Effects of Q&P Processing Conditions on Austenite Carbon Enrichment Studied by In Situ High-Energy X-ray Diffraction Experiments
Abstract
:1. Introduction
2. Materials and Methods
2.1. Studied Alloy
2.2. Diffraction Set-Up and Data Processing
2.3. Quenching & Partitioning (Q&P) Processing Conditions
3. Results
3.1. Evolution of Phase Fractions
- (i)
- During the first cooling step, the alloy remained fully austenitic above Ms.
- (ii)
- Primary martensitic transformation below initial Ms: The initial Ms temperature was estimated to be 295 °C from an extrapolation of the transformation kinetics. The final fraction of martensite before reheating was 65% (note that at this stage, the bcc phase was implicitly but obviously identified as martensite).
- (iii)
- Reheating step, with a duration of a few seconds, because of the regulation procedures of the heating device: The duration of this transient regime was lower than 5 s, before temperature stabilization for the partitioning step. Figure 3b reveals that during the reheating step, the phase fractions remained constant. For all studied conditions, the phase fractions remained constant during reheating up to about 370 °C.
- (iv)
- Partitioning step: Above 370 °C, a significant increase in the bcc phase fraction was observed during the partitioning step. The kinetics were initially fast during the first 50 s, progressively slowing down. Due to the experimental limitations in revealing tetragonality, the exact nature of this bcc phase was debatable as it could have corresponded to either bainite and/or athermal martensite. Indeed, there was strong evidence of both bainite and athermal martensite formation below Ms [7,31,32]. It was even stated that this isothermal product was neither purely martensitic nor purely bainitic [33]. One could even consider the simultaneous formation of both athermal martensite and bainite as an alternative. Indeed, the formation of athermal martensite could have a strong accelerating effect on the subsequent bainite formation by providing a higher density of potential nucleation sites. As a consequence, below Ms, the nature of the transformation products during isothermal holding was unclear. In addition, lower bainite and athermal martensite exhibited morphological similarities. The morphological criteria used by Somani et al. to distinguish bainite from athermal martensite (laths with wavy boundaries and ledge-like protrusions) [7] was discussed very recently by [34] in a convincing way. According to their analysis, below Ms, the driving force for bainite nucleation is so high that small units of bainite may grow from the initial martensite laths in the form of ledge-liked protrusions. Furthermore, bainitic ferrite can grow from the prior athermal martensite, maintaining a similar orientation relationship that could have contributed to the formation of the ledge-like protrusions that could have, in turn, given rise to a wavy appearance of the boundaries.
- (v)
- During the first part of the final cooling step, the fraction of the bcc phase identified as bainite remained constant down to 120 °C.
- (vi)
- Final martensitic transformation, evidenced by a 2% increase in the bcc phase fraction below 120 °C: Note that again, similarly to step (ii), the increase in the bcc phase was obviously attributed to martensite formation. The final fraction of retained austenite at RT was about 27%.
3.2. Evolution of Austenite Lattice Parameter
4. Discussion
4.1. Carbon Mass Balances
4.2. Critical Assessment of the Methods for Estimating Austenite Carbon Enrichment
5. Conclusions
- -
- After the first martensitic transformation during cooling down to the QT, no significant evolution is observed during the re-heating step-up to the PT.
- -
- During holding at the PT, an increase in the austenite lattice parameter is observed, resulting from the carbon redistribution from martensite and formation of ferritic bainite.
- -
- A final increase of the austenite lattice parameter is observed during the final quench, attributed to internal stresses resulting from differences in the thermal expansion between the different phases present (and potentially improving the TRIP ability of the steel).
Acknowledgments
Author Contributions
Conflicts of Interest
References
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Variation in Δaγ | Reference Cycle QT = 230 °C/PT = 400 °C | High Partitioning Temperature (PT) QT = 230 °C/PT = 450 °C | Low Quenching Temperature (QT) QT = 200 °C/PT = 400 °C |
---|---|---|---|
End of step (ii) | −0.0055 Å | −0.0060 Å | −0.0072 Å |
During step (iv) | +0.0127 Å | +0.0106 Å | +0.0166 Å |
During step (vi) | +0.0078 Å | +0.0087 Å | +0.0089 Å |
Studied Cycles | Reference Cycle QT = 230 °C/PT = 400 °C | High PT QT = 230 °C/PT = 450 °C | Low QT QT = 200 °C/PT = 400 °C |
---|---|---|---|
Fγ (%) | 27 | 30 | 21 |
Cγ (wt %) | 0.70 | 0.67 | 0.80 |
Fγ × Cγ (wt %) | 0.19 | 0.20 | 0.17 |
C0 − Fγ × Cγ (wt %) | 0.11 | 0.10 | 0.13 |
Studied Cycles | Reference Cycle QT = 230 °C/PT = 400 °C | High PT QT = 230 °C/PT = 450 °C | Low QT QT = 200 °C/PT = 400 °C |
---|---|---|---|
aγ (Å) at RT | 3.6030 | 3.6013 | 3.6054 |
Cγ (wt % using the Toji et al. formula) | 0.92 | 0.87 | 0.99 |
C0 − Fγ × Cγ (wt %) | 0.05 | 0.04 | 0.09 |
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Allain, S.Y.P.; Geandier, G.; Hell, J.-C.; Soler, M.; Danoix, F.; Gouné, M. Effects of Q&P Processing Conditions on Austenite Carbon Enrichment Studied by In Situ High-Energy X-ray Diffraction Experiments. Metals 2017, 7, 232. https://doi.org/10.3390/met7070232
Allain SYP, Geandier G, Hell J-C, Soler M, Danoix F, Gouné M. Effects of Q&P Processing Conditions on Austenite Carbon Enrichment Studied by In Situ High-Energy X-ray Diffraction Experiments. Metals. 2017; 7(7):232. https://doi.org/10.3390/met7070232
Chicago/Turabian StyleAllain, Sébastien Yves Pierre, Guillaume Geandier, Jean-Christophe Hell, Michel Soler, Frédéric Danoix, and Mohamed Gouné. 2017. "Effects of Q&P Processing Conditions on Austenite Carbon Enrichment Studied by In Situ High-Energy X-ray Diffraction Experiments" Metals 7, no. 7: 232. https://doi.org/10.3390/met7070232