Study of bainite and martensite tempering in a medium C high Si steel. microstructural disparities and equilibrium convergence

This study investigates the tempering behavior of bainite and martensite in a medium carbon, high silicon steel, with a focus on the microstructural evolution and the attainment of equilibrium, over a temperature range of 200-650 • C. The dissimilarities between the characteristics of the two initial microstructures, both comprising a C-saturated tetragonal ferrite matrix and retained austenite, are reflected in the differences observed in their evolution towards equilibrium as the tempering temperature increases.Therefore, while retained austenite plays a pivotal role in the bainitic microstructure, in the martensitic microstructure it is the ferritic matrix, which is highly dislocated and enriched in carbon, that plays a determinant role.The findings demonstrate that while both bainite and martensite can converge towards the same equilibrium state upon high-temperature tempering (600-650 • C), the pathway to this convergence is markedly different, with bainite exhibiting a slower transformation rate.This path to equilibrium has been characterised by means of high-resolution dilatometry, scanning electron microscopy and detailed X-ray diffraction analysis.This has provided a dynamic and detailed picture of the most relevant microstructural changes and tempering mechanisms in high silicon steels.It has also provided a foundation for tailoring heat treatment processes to optimise the mechanical properties of advanced bainitic and martensitic steels.