Cécile RAMPELBERG: Characterization and modeling of Carbide-Free Bainite transformations along isothermal and anisothermal heat treatments

Abstract:
Carbide-Free Bainites are multiphase microstructures obtained from austenite decomposition at low temperatures (typically between 450 °C and 200 °C) in alloyed steels. These microstructures are very attractive owing to their high mechanical properties and good toughness especially for forged parts dedicated to the automotive market. They are made of a fine ferritic matrix without carbide thanks to a judicious chemical composition, retained austenite stabilized by carbon partitioning during the transformation and martensite. The high fraction of retained austenite may transform in martensite during further mechanical solicitations at room temperature (strain induced transformation). These microstructures have been studied since many years, but their formation mechanisms are still a subject that continues to divide the metallurgy community, between diffuse and diffusionless approaches. The incomplete transformation phenomenon encountered in this process is one of the bones of contention. One of the great novelties of this work was to elucidate the mechanisms of formation of these microstructures in continuous cooling conditions.
In this work, we have investigated the evolution of microstructures along different thermal treatments (isothermal holdings, multi-steps and continuous cooling treatments) by in situ High Energy X-Ray Diffraction (HEXRD) on synchrotron beamlines. Such experiments make possible the simultaneous measurement of phase transformation kinetics, of the lattice parameters of the different phases and the detection of possible carbide precipitation processes. On this basis, very precise carbon mass balances between the constituting phases have been established for the first time leading to the conclusions that the ferritic bainite is even more supersaturated in carbon that expected. The multis-step and continuous cooling experiments have also proved that the bainitic transformation doesn’t respect the additivity rule of purely diffusive transformations and is highly sensitive to the transformation sequences. The microstructures after thermal treatments have been systematically studied post mortem by Scanning Electron microscopy (SEM) coupled with Electron Back Scattered Diffraction (EBSD). It has served to explain the observed microstructures after continuous cooling which show large distributions of size, morphology and microtexture as they are formed progressively at different temperatures.
A phase transformation model based on the diffusionless-type approach of Van Bohemen (2019) was finally developed and calibrated on available experimental data. This model is not only able to simulate bainite kinetics along isothermal holding and continuous cooling but also the respective compositions of the phases. The capabilities and limits of the new approach are analyzed and discussed.

Composition du jury:
> Reporters:
- Francisca GARCIA CABALLERO, Directrice de recherche, CENIM-CSIC, Madrid, Espagne,
- Mohamed GOUNE, Professeur, Université de Bordeaux, France
> Examiners:
- Sabine DENIS, Professeure, Institut Jean Lamour, Nancy, France
- Hugo VAN LANDEGHEM, Chargé de recherche du CNRS, SIMAP, Grenoble, France
> Direction of thesis:
- Sébastien ALLAIN, Directeur de thèse, Professeur, Institut Jean Lamour, Nancy, France,
- Julien DA COSTA TEXEIRA, Co-directeur de thèse, Chargé de recherche du CNRS, Institut Jean Lamour, Nancy, France
> Guests:
- Guillaume GEANDIER, Chargé de recherche du CNRS, Institut Jean Lamour, Nancy, France
- Thomas SOURMAIL, Docteur-Ingénieur, Ascometal (CREAS), Hagondange, France