in cutting, the physical and mechanical properties of the surface metal are changed because of the effect of cutting force and cutting heat. The most important change is the change of the microhardness of the surface metal, the change of the metallographic structure and the production of the residual stress. Because the plastic deformation and cutting heat of the grinding process are more serious than the cutting edge of the cutting edge, the changes of the three physical and mechanical properties of the machined surface layer after grinding are very great.

    the plastic deformation caused by the action of cutting force in the process of machined , which causes the lattice distortion, distortion, the shear slip between the grains, the grain being elongated and fibrosis, even broken, which all make the hardness and strength of the surface layer metal increase. This is the case. The image is called cold hardening (or strengthened). The strengthening of the surface layer will increase the resistance of the metal deformation, reduce the plasticity of the metal, and change the physical properties of the metal.

    the metal being hardened by cold is in the unstable state of the high energy position, and only one possible, the unstable state of the metal is to be converted to a relatively stable state. This phenomenon is called weakening. The magnitude of the weakening depends on the temperature, the duration of the temperature and the degree of enhancement. As the metal is subjected to both force and heat during the machining process, the final properties of the surface metal after processing depend on the results of the consolidation and weakening.

    there are three indicators for evaluating cold work hardening, namely the microhardness HV of the surface metal, the hardened layer depth h and the hardening degree N.