WEAR OF COATED CARBIDE TOOLS WHEN MACHINING AN NI-CR-MO (817M40) STEEL

Two different grades of coated cemented carbide inserts (composite lay ers of Ti compound +TiC+Al2O3 and Ti compound +TiC+Al2O3+TiN) were use d to machine 817M40 steel at various cutting speeds in order to assess their performance. Flank wear, surface finish and cutting forces were recorded during the machining trials. Analysis of the test results su ggests that the Ti compound+TiC+Al2O3 coated tools gave better perform ance at higher speed conditions (>200 rn/min.) due to their finer grai n size and the lower cobalt content as well as their higher TiC and Ta C content which provide greater resistance to fracture during machinin g. Lower flank wear rate and improved surface finish were, however, ob tained when machining with the Ti compound+TiC+Al2O3+TiN coated tools at lower speed conditions due to the effect of the TiN outer layer whi ch enhanced lubrication at the tool-chip interface. Flank wear, contro lled mainly by the attrition wear mechanism, was the dominant failure mode at lower cuffing speeds while attrition wear and diffusion wear m echanisms were singly or jointly responsible for premature fracture of the cutting tools at high speed conditions.