable, because in some cases, the RCL signal does not achieve a clear steady-state, but it remains increasing or decreasing with a low but constant slope;
- If treatments are performed for much longer times than necessary, R4, further changes in the microstructure can take place, e.g., autotempering and decomposition of the bainitic microstructure [21,22,23], and discontinuous lines in Figure 1b intend to highlight the fact that the occurrence of those can lead to different outcomes in the dilatometric curve. For example, decomposition of residual austenite into a mixture of ferrite and cementite can lead to a net contraction or expansion depending on the amount of C in solid solution [29], whereas carbon partitioning i.e., further C enrichment of austenite, due to autotempering of bainitic ferrite, will introduce a contraction, and the precipitation of carbides from austenite and bainitic ferrite will induce an expansion and a contraction, respectively.
3. Methods for the Estimation of the End of the Bainite Transformation (EBT). Results and Discussion
3.1. Method 0. Interrupted Isothermal Tests
3.2. Method I. Graphical Method
3.3. Method II. Dilatometric Curve Simulation by Atomic Volumes
- From XRD data, bainitic ferrite fraction and lattice parameters, and , are calculated from the lattice parameters, and its C content () is derived using [52]. It has been proven that bainitic ferrite is fairly stable through the whole transformation regardless of the degree of transformation, and therefore, and , i.e., , will remain constant from the beginning to the end of the transformation [24].
- For = 1%, the lever rule is applied to calculate for that amount of transformed bainitic ferrite.
- With all that information, and using Equation (4), the lattice parameters at the studied temperature are calculated. Then, Equation (5) is used to calculate , associated with the formation of 1% of bainitic ferrite.
- The process is repeated for different values of up to the fraction provided by the XRD experiment, .
- The newly obtained results are then normalized, as it is also the experimental curve.
- From the normalized , a match is found in the—also normalized—dilatometric curve, and therefore, its corresponding experimental time is established.
- Finally, the EBT time would be that at which the calculated , but a less conservative value will be obtained allowing for the XRD uncertainty (±3%), so the EBT can also be calculated as that at which − 1% or that when − 2%.
3.4. Method III. Rate of Transformation Approach
4. Conclusions
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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Alloy | C | Si | Mn | Cr | Mo | Ni | Cu | Ms |
---|---|---|---|---|---|---|---|---|
Steel 1 | 0.43 | 3.05 | 0.71 | 0.98 | 0.21 | 0.09 | 0.14 | 280 |
Steel 2 | 0.99 | 2.47 | 0.74 | 0.98 | 0.02 | 0.12 | 0.19 | 173 |
Steel 3 | 0.31 | 1.52 | 2.44 | 320 |
Alloy | TIso/°C | aγ/Å | aα/Å | cα/Å |
---|---|---|---|---|
Steel 1 | 300 | 3.612 | 2.852 | 2.876 |
325 | 3.614 | 2.853 | 2.875 | |
350 | 3.609 | 2.854 | 2.877 | |
Steel 2 | 250 | 3.627 | 2.852 | 2.877 |
300 | 3.626 | 2.850 | 2.876 | |
350 | 3.621 | 2.854 | 2.874 |
Alloy | TIso (°C) | Method 0 | Method I | EBT/s | |||||
---|---|---|---|---|---|---|---|---|---|
Method II | Method III % of the Maximum DRCL | ||||||||
0% | 1% | 2% | 4% | 3% | 2% | ||||
Steel 1 | 300 | 700 | 1700 | 3600 | 1252 | 1123 | 694 | 777 | 932 |
325 | 470 | 500 | 3596 | 3596 | 3596 | 480 | 490 | 530 | |
350 | 340 | 1100 | 3347 | 1213 | 705 | 378 | 406 | 475 | |
Steel 2 | 250 | 21,000 | 42,014 | 50,274 | 37,050 | 27,581 | 25,458 | - | - |
300 | 6500 | 12,500 | 28,764 | 15,004 | 11,455 | 8020 | 8690 | - | |
350 | 5200 | 11,000 | 23,284 | 10,599 | 7859 | 7503 | 8032 | - | |
Steel 3 | 400 | 630 | 1264 | 1988 | 870 | 688 | 653 | 724 | 818 |
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Santajuana, M.A.; Eres-Castellanos, A.; Ruiz-Jimenez, V.; Allain, S.; Geandier, G.; Caballero, F.G.; Garcia-Mateo, C. Quantitative Assessment of the Time to End Bainitic Transformation. Metals 2019, 9, 925. https://doi.org/10.3390/met9090925
Santajuana MA, Eres-Castellanos A, Ruiz-Jimenez V, Allain S, Geandier G, Caballero FG, Garcia-Mateo C. Quantitative Assessment of the Time to End Bainitic Transformation. Metals. 2019; 9(9):925. https://doi.org/10.3390/met9090925
Chicago/Turabian StyleSantajuana, Miguel A., Adriana Eres-Castellanos, Victor Ruiz-Jimenez, Sebastien Allain, Guillaume Geandier, Francisca G. Caballero, and Carlos Garcia-Mateo. 2019. "Quantitative Assessment of the Time to End Bainitic Transformation" Metals 9, no. 9: 925. https://doi.org/10.3390/met9090925