Understanding bainite formation in steels

Abstract

The formation of bainite in steels is one of the most intensely researched topics in the field of metallurgy. Despite the importance of bainite among the modern-day steels, even a qualitative theory to explain the bainite formation still remains a subject of controversy due to the complexity of its formation mechanism. Currently, two competing theories have been proposed by the scientific community regarding the mechanism of bainite formation. One theory suggests that the bainite growth occurs via diffusionless and displacive mechanism while the other argues that bainite growth is a diffusional process. In this work, the kinetics of bainite formation is studied in detail through the lens of the displacive theory of bainite formation. Using this theory, a novel physically-based model is developed to describe the kinetics of bainite formation. This kinetic description is validated using experimentally obtained kinetic data. The effect of martensite/austenite interfaces, ferrite/austenite interfaces and cementite/austenite interfaces on the kinetics of bainite formation is also understood with the help of customized set of heat treatments. The results obtained in this doctoral thesis provide significant insight into the effect of various parameters which control the rate of bainite formation in steels. From a technological perspective, the results also open up new avenues for designing efficient heat treatment routes for the development of multi-phase advanced high strength steels involving bainitic microstructures.