Cutting tool wear in the machining of hardened steels: Part I: alumina/TiC cutting tool wear

A study was undertaken to investigate the mechanisms of alumina/TiC cutting tool wear in the finish turning of hardened steels with particular cognisance of the work material inclusion content. A six-fold variation in tool life was observed when machining different heats of BS 817M40 steel (similar to AISI 4340) of 52 HRC. In machining steels containing Ca-bearing mixed oxide inclusions, a reaction between the alumina phase of the tool and oxide inclusionary deposits is the dominant wear mechanism. In machining steels containing very low levels of Ca or steels with a very low inclusion content, tool wear appears to be largely based on superficial plastic deformation of the tool surface. The rate of tool wear appears to be determined by the hard inclusion content or alloy carbide content of the work material. Impingement of hard particles against the tool surface are thought to result in the generation of transient localised stresses which exceed the average contact pressures and, thus, either facilitate the operation of additional slip systems or overcome the increases in the critical resolved shear stresses on active slip systems due to prior strain. The influence of saw-tooth chip formation on cutting tool wear is also considered.