Investigation of bainitic phase transformation and phase evolution during tempering of high strength medium carbon steels

Abstract

In this study, the bainitic phase transformation in a medium-carbon low-alloy steel was investigated, followed by tempering. The focus was on the manufacturing process of large steel blocks from both industrial and scientific perspectives. The bainitic phase transformation was investigated under both continuous cooling and austempering conditions. For the manufacturing of small steel blocks, continuous cooling followed by double tempering was replaced with the conventional austempering process followed by a single tempering cycle. Experimental results from both laboratory-scale studies and industrial trials demonstrated the success of this new approach. Austempering at temperatures ranging from 380 to 450 °C was investigated, and the resulting microstructures were characterized in terms of phase volume fractions, individual phase characteristics, and both overall and local hardness. In the continuous cooling bainitic transformation, the effect of prior austenite grain size during slow cooling was investigated. Under these conditions, a multi-stage transformation was observed. In the first stage, both granular and plate-like bainite formed, followed by a second stage involving martensite formation accompanied by auto-tempering and the tempering of bainite. The significant differences in phase volume fraction were attributed to retained austenite, which increased from 10% to 24.6% as the prior austenite grain size increased from 101 to 304 μm. The effect of temperature non-uniformity during the tempering process was investigated in large steel blocks. Variations in the time required to reach the target tempering temperature resulted in significant differences in both microstructure and hardness. Furthermore, the results indicate a pronounced influence of tempering time on the reduction of low-angle grain boundaries, suggesting that this mechanism plays a primary role during tempering in this class of steels. Additionally, the carbides exhibited notable resistance to coarsening, even after 14 hours of tempering. The findings of this study highlight the significant influence of austempering temperature on bainitic phase transformations, as well as the effect of prior austenite grain size on the phase transformation during continuous cooling. Additionally, the results underscore the impact of temperature non-uniformity during the tempering of thick sections on the evolution of microstructure.

Date	20 Aug 2026
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Mohammad Jahazi (Supervisor) & Carlos Garcia-Mateo (Co-supervisor)