Influence of initial microstructure and heating rate on kinetics of phase transformation in medium carbon low alloy steel

Abstract

This thesis investigates the critical influence of initial microstructure and heating rate on the kinetics of austenite formation in medium carbon low alloy steels, which are vital materials for various industrial applications such as landing gear and hydraulic turbines. Understanding and precisely controlling these phase transformations are paramount for optimizing thermal processing, achieving desired microstructures, and predicting mechanical properties. Despite their significance, a comprehensive understanding of the kinetics of austenite formation from initial bainite and martensite microstructures, particularly how prior austenite grain size (PAGS) and heating rate interact, presents a significant research challenge. The research addresses this gap by employing a multi-faceted methodology. High-resolution dilatometry was utilized for continuous heating experiments to quantify transformation kinetics and critical temperatures. Intermittent quenching tests, coupled with advanced microstructural characterization techniques including optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD), were performed to elucidate the underlying nucleation and growth mechanisms of austenite. Furthermore, the study developed and optimized mathematical models, rooted in diffusion-controlled nucleation and growth theories and refined using a genetic algorithm, to accurately describe the observed kinetic behaviors. The findings reveal that for an initial martensite microstructure, grain refinement, specifically a reduction in PAGS from 330 μm to 117 μm, significantly accelerates the rate of austenite formation without altering the critical austenite formation temperature (Ac1). Nucleation of austenite in these martensitic samples predominantly occurs at prior austenite grain boundaries. Extending the investigation to an initial bainite microstructure, a clear distinction in critical temperatures and transformation kinetics was observed, with bainite exhibiting slower kinetics compared to martensite. The impact of heating rate was found to be more pronounced on bainite than on martensite, and differences in growth rates between the two initial microstructures are attributed to varying carbon distributions within their respective matrices. This work successfully developed and validated mathematical models capable of predicting the extent of austenite formation as a function of time, heating rate, and initial microstructure (martensite and bainite), providing valuable insights for the precise control of thermomechanical processing in these critical steel alloys.

Date	4 Jun 2026
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Mohammad Jahazi (Supervisor), Henri Champliaud (Co-supervisor) & Pinaki Bhattacharjee (Co-supervisor)