Tool life and surface alteration in high speed machining of a hardened Ni-Cr-Mo base steel with PVD coated carbide tool

摘要

A hardened Ni-Cr-Mo base steel was machined with a (TiN/TiCN/TiN) PVD coated carbide tool at high speeds up to 150m/min using different cutting fluids (at 3% concentration) delivered conventionally at a flow rate of 5 l/min. Tool life, surface roughness (Ra), hardness values as well as the integrity of the machined surfaces were recorded and analyzed. The machining results show that acceptable tool life and surface finish can be achieved when machining at cutting speed up to 100m/min speed. Machining at cutting speeds in excess of 100m/min resulted to drastic reduction in tool life and the formation of harder, plastically deformed and martensitic white layers beneath the machined surfaces. Lower flank wear rate, hence longer tool life, and component forces were generally obtained when machining at speeds up to 100m/min due to the high resistance of the TiN/TiCN/TiN coating to wear as well as the stability of the coolant composition at lower cutting conditions. The coolants tend to lose their effectiveness at higher cutting speeds resulting in a “quenching” effect on the cut surface and severe chipping of the sharp cutting edge and crack formation along the tool flank face.