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Steels for machine structural use are required to have superior machinability to enhance the productivity because various kinds of products are produced by machining operations. In terms of machinability, tool life is regarded as one of the most important factors since it gives a direct influence on an exchange frequency of cut tools in mass-production lines. So far, various studies on the mechanism of tool wear have been conducted to improve the tool life. Among them, it has been considered that two kinds of wear mechanism, diffusion wear which is caused by diffusion of particular tool constituents into the chip and abrasive wear in which the abrasive hard particles in steels scrape off the tool surface, mainly influence the rake face wear of carbide tools. It has been widely known that inclusions in steels strongly influence the machinability. A large number of studies on the mechanism of wear reduction effects, particularly in Ca-added steels, have been carried out since it was recognized at the early 1960s in Germany. Many reports have stated that the rake face wear at high cutting speeds is reduced by an adhesion of oxide inclusion containing calcium to the carbide tool surface, which could work as the so-called “belag”, since this oxide layer covers the tool surface and protects the tool from the diffusion wear. It has been also reported that the oxide inclusion containing calcium is softer than a typical oxide like Al2O3, and that (Mn, Ca)S, which crystallized around the oxide containing calcium, reduces the abrasive effects of this composite oxide. Furthermore, in case of machining a Ca-added steel containing higher amount of sulfur with a relatively high cutting speed, an inclusion layer, which is mainly composed of manganese sulfide, forms on the tool surface, avoiding the direct contact between the tool surface and the chip, and it works to prevent the diffusion wear. Though there have been a lot of studies on the wear reduction effect by calcium addition, some aspects are still remained to be clarified, such as the roles of the oxide inclusion containing calcium for enhancing the formation of the inclusion layer. In this paper, the influence of calcium and/or magnesium addition on the rake face wear of carbide tools under a high cutting speed was investigated. The role of inclusions for the wear reduction in the turning operation was examined by detailed observation of the inclusion layer formed on the tool surface by cross sectional transmission electron microscopy.

Wear Reduction of Carbide Tools Observed in Cutting Ca-added Steels for Machine Structural Use