Cutting tool is much harder than the work piece. Yet the

Cutting tool is much harder than the work piece. Yet the tool wears out during the tool-work interaction, because

1. extra hardness is imparted to the work piece due to coolant used
2. oxide layers on the work piece surface impart extra hardness to it
3. extra hardness is imparted to the work piece due to severe rate of strain
4. vibration of induced in the machine tool

Metal cutting takes place in plastic deformation zone where work hardening takes place as chip get strained.