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Surface layer structure
During machining, stresses arise in the surface layer of the part, which remain even after the end of the machining process. The reasons for their occurrence are the temperature gradient in the surface layer of the part, the formation of new structures, plastic deformation of the material, etc. Stresses in the surface layer arising after
mechanical processing, can reach significant values and exceed the ultimate strength of the material. For example, cracks in steel parts after grinding occur under the action of just such stresses.
During the processing of a part, the material of the surface layer undergoes structural and phase changes. For example, crystalline grains of metal randomly located in the original structure after plastic deformation take a certain orientation - texture . Steel parts after heat treatment often have a hardness gradient in depth. During cold plastic deformation of metal parts, hardening is observed - hardening of the surface layer, accompanied by an irreversible change in the structure of the metal.
The surface layer of a metal part has a specific structure (Fig. 3.5.).
The outer layer 1 consists of an adsorbed film of gases, moisture, coolant components, etc. This film can only be removed by heating the part in a vacuum. Below it is layer 2, consisting of crystals with a lattice distorted as a result of deformation, decarburized under the action of high temperatures that occur during grinding. Oxides are concentrated in it, as well as voids and cracks, so its structure approaches amorphous. Layer 2 with a thickness of 2-8 nm is called the Bayleby layer . Layer 3 consists of highly deformed grains containing cementite 4, which was formed under the action of high temperatures. Next is metal 5 with the original structure.
Anisotropy is the dissimilarity of the physical properties of a medium in different directions. The surface layer, the structure of which is shown in Fig. 1.5., anisotropic, because its strength is not the same in depth. The distribution of strength in the surface layer of a metal part is estimated from the change in microhardness. Microhardness - the hardness of metal microvolumes, measured by the resistance to the penetration of a diamond pyramid into them
under the influence of small (0.1N) loads. It sharply decreases with distance from the surface of the part, especially within layer 2 with destroyed crystals. The thickness of the surface layer of steel parts: after turning - 0.25-2 mm, normal grinding - 12-75 microns, thin - 2-25 microns, after polishing - 0.2 microns.