Abstract. Low-temperature bainite, obtained by the transformation of austenite at temperatures as low as 200ºC for times as large as several days, has been reported to have extraordinary mechanical properties including the highest reported hardness of any bainitic steel. The unusual properties are a consequence of the fine scale of the microstructure, which contains bainite plates with thickness in the range 20-40 nm. The microstructure also contains carbon-enriched retained austenite which contributes to the properties via a number of mechanisms. In this work, the microstructure of a high carbon bainitic steel with Si to avoid cementite precipitation and Co to accelerate the transformation has been studied using Mössbauer spectroscopy for a series of samples transformed isothermally at 200ºC for time periods of 26, 34 and 96 hours. The total austenite content is almost identical (~13 wt%) for these samples although the carbon concentrations of the phases differ as a function of transformation time. The austenite increases its carbon content from 5.4 atomic % after 26 h transformation to 6.3 at. % after 96 h, while the final bainitic phase retains about 2.2 at. % of C. These results are consistent with data obtained using atom probe tomography for samples transformed isothermally for 12 days
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