Characterization of carbides in Q&P steels using a combination of high-resolution methods

Abstract

Current strategies for the 3rd generation of advanced high strength steels focus on the creation of a multiphase microstructure containing substantial amounts of retained austenite to enhance the mechanical properties. In the case of quenching and partitioning (Q&P) steels, the stabilization of austenite is achieved by carbon diffusion from the supersaturated martensite into the austenite, but carbon partitioning is often jeopardized by competing reactions such as carbide formation. In the present study, in-situ high-energy X-ray diffraction (HEXRD) was used to study the transformation kinetics during Q&P processing, and especially carbide formation at higher partitioning temperatures. It was found that this carbide precipitation resulted mainly from martensite tempering and partially also from a decomposition of the austenite. The detection of minor diffraction peaks that appeared during the partitioning step was assigned to θ- and χ-carbide formation, which was supported by correlative atom probe tomography (APT) and transmission electron microscopy (TEM). Additionally, NbC was detected by APT, but the amount was obviously too low to be detected by HEXRD. The applied methods are finally compared with regard to their applicability for carbide identification in Q&P steels.