Investigations on machinability aspects of hardened AISI 4340 steel at different levels of hardness using coated carbide tools

This study investigates the effect of workpiece hardness, cutting parameters and type of coating (coated tool) on different machinability aspects like, the tool life, surface roughness, and cutting force and chip morphology during turning of hardened AISI 4340 steel at different levels of hardness. Cutting forces observed to be higher for harder workpiece and for CVD applied multi-layer MT-TiCN/Al2O3/TiN coated carbide tool. Better surface finish observed for harder workpiece and for PVD applied single-layer TiAlN coated carbide tool. However, better tool life obtained by CVD coated tool can be attributed to its thick coating and the protective Al2O3 oxide layer formed during cutting, which has protected the tool from severe abrasion at elevated temperatures. Modified Taylor tool life equation indicated that the workpiece hardness followed by the cutting speed and depth of cut as the most influencing factors on tool life. The better performance of CVD coated tool under study is obtained by limiting the cutting speed to 300 and 180 m/min for workpiece hardness of 35 and 45 HRC, respectively. However, the upper limit is of 200 m/min when using PVD coated tool. It has been observed that the tool wear form and the wear mechanism(s) by which the tool wear occurred are influenced by the workpiece hardness, cutting conditions and the type of tool.

► Higher cutting forces for harder workpiece and CVD multi-layer TiCN/Al2O3/TiN coated tools
► Thick CVD coating resulted in longer tool life as against thin PVD-TiAlN coating.
► Tool life mostly influenced by workpiece hardness followed by speed and depth of cut
► PVD coated carbide tools produced better surface finish and minimum cutting forces.
► Coated WC tools unable to turn 55 HRC workpiece even at lowest cutting conditions