Marion BREGEAULT – Bainitic transformation in continuous cooling in low-alloy steel forgings: effect of local composition on the kinetics and the final microstructure

Jury

Sébastien Allain, Professeur, Université de Lorraine, Reviewer
Christopher Hutchinson, Distinguished Professor, Monash University, Reviewer
Yves Bréchet, Associate Professor, Monash University, Examiner
Sophie Cazottes, Professeure, Institut National des Sciences Appliquées (INSA) de Lyon, Examiner
Alexis Deschamps, Professeur, Université Grenoble Alpes (UGA), President of the jury
Muriel Véron, Professeure, Université Grenoble Alpes (UGA), PhD Supervisor
Hugo Van Landeghem, Chargé de Recherches HDR, CNRS, PhD Co-Supervisor, Invited
Hatem Zurob, Full Professor, McMaster University, Invited
 

Abstract

Large low-alloy steel forgings used for nuclear reactors present chemical heterogeneities at the mesoscale and degrading resilience properties. The aim of this thesis is understand the influence of local composition variations on the bainitic transformation during thermal treatment. The microstructures obtained in continuous cooling are characterized quantitatively. Controlled compositional gradients are fabricated to study local transformation kinetics using in-situ synchrotron X-ray diffraction. The sequential nature of the transformation is rationalized in terms of carbon partitioning regimes. Transformation mechanisms at the scale of the bainite plate are investigated through in-situ Laser Scanning Confocal Microscopy. Finally, a physical-based model for overall transformation kinetics is proposed. The role of cooling rate and gradual stabilization are well reproduced. Substitutional elements are thought to inhibit nucleation sites activation at parent grain boundaries.