This paper deals with influence of chemical compositions of work materials and cutting tools on machinability of BN added steels. In this study, work materials including different levels of Al, B and N were turned with carbide and cermet tools which also included different levels of Ti at high speeds. Furthermore, the free-machining mechanisms were discussed on the results obtained from cutting tests. Machining tests showed that BN added steel including Al had very good machinability when they were machined with P-grade carbide tools. The machinability, however, decreased when the work material had low content of Al, B or N. Up to 0.06% of Al in work material, it showed that the larger amount of Al the bigger effect of machinability. The machinability of BN added steel was influenced by Ti content of cutting tool, that is, the higher content of Ti the smaller tool wear. When machining BN added steels at high cutting speed with Pgrade carbide tools, Al and N were detected as a layer, and the layer was considered as A1N by Xray diffraction analysis. AlN layer on the carbide tool surface prevented the inter-diffusion between cutting tool and work material. The effect of A1N on the diffusion was considered as the mechanisms of free-machining of BN added steels.
