The Plastic Deformation of Mild Steel in Metal Cutting

The effect of subcritical quenching and room temperature aging on the cutting behavior of mild steel has been studied by optical and electron microscopy. Carbon and nitrogen in supersaturated solid solution and/or as submicroscopic precipitates greatly increase the dynamic strength of the steel and, hence, reduce the rate of metal removal when machining with high speed steel tools. When compared with a steel with a carbon and nitrogen-depleted or precipitation-free ferrite matrix, it is clear that the effect is caused by an increase in the high-strain work-hardening rate and a reduction in dynamic recovery. Because these submicroscopical precipitates are redissolved upon straining, variation of particle size and spacing has little effect upon the rate of metal removal, hence there are no significant differences between the machining behaviour of the steel in the solution heat treated, quench-aged and overaged condition. Transmission and scanning electron microscopy were used to examine the high-strain deformation structure of chips. Subzero cutting, at liquid nitrogen temperature, was used to control the temperature in the primary shear zone and to “freeze” pre-existing structure in the flow-zone.