Convergence of Pathways to Equilibrium During Tempering of Bainitic and Martensitic Structures in a High-Si Medium Carbon Steel.

This research reveals the convergent nature of bainitic and martensitic microstructures during high-temperature tempering in a high-silicon medium carbon steel, while examining their distinct transformation pathways across temperatures ranging from 200°C to 650°C. Despite their different initial states - both containing carbon-saturated tetragonal ferrite matrix and retained austenite - these microstructures follow unique routes to a common equilibrium state. The investigation demonstrates that while retained austenite governs the tempering response in bainitic structures, the highly dislocated, carbon-enriched ferritic matrix controls the transformation in martensitic structures. Through comprehensive analysis utilizing high-resolution dilatometry, scanning electron microscopy, and X-ray diffraction techniques, we establish that both microstructures ultimately achieve identical equilibrium states at elevated tempering temperatures (600-650°C), albeit with significantly different kinetics - bainitic structures exhibiting slower transformation rates. This study maps the critical tempering mechanisms in high-silicon steels and quantifies the transformation pathways to equilibrium. These findings provide fundamental insights for optimizing heat treatment protocols, demonstrating that despite different starting microstructures, carefully controlled tempering can yield convergent final states in advanced steel processing. This research is funded by the European Union under the RFCS project WarP – Grant Nº 101112425.