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. The free-cutting phase (FCP) plays a crucial role in the machinability of steel materials. When the Te/S ratio is below 0.07, Bi-Mn(Te, S)-bridged two sulfides form, avoiding the formation of chain-like FCP. When the Te/S ratio is beyond 0.07, MnTe precipitates on MnS with more nucleation sites and Bi precipitates around Mn(Te, S), forming the large-sized spherical MnS-MnTe-Bi FCP.image (c) 2023 WILEY-VCH GmbH