COATING-WORKPIECE OPTIMIZATION OF FREE CUTTING STEELS - TURNING WITH TICN-COATED, ZRO2-COATED, AND ZRN-COATED CEMENTED CARBIDE TOOLS

The mechanism of selective transfer of non-metallic inclusions from a steel workpiece to the rake face of the cutting tool, occurring under specific conditions of simultaneous optimization of workpiece material and tool surface chemical composition, is studied by means of inclusi on control of the workpiece steel and by deliberate modifications of t he chemical composition at the tool surface. Two resulphurized, free c utting steels, one of which has been newly developed with special incl usion control, are selected as workpiece materials. We present metallo graphic evidence on protective transfer layer (built-up layer (BUL)) f ormation enhancement for TiCN-, ZrO2- and ZrN-coated K20-type, cemente d carbide cutting tools. BUL formation and stability are shown to be d ominated by the nature and continuity of the chemically active element on the tool surface. Among the coatings selected in this study, Zr-ba sed coatings are found to be most favourable for tool life enhancement . The sulphide formation enthalpy of the active transition element on the tool surface is concluded to be the major driving force for BUL fo rmation.