In the Case of Turning Cast Irons Yasuo YAMANE, Biing Hwa YAN, Hirosi USUKI and Norihiko NARUTAKI Due to its superior thermal and mechanical characteristics, a silicon nitride ceramics has been expected to be used for cutting tool materials. It has been reported that silicon nitride ceramic tools show good performance in machining cast iron, but large wear has been observed on the tools in machining carbon steel. In the case of cutting carbon steel, it has been assumed that the reaction between

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... Si and Fe is the major cause of the large wear. If the assumption is correct, the reason must be clarified as to why the silicon nitride ceramic tool shows good wear resistance to the cast iron consisting of Fe. The purpose of this report is to thus clarify the tool wear and the cutting performance of silicon nitride ceramic tools in turning cast iron, and to study mechanisms having good wear resistance to the cast iron. Gray cast iron FC 25, continuous-casted iron FC 25-CC, ductile cast iron FC 25 D and carbon steel S 50 C were turned in the laboratory air, and in a sealed chamber in which the atmosphere was changeable. The results obtained were as follows : (1) The wear of the silicon nitride ceramic tool in turning FC 25-CC or FCD 25 was as large as that of turning S 50 C. (2) In the case of turning FC 25 in the laboratory air, the wear of the silicon nitride tool decreased with an increase in the cutting speed, and the protective layers consisting of Al-Mn oxide were observed on the rake face of the tool at high cutting speeds. (3) In the case of turning FC 25 in a low oxygen content atmosphere, the wear of the silicon nitride tool increased, and adhesion of the chips was observed on the rake face. On the contrary, in the case of turning FC 25-CC, no differences between the wear in the laboratory air and the wear in the low oxygen content atmosphere were observed.