In this work, the carbon partitioning behavior during quenching and partitioning (Q&P) was investigated in a medium carbon Fe-CMnSiMo steel as a function of quenching temperature (QT) and partitioning time ( \({t}_{p}\) ), at a fixed partitioning temperature (PT) of 673 K (400 °C). The carbon content of untransformed austenite after partitioning at 673 K (400 °C) was predicted using different thermodynamic models, and the kinetics of carbon partitioning was simulated using DICTRA®. The actual carbon content of retained austenite was estimated through X-ray diffraction (XRD) analysis and three-dimensional atom probe tomography (APT) measurements. The carbon content of retained austenite was found to depend on the initial martensite fraction formed at the QT and showed significant deviations from the predicted values. Furthermore, the experimentally observed kinetics of carbon partitioning deviated significantly from DICTRA® predictions, with carbon homogenization within austenite occurring over longer times than predicted values. While these deviations are commonly attributed to carbide precipitation, the present study reveals additional phenomena such as incomplete carbon partitioning and segregation of carbon at dislocations inside martensite. These phenomena, which are usually ignored in the thermodynamic calculations and DICTRA® simulation, were found to significantly influence the carbon partitioning behavior. In addition, direct atomic-scale evidence of C and local Mn partitioning between martensite and untransformed austenite obtained using APT has also been provided in this work.
