This study investigates the in-situ evolution of the morphology of heated microstructures and sulfides, and to detect the machinability of Bi-bearing high S, low C free-cutting steels. The results show that Bi particles soften, melt, and vaporize during heating process. Its temperature varies slower than that of steel, and causes extensive local expansion of austenite grains. Heat absorbed by small sulfides leads to their volume expansion and formation of larger sulfides by fusion, whereas heat absorbed by large sulfides causes enlargement in dependent spherical or ellipsoidal shapes. During the heating process, at temperatures between 1500°C and 1550°C, the nominal coarsening speed of the tail wave is higher than that of the triple junction, but their movement speeds are similar. The addition of Bi particles to steel enhances its machinability. The cutting heat melts Bi particles to form liquid Bi films, resulting in rapid chip breaking during cutting.

In the domain of fire-resistant steels, the characteristics of precipitates significantly influence material properties. This study developed a novel heat treatment protocol to concurrently achieve both interphase precipitation and random precipitation. Samples were subjected to isothermal treatments at various temperatures and durations, while techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to thoroughly analyze the coarsening behavior of the two types of precipitate and reveal their thermal stability differences. The results show that the growth and coarsening rates of interphase precipitates are substantially lower than random precipitates. Coarsening kinetics analysis reveals that the radius of random precipitates follows a 1/3 power law with time at 600 °C and 650 °C, whereas the radius of interphase precipitates adheres to a 1/6 power law at 600 °C and a 1/5 power law at 650 °C. Furthermore, interphase precipitation demonstrates excellent size uniformity, which hinders the formation of a concentration gradient, thereby reducing the coarsening rate and enhancing thermal stability. After prolonged tempering treatment, interphase precipitation maintains a higher strengthening contribution than random precipitation. This study provides novel insights and theoretical foundations for the design and development of fire-resistant steels.