Cutting tool wear in the machining of hardened steels: Part II: cubic boron nitride cutting tool wear

A study was undertaken to investigate the wear mechanisms of CBN/TiC cutting tools in the finish machining of BS 817M40 (AISI 4340) steel of 52 HRC. A fourfold variation in tool wear rate was observed in the machining of three different heats of this steel. One of the primary characteristics of the tool wear surfaces is the manner in which the TiC phase stands proud of the CBN phase. The relative abundance of different elements on the wear surfaces of the tools, which are present in the work material in small (Mn, Si) or very small (Al, S, O) quantities, suggests that the dominant wear mechanism of CBN/TiC cutting tools is chemical in nature. In considering the relative wear rates of the tools used to machine the different heats of steel, a reasonable correlation is noted between the work material Al and S content and the corresponding tool wear rate. Examination of built up layers at the trailing edge of the tool, however, suggests that work material Al content is rate-determining with regards to tool wear. Following these observations, a new mechanism is proposed to account for the (widely acknowledged) superior wear resistance of CBN/TiC composites in comparison to high-content CBN tools.