Free Machining Steels—The Behavior of Type I MnS Inclusions in Machining

Experimental results obtained by in situ machining within a Scanning Electron Microscope of a resulphurized steel containing Type I manganese sulphide inclusions are presented. The data obtained from slow speed machining tests show that chip formation in these steels involves decohesion and cavitation at the sulphide-matrix interface as well as fracture of sulphide inclusions. While these processes promote fracture in the shearing region during chip formation (confirming an earlier hypothesis due to Tipnis and Cook), it is argued that these phenomena cannot be responsible for better tool life, surface finish, and lowered power requirements usually encountered with free machining alloys under typical industrial cutting conditions. The role of thermal dissipation during machining and its effect in altering the chip formation through thermally induced changes in plastic flow behavior of Type I sulphide inclusions are discussed.