Quenching and partitioning - a new steel heat treatment concept

Steel continues to be one of the primary materials in use today, however, even after 200 years of published research, innovative methods are being discovered and exploited to produce steel with enhanced properties. One such recent discovery has been termed Quenching and Partitioning (Q&P). The Q&P heat treatment process is reported to produce a multiphase microstructure consisting of retained austenite
stabilised to room temperature, and a harder martensitic phase. This combination is prospected to form a new generation of high strength, formable sheet steel well
suited to the demands of the automotive industry. Previous research has reported on the application of Q&P to commercial grades of steel, however, the temperatures
required have hampered study of the process. In this investigation a model alloy has enabled separation of Q&P into its individual stages for closer inspection. Standard
metallographic techniques were used to examine the microstructure of the material, followed by X-ray diffraction measurements to determine the stability of the
austenite phase fraction before and after partitioning, confirming the effectiveness of the partitioning process in stabilising retained austenite. Estimations of carbon
concentration via lattice parameter measurements have provided evidence of austenite carbon enrichment, but also suggest that some carbon remains unaccounted for, possibly as an equilibrium distribution between epsilon carbides and strained interstitial sites. In-situ partititioning in a neutron diffractometer has been utilised to obtain real-time measurements of the partitioning process. These measurements suggest that redistribution of carbon within martensite occurs before partitioning to austenite becomes kinetically significant. The trapping of carbon within
energetically favourable locations is postulated to be a major factor in retarding carbon migration from martensite to austenite when compared to mathematical models of the partitioning process. Neutron diffraction measurements of carbon interstitial occupancy in austenite have also provided further evidence of austenite carbon enrichment during the partitioning process.