Bainite and Martensite in Steels: Phase Transformations, Tempering and Pathways to Equilibrium

This study provides an in-depth educational resource on phase transformations in steel. It begins by establishing fundamental concepts, including equilibrium and non-equilibrium transformations, various crystal structures, and the role of phase diagrams. A core focus is placed on bainite and martensite, detailing their displacive and diffusionless transformation mechanisms, thermodynamics, characteristic morphologies, and the intricate distribution of carbon and second phases like retained austenite. The indispensable dilatometry technique is thoroughly introduced, demonstrating its application in tracking dimensional changes caused by temperature, chemical composition, and critical phase transformations. The culmination of the study is a case-study section, presenting a comparative study on the tempering behaviour of bainitic and martensitic microstructures in a medium carbon, high silicon steel. This study highlights how initial microstructural disparities — specifically the role of retained austenite in bainite and the highly dislocated, carbon-rich ferritic matrix in martensite — dictate their distinct tempering evolutions. Crucially, it reveals that while both microstructures ultimately converge to a common equilibrium state at elevated tempering temperatures (600-650 °C), they follow significantly different kinetic pathways. This comprehensive overview aims to enhance understanding of microstructure- performance control in advanced high-performance alloys. This research is funded by the European Union under the RFCS project WarP – Grant Nº 101112425.