On the decomposition of austenite in a high-silicon medium-carbon steel during quenching and isothermal holding above and below the Mₛ temperature

Abstract The microstructural mechanisms operating during the decomposition of austenite 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 (Mₛ) temperature for times up to 1 h have been investigated using a Gleeble 3800 thermomechanical simulator. Dilatometry and metallography using laser scanning microscopy, transmission electron microscopy, scanning electron microscopy together with X-ray diffraction and hardness measurements have been employed. Treatment with the quench stop and isothermal hold both above and below Ms lead to the formation of both martensite and bainite. In the case of isothermal treatment above Mₛ, at 350 and 300 °C, high‑carbon martensite is formed during the final cooling to room temperature. In the case of isothermal treatment at temperatures below Mₛ, at 250 and 200 °C, the initial martensite formation and subsequent carbon partitioning to austenite is followed by the formation of bainite containing carbides and high‑carbon martensite that forms during the final cooling to room temperature. Despite the presence of the high silicon content, carbides are able to form even at the lowest temperature studied (200 °C). All treatments led to the presence of carbon enriched retained austenite, which ranged from 4 to 18 vol%. The variation of room temperature hardness as a function of isothermal holding time at the various temperatures is rationalized in terms of the microstructural development