Microstructural evolution in a high-silicon medium carbon steel following quenching and isothermal holding above and below the Ms temperature

In this study, the microstructural features evolved in a high-Si, medium-carbon steel (Fe-0.53C-1.67Si-0.72Mn-0.12Cr) subjected to quenching and isothermal holding at temperatures above and below the martensite start temperature (Ms) for one hour have been examined. Both laser scanning confocal and transmission electron microscopy were employed for detailed microstructural characterization, supported by dilatometry, X-ray diffraction, and hardness measurements. In the case of isothermal treatment above Ms at 300 °C, besides bainite transformation marked by typical S-shaped dilatation curve, high-carbon martensite is formed during the final cooling to room temperature. In the case of isothermal treatment below Ms at 250 °C, the initial martensite formation and subsequent carbon partitioning to austenite is followed by the formation of bainite containing carbides and some high-carbon martensite that forms during the final cooling to room temperature. Also, selected area diffraction patterns (SAED) for both of Q&B and Q&P heat treated samples showed there are extra weak diffraction spots, presumably due to the presence of omega phase (ω) as an intermediate phase during fcc (face- centered cubic) austenite to bcc (body- centered cubic) martensite transformation and is considered as a common substructure in bcc metals and alloys with a coherent interface with the matrix: aω = √2 × abcc and cω = √3/2 × abcc that appeared in twinning martensite or martensite regions with dislocations as a substructure.