Numerical Investigations of the Effects of Substitutional Elements on the Interface Conditions During Partitioning in Qu
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quenched and partitioned steels, carbon partitioning is considered to be driven by a constraint para-equilibrium at the martensite/austenite interface. Using Thermo-Calc calculations, we investigated the effect of non-partitioned elements on the resulting interface condition. Among all tested elements, only aluminum and chromium have significant effects. From this numerical study, a practical composition- and temperature-dependent relationship describing interface tie lines was derived and calibrated for Fe-C-2.5Mn-1.5Si-X wt pct alloys (X = Cr or Al). The quenching and partitioning (Q&P) process was invented by Speer et al., to meet the needs of the automotive sector for the development of a third-generation advanced high-strength steel.[1,2] Q&P steels show mainly duplex ultrafine microstructures made of martensite and residual austenite. These typical microstructures are obtained by (1) an initial quench after austenitization down to a temperature QT (quench temperature) between the martensite start (Ms) and finish (Mf) temperature of the alloy to induce a partial martensitic transformation, (2) an isothermal holding
STEVE GAUDEZ, JULIEN TEIXEIRA, SE´BASTIEN Y.P. ALLAIN, and GUILLAUME GEANDIER are with the Institut Jean Lamour, UMR CNRS-UL 7198, Nancy, France. Contact e-mail: [email protected] MOHAMED GOUNE´ is with the Institut de Chimie de la matie´re Condense´e de Bordeaux, UPR 9048, Pessac, France. MICHEL SOLER is with the ArcelorMittal Maizie´res Research SA, Maizie´res-le´s-Metz, France. FRE´DE´RIC DANOIX is with the Groupe de Physique des Mate´riaux, UMR CNRS-INSA-UR 6634, Saint Etienne du Rouvray, France. Manuscript submitted November 15, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
treatment at the partitioning temperature (PT), identical to or higher than the QT to promote carbon diffusion out from martensite to stabilize austenite, and (3) a final quench from the partitioning temperature to room temperature. During step (2), at PT, carbon partitioning from martensite to austenite is one of the key phenomena that occur during the Q&P process. Its kinetics is governed by carbon diffusion in martensite and austenite, respectively, and by the carbon concentrations on each side of the martensite/austenite interface. This boundary condition is often assumed to be governed by constrained para-equilibrium (CPE). This CPE imposes first no partition of substitutional elements (accounting for their supposed low diffusivity at PT), continuity of the carbon chemical potential across the interface, no interface mobility and, finally, absence of any carbide precipitation.[1] In this article, we focus only on the thermodynamic conditions at the interface to determine all the possible tie lines for a given temperature and alloy composition, i.e., the relations between carbon concentrations at interfaces in both austenite and martensite. As shown by Speers,[1] a carbon mass balance and the assumption of interface mobility permit calculating the final state after partitioning. Few previous works have proposed prac
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