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

A study was undertaken to investigate the wear mechanisms of CBN/TiC cuttin g 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 thre e 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 surfac es of the tools, which are present in the work material in small (Mn, Si) o r very small (Al, S, O) quantities, suggests that the dominant wear mechani sm of CBN/TiC cutting tools is chemical in nature. In considering the relat ive 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 th e trailing edge of the tool, however, suggests that work material Al conten t is rate-determining with regards to tool wear. Following these observatio ns, a new mechanism is proposed to account for the (widely acknowledged) su perior wear resistance of CBN/TiC composites in comparison to, high-content CBN tools. (C) 2001 Elsevier Science B.V. All rights reserved.