Steel characteristics and their link to chip breaking and tool wear in metal cutting

The vision of this thesis is to study how it is possible to obtain optimised workpieces during metal cutting processes in industry. Specifically, the work is aimed to increase the understanding between the steel characteristics and their link to the chip breaking and tool wear during metal cutting. The emphasis is on the influence of the cleanliness and the characteristics of non-metallic inclusions in the workpiece on the machinability of carburising steel grades. The machinability of a case hardening steel is improved by a M-treatment (additions of Ca). Also, the improved machinability of the M-steels offers an attractive potential to save money which makes it possible to reduce the tooling costs with up to 50%. The improved machinability of Ca-treated steels is correlated to the formation of lubricating slag layers consisting of Ca-enriched sulfide inclusions and oxy-sulfide inclusions, which are formed on the rake face during the machining operation. It is proposed that the formations of slag layers from the workpiece constituents are essential to minimise the chemical degradation of the tool edge due to a contact with the chip. During this process, sulfur minimises the material transfer from the chip flow, whilst Ca-treated impurities have a stabilising effect on the protective deposits made of slag layers. Since there is a remaining industrial need to increase the production rate, whilst maintaining a high quality of the finished parts, the future production will continue to require extreme demands on the quality of workpieces. If the emphasis is focused on the workpiece, it should be possible to obtain a robust manufacturing process. Therefore, the challenge for future steel metallurgists is to develop high performance grades with optimised combined properties. Nyckelord: skärbarhet, stål, icke-metalliska inneslutningar. v Preface This thesis summarises the knowledge I gained during my period as a Ph.D. candidate within the research area of applied process metallurgy. The work during the first two years was carried out partly Sweden. The work has been part of projects which was focused on the influence of non-metallic inclusions and microstructure on the machinability of carburising steel grades for soft and hard machining. The first project was conducted within the Member Research Consortium Materials in Machining at Swerea KIMAB. The second part was conducted within a VINNOVA funded project within the  Some metallurgical aspects of modification and control of non-metallic inclusions to improve the machinability of steel.  The effect of inclusion composition, size, …