Steel characteristics and their link to tool wear in hard part turning of transmission components

This study describes the influence of the steel characteristics of Ca-treated carburising steel grades during hard part turning of synchronising rings in gearbox production. The main focus was on the chemical composition of the non-metallic inclusions in the evaluated workpieces and their effect on the PCBN tool wear. In addition, a Ca-treated carburising steel grade was compared to a standard steel grade. Machining tests were performed at the transmission machining site at Scania in order to evaluate the PCBN cutting tool life as defined by the generated surface roughness during actual production. The progression of flank and crater wear was evaluated by using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) and a secondary electron (SE) detector. The Ca-treated steel showed a more than doubled tool life than that of the standard steel grade. The superior machinability was linked to the formation of a Ca-enriched slag barrier composed of (Mn,Ca)S and (Ca,Al)(O,S). It is believed that the stability of the protective deposits is essential to minimise diffusion-induced chemical wear of the PCBN tool. Furthermore, the improved machinability corresponds to a reduced tooling cost of 50% during an industrial production of transmission components at the site of Scania. Therefore, to implement the M-steel on a wider range of components would lead to a significantly reduced manufacturing cost per produced component. However, the capability of Ca-treated steels through the complete production route must be further investigated in order to allow for a large scale introduction of Ca-treated steels in the production.