Design of steels for high speed machining

Abstract

Historical analysis of metal cutting shows that metal removal rates have been increasing in the course of the century, predicated by the advancement in tool materials but the steel design has lagged behind. This paper examines the mechanisms of chip formation and tool wear as a function of cutting speed in metal cutting. Chemical wear is identified as the dominant mechanism of tool wear at high cutting speeds caused by temperature rise due to shear localisation in the primary and secondary shear zones of chip. Shear localisation in the primary shear zone is shown to be influenced by both microstructural parameters, i.e. matrix hardening and second phase particles, and metal cutting variables, i.e. cutting speed (strain rate) and feed (pressure). Shear localisation in the secondary shear zone is caused by the tribological conditions of seizure at the tool/chip interface. Chemical crater wear is caused by the dissolution of tool into the workpiece (chip) by diffusion mechanism and can be prevented by suppressing the tribological condition of seizure. The design of steel for high speed machining is based on engineering glassy oxide inclusions in steel, which are designed to form a viscous layer in situ at the tool/chip interface at high cutting speeds. The viscous layer lubricates the tool/chip interface and prevents the occurrence of seizure, thereby suppressing chemical crater wear. In comparison with the large volume fraction of inclusions required for promoting ductile fracture at low cutting speeds, the amount of inclusions required for lubricating the tool/chip interface is very small and is in the range that is typical of clean steel. Thermodynamic modelling is shown to be a powerful tool to engineer glassy oxide inclusions in steel