The Micromechanisms of Cemented Carbide Cutting Tool Wear

Abstract

<p>Instrumental Neutron Activation Analysis has been used to measure the materials loss from the cutting tools during machining. Using this technique, the contributions from mechanical and dissolution wear have been individually quantified, and the dominance of dissolution wear during the high speed machining of a medium carbon steel using uncoated tungsten carbide-cobalt tools has been established.</p> <p>SIMS analysis of the chip material next to the tool-chip interface showed concentration profiles of tungsten and cobalt to a depth of about 0.5 micron. The maximum concentration of tungsten is seen to increase with cutting speed, in agreement with the principle of dissolution wear. There is consistency between the results of the two independent analytical techniques (INAA and SIMS), used to study the dissolution wear process.</p> <p>The microstructure of the secondary shear zone of the chips was studied by Scanning Electron Microscopy and Transmission Electron Microscopy. In the immediate vicinity of the tool-chip interface of water quenched chips, a changed ultrafine equiaxed grain structure (0.2μ) was observed. Enhancement of the diffusivity of tungsten resulting from this ultrafine structure was estimated and incorporated into a thermokinetic model for dissolution wear. However, the dissolution wear predicted by the model was lower than the experimentally measured values at all the speeds; e.g. the predictions based on complete austenitization and a 0.2 μm grain size in the immediate vicinity of the tool-chip interface accounted for 34.7% of the experimentally measured value at 240 m/min. It is proposed that entire dissolution wear could possibly be accounted for by the thermokinetic model based on further enhancement in the diffusivity due to grain boundary migration or the presence of approximately 10nm size grains during the tool-chip contact. The joint role of solubility of the tool material into the chip and the enhanced diffusivity has been shown to be important in determining dissolution wear, and the performance of HfN coated tools is discussed in light of the above model. The 'hard' alumina inclusions lead to an increase in the mechanical wear rate of the cutting tool, and this effect is demonstrated quantitatively by the INAA technique.</p>