Factors affecting the physical and mechanical properties of the machined surface layer
Cold hardening of surface layer
In the process of mechanical processing, the plastic deformation caused by the action of cutting force causes the distortion and distortion of the character, the shear slip between the grains, the elongating and fibrosis of the grain, even breaking, which all make the hardness and strength of the surface metal increased. This phenomenon is called the cold hardening (or strengthening). The main factors affecting cold hardening are: the cutting edge blunt radius increases, the extrusion effect on the surface metal is enhanced, and the plastic deformation is intensified, resulting in hardening. The tool flank wear increases, the friction between the flank and the machined surface increases, and the plastic deformation increases, resulting in the hardening of the chill. With the increase of cutting speed, the working time of tool and workpiece is shortened, the depth of plastic deformation is reduced, and the depth of chill layer decreases. When the cutting speed increases, the cutting heat will also shorten the working time on the surface layer of the workpiece, which will increase the degree of chill. With the increase of feed rate, the cutting force also increases, and the plastic deformation of the surface metal increases, and the hardening effect strengthens. The greater the plasticity of the workpiece material, the more serious the chill phenomenon.
Metallographic change of surface material
The metallographic structure of the surface metal will change when cutting heat exceeds the temperature of the transformed surface. There are mainly three kinds of grinding burn, quenching burn and annealing burn. There are two ways to improve grinding burn: one is to reduce the grinding heat as much as possible; the two is to improve the cooling condition and minimize the amount of heat generated. Proper selection of grinding wheels, reasonable selection of cutting parameters and improvement of cooling conditions.
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