In Situ Quantitative Assessment of the Role of Silicon During the Quenching and Partitioning of a 0.2C Steel

Silicon is an essential alloying element added in quenching and partitioning (Q&P) steels to delay and/or suppress carbide precipitation. However, there is a strong industrial interest to reduce the silicon content as it has detrimental effects on the metallurgical route. This work investigates by means of in situ high-energy XRD (HEXRD) the effect of silicon on the microstructural evolution during quenching and partitioning of a commercial 0.2C-2.3Mn grade. The results of this study highlight the role of the bainite transformation during the reheating and partitioning steps for effective austenite retention. Silicon influences the kinetics of austenite decomposition into bainite and finally promotes the stabilization of austenite. This is explained by the ability of silicon to suppress carbide precipitation (i) at the interface between bainite and austenite and (ii) in the martensite matrix. Carbide precipitation at the bainite/austenite interface decreases the amount of carbon that diffuses from bainite to austenite, subsequently accelerating the bainite transformation kinetics and preventing austenite stabilization. Carbide precipitation in martensite reduces the amount of carbon available for partitioning in austenite, further preventing its stabilization. Additions of elements such as Cr or Mo could be therefore considered in order to reduce the austenite decomposition in low-silicon steel grades.