Machinability and Surface Integrity Characterization in Hard Turning of AISI 4320 Bearing Steel Using Different CBN Inserts

The thermomechanical interaction between the cutting tool and the workpiece in any material removal process is a key factor in defining the efficiency and productivity of the process. It is especially important in cutting hard-to-machine materials such as titanium- and nickel-based alloys or hardened steels. The productivity of the process can be defined in terms of machining time, intervals of tool change (due to tool wear) and surface integrity characteristics such as dimensional tolerances, surface roughness and the machining-affected zone. The objective of this work is to conduct a comprehensive study of the performance of three different industrial-grade CBN cutting inserts during hard turning of AISI 4320 case-carburized steel. The CBN inserts all had the same geometry but used different edge preparation and coating technologies. To quantify tool performance, the cutting forces and tool wear were measured at consistent intervals and the surface integrity of the workpiece was characterized in terms of surface roughness, white and dark layer depths, micro-hardness and residual stresses. Results show that the stability of the cutting edge on two of the tested inserts plays an important role in determining tool life (where 60% higher tool life was achieved compared to the other tested insert with unstable cutting edge), cutting force stability and the formation of compressive residual stresses on the surface of the turned workpiece