In situ study of carbide-free bainite transformation kinetics

Abstract

To optimize the properties of TRIP-assisted steels, a comprehensive understanding of the kinetics of carbide-free bainite transformation is essential. This study primarily focuses on two critical aspects: the evolution of phase fractions, specifically bainitic ferrite and austenite, over time, and the partitioning of carbon into diƯerent austenite regions. These austenite regions include untransformed “blocky” austenite and film-like austenite located between ferritic subunits formed during bainite transformation. Due to the potential transformation of some austenite into martensite during quenching to room temperature, it is challenging to accurately determine the carbon content and phase fraction of austenite at specific points during the transformation using ex-situ investigations. To address this challenge, in situ X-ray diƯraction (XRD) measurements are employed to study these aspects during isothermal bainite transformation of a steel containing 0.4 wt% C, 1.5 wt% Mn, and 1.5 wt% Si at various temperatures. These measurements enable the observation and analysis of phase fractions and carbon distribution with high temporal resolution. This provides precise data on the microstructural development. The samples are heat-treated with a dilatometer during the in-situ investigation to ensure precise control of the thermal conditions.