Design of steels for high speed machining

Historical analysis of metal cutting shows that metal removal rates have be en increasing in the course of the century, predicated by the advancement i n tool materials but the steel design has tagged behind. This paper examine s the mechanisms of chip formation and tool wear as a function of cutting s peed in metal cutting. Chemical wear is identified as the dominant mechanis m of tool wear at high cutting speeds caused by temperature rise due to she ar localisation in the primary and secondary shear zones of chip. Shear loc alisation in the primary shear zone is shown to be influenced by both micro structural parameters, i.e. matrix hardening and second phase particles, an d metal cutting variables, i.e, cutting speed (strain rate) and feed (press ure). Shear localisation in the secondary shear zone is caused by the tribo logical conditions of seizure at the tool/chip interface. Chemical crater w ear is caused by the dissolution of tool into the workpiece (chip) by diffu sion mechanism and can be prevented by suppressing the tribological conditi on of seizure. The design of steel for high speed machining is based on eng ineering glassy oxide inclusions in steel, which are designed to form a vis cous layer in situ at the tool/chip interface at high cutting speeds. The v iscous layer lubricates the tool/chip interface and prevents the occurrence of seizure, thereby suppressing chemical crater wear. In comparison with t he large volume fraction of inclusions required for promoting ductile fract ure at low cutting speeds, the amount of inclusions required for lubricatin g 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 toot to engineer glassy oxide inclusions in steel. I&S/1480.