Response of ferrite, bainite, martensite, and retained austenite to a fire cycle in a fire-resistant steel

Abstract

Understanding the kinetics of microstructural degradation during the event of a fire is of major relevance to future optimization of fire-resistant steels (FRS). In this work, we use in situ synchrotron X-ray diffraction to assess the rapid thermally-assisted degradation of different starting microstructures, such as (i) ferrite + pearlite; (ii) bainite + retained austenite, and (iii) martensite + retained austenite, during the simulation of a fire cycle in a Fe-0.13C-0.11Cr-0.38Mo-0.04V FRS. Our results show that retained austenite is the most unstable phase, especially when generated by faster cooling rates, decomposing at temperatures as low as 180 °C during fire simulations. Bainite and martensite are both unstable and undergo recovery and carbon desaturation via secondary precipitation of cementite. However, bainite is comparatively more stable than martensite since its decomposition starts at 400 °C, while for martensite it occurs at 320 °C. We also present a methodology to deconvolute the effect of temperature on the increased background and signal intensities of the X-ray spectra, allowing the direct observation of the kinetics of secondary cementite precipitation. • Different starting microstructures were submitted to a simulated fire cycle. • The kinetics of microstructural decomposition was studied using synchrotron X-ray diffraction. • Bainite, martensite and retained austenite underwent carbon desaturation during heating. • The kinetics of secondary precipitation of cementite was revealed for each microstructure .