Regular article In-situ investigation of quenching and partitioning by High Energy X-Ray Diffraction experiments S.Y.P. Allain a , G. Geandier a , J.C. Hell b , M. Soler b , F. Danoix c , M. Gouné d, ⁎ a Institut Jean Lamour, CNRS - Université de Lorraine, Parc de Saurupt - CS 50840, 54011 Nancy Cedex, France b Automotive Products, ArcelorMittal Maizières Research, Voie Romaine, BP 30320, 57283 Maizières-lès-Metz, France. c Normandie Univ, UNIROUEN, INSA Rouen, CNRS, Groupe de Physique des Matériaux, 76000 Rouen, France d ICMCB-CNRS-Université de Bordeaux, 87 avenue du Docteur Schweitzer, 33609 Pessac, France abstract article info Article history: Received 23 October 2016 Received in revised form 16 December 2016 Accepted 16 December 2016 Available online xxxx We report the first ultra-fast time-resolved quantitative information on the quenching and partitioning process of conventional high strength steel by in-situ High Energy X-Ray Diffraction experiment. The time and tempera- ture evolutions of phase fractions, their carbon content and internal stresses were determined. The austenite to martensite transformation below Ms is followed by a stagnant stage during which microstructural state remained unchanged. Afterwards, a fast kinetics of carbon enrichment of austenite during the partitioning step at 400 °C is highlighted. The analysis proposed supports the carbon diffusion from martensite to austenite as the main mechanisms responsible for this enrichment. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: In-situ High Energy X-Ray Diffraction Quenching and partitioning Phase transformation Partitioning Steels The need to improve fuel efficiency and safety has led to a high and growing demand for high-strength steels in the automotive industry. The potential of weight reduction directly depends on mechanical prop- erties improvement, which are in turn controlled by the microstructural features [1]. Recently, a novel steel heat treatment termed “quenching and partitioning” (Q&P) has been proposed as an alternative way to ob- tain attractive properties [2,3]. The process involves quenching below the martensite start temperature (Ms) followed by a rapid heating and ageing above the initial quench temperature. The ageing step, usu- ally performed between 300 °C and 500 °C, is also termed “partitioning step” since the carbon enrichment in austenite is expected to occur dur- ing this stage. The benefits of such a treatment in terms of improved me- chanical properties have been clearly demonstrated [3,4]. However, despite the large amount of knowledge acquired in this decade, the Q&P is still a matter of debate. Indeed, the Q&P microstructures are ex- tremely difficult to characterize by traditional metallographic methods since their constituents are on the submicron scale and respond in a similar way to etching [5]. In addition, the mechanism of carbon enrich- ment in retained austenite during the partitioning step is still controver- sial. Indeed, strong experimental evidences of carbon partitioning from martensite to austenite exist [6,7], but the possible carbon partitioning from supersaturated bainitic ferrite to austenite is still possible [8]. Moreover, from a kinetics point of view, it has been suggested that the temperature is too low for carbon diffusion and that carbon supersatu- ration in martensite can be eliminated by carbides precipitation during partitioning step [3,9]. Furthermore, the formation of bainite during partitioning cannot be completely ruled out and could explain the mea- sured of enrichment of carbon in retained austenite, as the tempera- tures are consistent with those for bainite formation [6]. It was recently demonstrated that in-situ High Energy X-Ray Diffrac- tion (HEXRD) is a powerful method to obtain time-resolved precise quantitative information about phase transformation during quenching of low carbon steels [10]. In the present work and for the first time, the microstructure evolution during the Q&P process of a conventional TRIP steel was investigated by in-situ HEXRD experiments. The results clarify both the time and temperature evolutions of microstructure and the mechanisms of carbon austenite enrichment during the Q&P process. A Fe-0.3 wt% C-2.5 wt% Mn-1.5 wt% Si-0.8 wt% Cr alloy was cast in a vacuum induction melting furnace. It was then hot rolled and cold rolled to a final gauge thickness of 1.4 mm. The samples investigated in the present study were machined from cold-rolled strip. They are 30 mm long in the rolling direction and their sections are 1.4 × 4.0 mm 2 . All the details about the sample preparation can be found in [11]. The experiments were performed at the European Synchrotron Ra- diation Facility (ESRF) in Grenoble, France, on beam line ID15B under powder diffraction configuration [11,12]. The high energy monochro- matic beam (E = 87 keV, λ = 0.14 nm) permits to work in transmission and the association with a fast 2D detector enables high acquisition rates (10 Hz) suitable to study “real time” process on bulk samples. In Scripta Materialia 131 (2017) 15–18 ⁎ Corresponding author. E-mail address: mm.goune@gmail.com (M. Gouné). http://dx.doi.org/10.1016/j.scriptamat.2016.12.026 1359-6462/© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat