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Open AccessFeature PaperArticle

Microstructure Evolution and Competitive Reactions during Quenching and Partitioning of a Model Fe–C–Mn–Si Alloy

1
CNRS, Univ. Bordeaux, Bordeaux INP, ICMCB, UMR 5026, F-33600 Pessac, France
2
Normandie Université, UNIROUEN, INSA Rouen, CNRS, Groupe de Physique des Matériaux, 76000 Rouen, France
3
Institut Jean Lamour, UMR CNRS-UL 7198, 54011 Nancy, France
4
Maizières Automotive Products, Arcelormittal Maizières Research SA, 57283 Maizières les Metz, France
*
Author to whom correspondence should be addressed.
Metals 2020, 10(1), 137; https://doi.org/10.3390/met10010137
Received: 23 December 2019 / Revised: 10 January 2020 / Accepted: 13 January 2020 / Published: 16 January 2020
(This article belongs to the Special Issue Advanced High Strength Steels by Quenching and Partitioning)
The mechanisms behind the carbon enrichment of austenite during quenching and partitioning are still a matter of debate. This work investigates the microstructural evolution during the quenching and partitioning of a model Fe–C–Mn–Si alloy by means of in situ high energy X-ray diffraction (HEXRD) atom probe tomography, and image analysis. The ultra-fast time-resolved quantitative information about phase transformations coupled with image analysis highlights the formation of carbide-free BCT bainite, which is formed within a very short range during the reheating and partitioning step. Its transformation rate, which is a better indicator than the intrinsic volume fraction, depends on the quenching temperature (QT). It is shown to decrease with decreasing QT, from 45% at QT = 260 °C to 20% at QT = 200 °C. As a consequence, a significant part of the carbon enrichment observed in austenite can be attributed to bainite transformation. Furthermore, a large part of carbon was shown to be trapped into martensite. Both the formation of Fe2.6C iron carbides and the segregation of carbon on lath boundaries in martensite were highlighted by atom probe tomography. The energy for carbon segregation was determined to be 0.20 eV, and the carbon concentration on the lath boundaries was obtained to be around 25 at %. Therefore, the carbon enrichment of austenite is the result of competitive reactions such as carbon partitioning from martensite, bainite transformation, and carbon trapping in martensite. View Full-Text
Keywords: steels; quenching and partitioning; partitioning; high-energy X-ray diffraction; atom probe tomography steels; quenching and partitioning; partitioning; high-energy X-ray diffraction; atom probe tomography
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Aoued, S.; Danoix, F.; Allain, S.Y.; Gaudez, S.; Geandier, G.; Hell, J.-C.; Soler, M.; Gouné, M. Microstructure Evolution and Competitive Reactions during Quenching and Partitioning of a Model Fe–C–Mn–Si Alloy. Metals 2020, 10, 137.

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