On the performance of a silicon nitride cutting tool in machining steel

Abstract

The wear rate of a silicon nitride cutting tool used to machine gray cast iron measures two orders of magnitude lower than it does when the same cutting tool is used to machine AISI 1045 steel. The extremely high tool wear rate occurring with steel is attributed in great part, to both the diffusion of iron into and the diffusion of silicon and yttrium out of the silicon nitride tool—as indicated by an energy dispersive analysis of X-rays (EDAX) analysis of the cutting tool after machining, in a direction away from the cutting edge. This results in a depletion of the tool material close to the interface of silicon and yttrium. By comparison, the extent of diffusion when machining cast iron is much smaller than when machining steel. Machining in an inert atmosphere obtained by directing a jet of argon at the cutting area proved to have no effect on the wear rate. But a jet of prechilled carbon dioxide that also was directed at the cutting area reduced the waer rate for some time. During that period, the chips formed while machining the steel changed from continuous to segmented chips; they again became continuous, however, when the cooling effect was counteracted by the excessive heat generated by the machining process.