EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

In this study, the thermal stability of bainitic ferrite and carbon-enriched retained austenite has been studied in in two steels containing 0.6 C (wt %) and microallowed with V and Mo. For this goal, a nanobaintic microstructure was developed in both steels by a thermal treatment consisting of austinitization at 1150°C for 180 s and a bainitic transformation at 250°C for 14h. High resolution temperature dilatometric studies consisting on heating cycles up to the Ac1 temperature or tempering treatments for 1h at temperatures ranging from 450 to 650°C showed a sequence of different decomposition events, that were characterized by both, X-ray diffraction and microstructural observations with the scanning electron microscope. The first event observed was associated to the precipitation of very fine cementite particles in the austenite thin films. For tempering temperatures higher than 500°C, it was identified another decomposition step in both steels, that was accompanied by a decrease on both, the austenite mass fraction and its lattice parameter. As the carbon content in this phase decreased, this transformation was associated to the precipitation of carbide particles in the retained austenite blocks. This process give raise to the formation of fresh martensite from this lower carbon austenite during cooling to room temperature after the tempering treatment. In the last tempering stages, the remaining austenite decomposes into ferrite and cementite, and due to carbide precipitation the bainitic ferrite loses its tetragonality, its dislocation density is reduced and the bainitic laths coarsen.
This research was funded by Research Fund for Coal and Steel, grant number RFCS-754070.