In-situ microstructural evolution during quenching and partitioning of a high-carbon steel by high-temperature X-Ray Diffraction

Carbon partitioning from martensite to austenite is essential for austenite stabilization during quenching and partitioning (Q&P), while a few competitive phenomena, such as bainitic transformation and carbide precipitation, alter the microstructural evolution. So, there is a need of using in-situ in combination with ex-situ characterisation techniques to understand the C partitioning at high temperature in relation to simultaneous competitive phenomena that might occur during the partitioning stage.In this study, microstructural evolutions of a medium carbon steel ( 0.6C-1.6Si-1.25Mn-1.75Cr wt%) during Q&P treatment were investigated by using an in-situ High-Temperature X-Ray Diffraction (HTXRD) equipment at three partitioning temperatures. Results confirmed that carbon enrichment of austenite at 280 and 400 degrees C originates from partial carbon depletion from martensite and bainitic transformation, while partitioning at 500 degrees C results in the complete depletion of carbon from initial martensite and ferrite formation. Short diffusion distance (~0.13 mu m) of carbon at 280 degrees C caused a poor carbon homogenization of austenite and formation of 8 vol% fresh martensite after final quenching. High Si content of the steel stabilized transitional carbides and, concurrently, suppressed Fe3C formation during Q&P. The outcome of this study could contribute to the design of suitable chemistry and process parameters for producing quenched and partitioned steels.