Thermal stability of austenite retained in bainitic steels

A. Saha Podder and H. K. D. H. Bhadeshia


Steels with a microstructure consisting of a mixture of bainitic ferrite and carbon-enriched retained austenite are of interest in a variety of commercial applications because they have been shown to exhibit good combinations of strength, toughness and ductility. However, their use at temperatures moderately above ambient requires a knowledge of the thermal stability of the austenite. The changes that occur during the tempering of a mixture of bainitic ferrite, carbon-enriched retained austenite and martensite have been characterised. An analysis of the volume change due to transformation shows that it is possible to distinguish the decomposition of austenite from the tempering of martensite. The nature of the carbides that form during the heat treatment is discussed as are the implications on the development of mathematical models accounting for calculating the strain during austenite decomposition and martensite tempering. It is found that the early stages of tempering reactions where the austenite content is not greatly reduced, can dramatically influence the stability of the austenite as it is cooled to ambient temperature.

Materials Science and Engineering A, 527 (2010) 2121-2128.

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The bainitic microstructure after transformation, but before tempering

The bainitic microstructure after transformation, but before tempering

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Tempered sample, showing bright field image of twinned martensite that has formed during the cooling of the sample from the tempering temperature

Corresponding dark field image of the twins. The austenite that remains at the tempering temperature becomes unstable to martensitic transformation.

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