Some recent research on the wear behaviour of carbide cutting tools

Abstract

Metallographic investigations have shown that a ferrite-austenite transformation occurs in the flow layer at the back of the chip when steel is machined with carbide tools. These transformation phenomena affect the wear reactions between the carbide tools and the steel. The alpha-gamma transformation starts at relatively low cutting speeds according to the selected cutting conditions. This results in a reduction of the formation of the built-up edge until, after complete transformation at higher cutting speeds no built-up edges occur. The flank wear varies in accordance with the size of the built-up edge. Within the range of high cutting speeds, considerable influence of the alpha-gamma transformation upon the crater wear can be observed. The carbon content of the resulting austenite phase depends upon the progress of the ferrite-austenite transformation. This influences the diffusion reactions between carbide and steel, and therefore quite considerably the strength of the resulting diffusion layers. It was possible to observe that in order to obtain good machinability, the most favourable structural states are those which result in an austenite phase with little carbon at the back of the chip. In this connection it should be stated that from the point of view of low crater wear, it is not the average carbon content which is important, but the lowest possible proportion of dissolved carbon in the individual austenite crystals. For practical purposes, these experiments show that the most favourable cutting conditions, from the point of view of wear, occur only if no built-up edge exists. The formation of the built-up edge is unfavourable, not only from the point of view of wear, but also from the point of view of the quality of the machined surface.