Phase Transformations in Steels during Quenching and Partitioning Heat Treatment

Abstract

Quenching and Partitioning (Q&P) processed steel is a promising candidate for the third-generation advanced high strengths steel(AHSS), where considerable amount of carbon-enriched austenite can be obtained in the steel microstructure. The Q&P process starts with full or partial austenisation of the steel, followed by a quench to form a desired amount of martensite and an isothermal holding (the partitioning step) for carbon to partition from martensite into austenite. The end point of the carbon partitioning is predicted by a model referred to as Constrained Carbon Equilibrium (CCE), which assumes a stationary martensite/austenite interface during the partitioning step and precludes any competing reaction, e.g. bainite formation and carbide precipitation. However, it remains controversial whether martensite/austenite interface remains stationary during partitioning and competing reactions have been reported to occur, which can influence the final steel microstructure. This thesis therefore aims to study the phase transformations accompanying carbon partitioning during the Q&P process. A steel was heat treated by the Q&P process to investigate the evolution of phase fractions during/approaching the stabilisation of carbon content using dilatometry and Synchrotron X-Ray Diffraction (SXRD). The steel is a model alloy of composition Fe–0.2C–3Mn–2Si (wt.%). From a set of preliminary heat treatments using dilatometry, 400°C was chosen as the optimal partitioning temperature for a secondary dilatometry tests and the in-situ SXRD. Results show that as the initial quenching stops, the austenite continues to transform during the short holding and reheating, which might be associated with the formation of martensite and then bainite. The bainitic transformation continues to the end of partitioning step. The consumption of austenite is also accompanied by the increased carbon level of austenite. Therefore, the carbon enrichment in austenite is achieved by the coupling of carbon partitioning from martensite and bainitic transformation. From the end of initial quenching to the end of partitioning step, 40%-50% of previously retained austenite was consumed. The rate of transformation is highest at the beginning of reheating. Microstructure consisting of tempered martensite with carbides, bainite and retained austenite is obtained through the Q&P process, with tempered martensite mainly as the BCC phase. The RA fraction can be adjusted from 6.9vol.% to 14.3vol.% with little or no formation of fresh martensite. The time required to acquire the multi-phase microstructure can be greatly reduced compared to austempering.