Retained austenite thermal stability in a nanostructured bainitic steel

• Nanostructured bainitic steel is not thermodynamically stable.
• Extensive austempering in these microstructures has not been reported before.
• Precipitation of cementite particles is unavoidable at longer austempering times.
• TEM, FEG-SEM and XRD analysis were used for microstructural characterization.

The unique microstructure of nanostructured bainite consists of very slender bainitic ferrite plates and high carbon retained austenite films. As a consequence, the reported properties are opening a wide range of different commercial uses. However, bainitic transformation follows the T0 criteria, i.e. the incomplete reaction phenomena, which means that the microstructure is not thermodynamically stable because the bainitic transformation stops well before austenite reaches an equilibrium carbon level. This article aims to study the different microstructural changes taking place when nanostructured bainite is destabilized by austempering for times well in excess of that strictly necessary to end the transformation. Results indicate that while bainitic ferrite seems unaware of the extended heat treatment, retained austenite exhibits a more receptive behavior to it.