The Forms and Evolution of Free‐Cutting Phase in Bi–Te–S Free‐Cutting Steel

The free‐cutting phase (FCP) plays a crucial role in the machinability of steel materials. This article investigates the existence forms of the Bi–Te–S FCP in steel according to 2D and 3D morphology and observes its evolution process through a high‐temperature confocal laser scanning microscope combined with thermodynamic calculation and crystallographic analysis. The forms of Bi–Te–S composite FCP are classified comprehensively. Six types of Bi–sulfide exist in this work, including separate (SP), cavity bridge (CB), acute angle coated (AAC), obtuse angle coated (OAC), and semicoated types (SC). Five types of Te–sulfide existence forms are found, including CB, AAC, OAC, SC, and complete‐coated types (CC). Regarding the existence forms of Bi–Te–sulfide, five types are identified, including SP, CB, OAC, SC, and CC. The evolution of Bi–Te–S composite FCP is closely related to the proportion of Bi, Te, and S. When the Te/S ratio is below 0.07, Bi–Mn(Te, S)‐bridged two sulfides form, breaking the chain‐like FCP and avoiding the formation of chain‐like FCP. Te mainly dissolves in the sulfide, promoting spherical sulfide formation. When the Te/S ratio is beyond 0.07, MnTe precipitates easily on MnS with more nucleation sites and grows to larger‐sized FCP; subsequently, Bi precipitates around Mn(Te, S), forming the large‐sized spherical MnS–MnTe–Bi FCP.