Factors influencing the austenite stability during tensile testing of Quenching and Partitioning steel determined via in-situ Electron Backscatter Diffraction Dorien De Knijf a,n , Cecilia Föjer c , Leo A.I. Kestens a,b , Roumen Petrov a,c a Department of Materials Science and Engineering, Ghent University, Technologiepark 903, B-9052 Zwijnaarde (Ghent), Belgium b Department Material Science and Engineering, TUDelft, Mekelweg 2, NL-2628 CD Delft, The Netherlands c ArcelorMittal Global R&D Gent, President J.F. Kennedylaan 3, B-9060 Zelzate, Belgium article info Article history: Received 30 March 2015 Received in revised form 24 April 2015 Accepted 25 April 2015 Available online 4 May 2015 Keywords: Quenching and Partitioning Retained austenite stability In-situ Electron Backscatter Diffraction (EBSD) Micro-tensile deformation abstract The effect of the microstructural characteristics of retained austenite on its transformation stability in steel after Quenching and Partitioning (Q&P) was studied via interrupted tensile tests and Electron Backscatter Diffraction measurements on a pre-determined zone of a micro-tensile test sample. The evolution of the retained austenite fraction was obtained as a function of the plastic strain. The dependence of the austenite transformation stability on the corresponding grain size, morphology, and local crystallographic orientation was discussed. Furthermore, the importance of the parameters on the austenite stability was analysed and it was shown that the austenite grains rotated, in addition to being transformed, constituting therefore an additional contribution to the ductility of Q&P steel. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Attaining an optimum balance of hard and ductile phases, e.g. martensite and austenite, is one of the most promising approaches being explored for the production of new advanced high strength steels (AHSS). This type of microstructure can be achieved via a Quenching and Partitioning (Q&P) heat treatment, which was introduced by Speer et al. [1–5]. This heat treatment consists of austenitisation, followed by quenching to a temperature below the martensite start tempera- ture (M s ), but above the martensite finish temperature (M f ), in order to partially transform the austenite to a controlled fraction of martensite. In the subsequent partitioning step, the steel is held isothermally between M s and M f or above M s in order to promote carbon diffusion from the supersaturated martensite to the untransformed austenite. The austenite grains, which are suffi- ciently enriched in carbon, are retained at room temperature after final quenching; the other austenite grains transform to high-C martensite, often referred to as fresh or untempered martensite. Various microstructures, and hence mechanical properties, can be obtained by varying the processing parameters. However, understanding the microstructure-mechanical property relation- ship is crucial to designing appropriate heat treatments. Knowl- edge of the microstructural features that influence the transfor- mation behaviour of retained austenite during deformation is also essential. A number of experimental studies performed on TRIP steels revealed that the transformation stability of austenite is affected by (i) the local carbon content in the austenite [6–9]; (ii) the grain volume of the retained austenite [9–14]; (iii) morphology [11,15,16]; (iv) the constraining effect of the phases surrounding the austenite [16–19] and (v) the crystallographic orientation [6,20,21] of the austenite with respect to the loading direction. In addition to transforming to martensite, austenite grains also rotate to accommodate plastic deformation [6,22]. During straining of Q&P steel, the overall C-content of the remaining austenite is observed to increase due to transformation of austenite grains with low C-concentrations [23]. Intrinsic to the slow C-diffusion in austenite [24,25], larger grains are generally linked with lower C-contents compared to the smaller counterparts with higher C-contents. This can be observed as well from their (partial) transformation to fresh martensite due to incomplete stabilisation during partitioning [26]. Recent investigations revealed however that stabilized blocky austenite grains were enriched more in C than their thin film-like counterparts [27,28], which combined with the fact that larger, blocky austenite grains transform at Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A http://dx.doi.org/10.1016/j.msea.2015.04.075 0921-5093/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ32 9 331 04 41. E-mail addresses: dorien.deknijf@ugent.be, doriendeknijf@hotmail.com (D. De Knijf). Materials Science & Engineering A 638 (2015) 219–227