Limiting conditions for application of PVD (TiAlN) and CVD (TiCN/Al2O3/TiN) coated cemented carbide grades in the turning of hardened steels

The development of new coated cemented carbide tool grades, based on grain refining and the unidirectional orientation of crystals in the coating layer, allows this material to withstand the severe tribological conditions imposed in hard turning. Previously, this was only possible using PCBN or oxide ceramic tool materials. The aim of this research was to determine the limiting conditions for two coated cemented carbide grades (with MT CVD and PVD coatings) aimed at allowing them to withstand at least a machining time of 15 min in the turning of hardened steels, considering the cutting parameters usually adopted for PCBN tools. Turning experiments were performed on AISI 4340, AISI 52100 and AISI D2 hardened steels, and the worn cutting edges were analyzed using focus variation microscopy (FVM) applying the three-dimensional wear parameters. The use of the PVD-coated cemented carbide grade was feasible in the case of AISI 4340 steel with a level of hardness up to 55 HRC, allowing a machining time longer than 30 min. For AISI 52100 and D2 steels, feasibility was only observed with hardness levels up to 50 and 45 HRC, respectively. Interestingly, in these cases, longer machining times were achieved using the MT CVD-coated cemented carbide grade. Wear and consequent coating layer deterioration were found to be the limiting factors for tool life, and the three-dimensional wear parameters allowed these to be identified. The results indicated the appropriate limiting conditions for the application of coated cemented carbide grades in hard turning as a function of the steel microstructure and level of hardness.