Microstructural engineering for hard turning

Finish machining of hardened steel surface, referred to as hard turning, is of considerable technological interest because of the potential to achieve excellent surface finish at reduced cost. However, tool life as determined by surface finish criterion has been a technological barrier and hence tool wear mechanisms were investigated. Thermal softening characteristic of the hardened material dominates over geometrical softening behaviour of the material to localize the shear in the primary shear zone, when high temperature is reached in the shear band particularly at high cutting speeds. The interaction of the primary shear zone with the cutting edge of the tool is shown to cause chemical dissolution wear of the tool cutting edge. The loss of cutting edge of the tool impairs surface finish and promotes residual stresses on the machined surface. Technological options to preserve the cutting edge of the tool are examined with emphasis on workpiece microstructural engineering.