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.
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