Austenite Stabilization Kinetics during Quenching & Partitioning Treatment by In Situ X‐Ray Diffraction Experiments

Quenching & partitioning provides a promising heat treatment approach to obtain high‐strength steels with improved formability properties. By exact temperature control, a multiphase microstructure containing tempered martensite and retained austenite is generated. However, this precise temperature control is only limitedly transferable to large industrial scale. Longer heat treatment times allow other microstructural features to occur. This paper deals with the impact of long‐term partitioning on the microstructural evolution for different partitioning temperatures. By observation of the X‐ray diffraction (XRD) pattern evolution during the treatment, the slower and longer‐lasting processes can be investigated. The final microstructure is compared with the estimations by the CCET model, which is an extension of the classical constrained carbon equilibrium (CCE) model, including bainitic phase transformations. Light microscopy and atom probe tomography are applied to certain samples for microstructural characterization. The experimental approach itself using in situ XRD measurement, is successful in monitoring the diffusion‐controlled processes and confirmed the microstructural evolution predicted by the CCET model. Low partitioning temperatures result in fine martensite laths and retained austenite films containing high carbon concentrations, whereas higher treatment temperatures favor more pronounced bainite formation and carbide precipitation.