Mössbauer Analysis of Low-Temperature Bainite.

Low-temperature bainite, obtained by the transformation of austenite at temperatures as low as 200°C 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 200°C 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. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]