University of Cambridge

Tensile behaviour of thermally-stable nanocrystalline bainitic-steels

G. M. A. M. El-Fallah and H. K. D. H. Bhadeshia


The deformation behaviour of two nanostructured bainitic steels designed to ensure the thermal stability of the retained austenite present alongside the bainitic ferrite, has been studied as a function of test temperature. One of the alloys is especially rich in silicon whereas the other in nickel. Surprisingly, the alloys are found to have greater ultimate tensile strengths and ductility when tested at 200 °C, compared with corresponding tests at ambient temperature. This is demonstrated to be a consequence of the more gradual deformation-induced transformation of the retained austenite at 200 °C. In contrast, there is a dramatic reduction in both strength and uniform ductility during testing at 450 °C. Some carbides precipitate during testing of the high-silicon alloy, thus making the austenite less stable to both thermally and mechanically. The high-nickel alloy suffers from the same fate but for different reasons, that the austenite actually grows at the 450 °C test temperature, leading to a reduction in its thermodynamic stability.

The experimental data on the stability of the retained austenite both at the test temperature and during cooling from that temperature have been modelled.

Materials Science and Engineering A 746 (2019) 145-153.

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