1. Bulk Nanocrystalline Steel Phase Transformations and Complex Properties Group
Transformation to bainite at temperatures around 200ºC produces a fine scale microstructure, giving high strength. This has been achieved by exploiting knowledge of solid-state phase changes to create a steel which can be transformed into slender plates of bainitic ferrite, embedded in austenite.
Transformation results in supersaturation and inhomogeneous distribution of carbon within the austenite, which has been characterised using X-ray and atom probe. The resistance of the structure to temperature has been investigated using hardness and transmission electron microscopy.
Cambridge, CORUS, MOD and Rolls Royce continuing research for aeroengine shafts and armour.
Austenite
1 mm
Ferrite
Fine scaled microstructure results from transformation at 200ºC for 1 week. Composition Fe-0.8C-1.6Si-2Mn-1.3Cr-0.3Mo-0.1V wt%.
Displacive transformation, fraction of bainite controlled by thermodynamics.
1 mm
Ferrite
Austenite
1.5 at.% Carbon
12 at.%
Carbon
15 nm
Tempering at 450ºC for 1 h, change occurs by decomposition of austenite and precipitation of fine carbides.
Supersaturation of carbon in ferrite and austenite has been observed with atom probe.