Hard PVD Coatings for Austenitic Stainless Steel Machining: New Developments
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Hard PVD Coatings for Austenitic Stainless Steel Machining: New Developments J.L. Endrino, A. Wachter, E. Kuhnt, T. Mettler, J. Neuhaus, C. Gey Balzers AG, SBU Tools. Balzers, FL-9496, Liechtenstein. ABSTRACT The machinability of austenitic stainless steels is, in general, considered to be a difficult process. This is due in great part to the high plasticity and tendency to work-harden of the workpiece, which normally results in extreme conditions imposed on cutting edges. Additionally, austenitic stainless steels have much lower thermal conductivities in comparison to plain carbon and tool steels; this inflicts high thermal loads within the chip-tool contact zone, which can significantly increase the wear rate. The complex machining of austenitic stainless steels can be, in part, relieved by use of a hard coating with low thermal conductivity and an adequate coating surface quality. This can lead to a low friction coefficient between the parts and an improved chip evacuation process. In this study, stainless steel plates were machined using a high speed finishing process and cemented carbide tools coated with four high aluminum containing coatings namely, AlCrN, AlCrNbN, fine grained (fg) AlTiN, and nanocrystalline (nc) AlTiN. Both AlTiN and AlCrN-based coatings are known for their high oxidation resistances and the formation of aluminum oxide surface layers during oxidation [1]. The coating surface textures before and after post-deposition treatment were analyzed by means of the AbbotFirestone ratio curves in order to study the influence of surface configurations leading to reduced tool wear. A maximum tool life of 150 m was observed for the tool coated with the nc-AlTiN coating. INTRODUCTION High work hardening rates even at low deformations and low thermal conductivities characterize austenitic stainless steels [2]. These two characteristics make austenitic stainless steels (γ-SS) more difficult to machine than carbon steels, low alloy steels and non-austenitic stainless steels. The high toughness and high ductility leads to the formation of long chips and the bonding of the material to the tool, which results in an increase adhesive wear. Moreover, high temperatures at the tool-chip interface result in an increased diffusion and chemical wear. In addition, the formation of build-up edges can also lead to variable machining forces and an increase in mechanical wear of the edge, which can cause tool breakage. To increase the machinability of γ-SS optimum cutting parameters such as an optimum cutting speed and feed rate can increase the tool life many times [3,4]. Due to the high tool-chip interaction, surface coatings play a major role in increasing the tool life. Numerous studies in the last two decades have shown that high aluminum containing nitride coatings (such as AlCrN’s and AlTiN’s) can provide better wear protection than non aluminum containing nitrides (CrN and TiN) at high temperatures due to their higher resistance to oxidation, higher hot hardness and lower thermal conductivities [5]. In our
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