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Detailed analysis of the quality defects of bearing parts after heat treatment
Source: China Bearing Network Time: 2013-02-17
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1. The outer decarburization bearing parts are in the heat treatment process; if it is heated in an oxidizing medium; the surface will have an oxidation effect to reduce the mass fraction of the surface carbon of the part; the outer surface decarburization will be formed. The depth of the outer decarburization layer exceeds the final processing allowance. The part will be invalidated. The determination of the depth of the decarburization layer can be determined by the metallographic method and the microhardness method in the metallographic examination. The surface hardness measurement curve of the surface layer is the standard; the ruling criterion can be made.
2. If the under-heat quenching temperature is low or the cooling is poor, a tortite arrangement that exceeds the specification rules occurs in the micro-arrangement; it is called an under-heating arrangement; it reduces the hardness; the wear resistance drops sharply; affecting the bearing life.
3. Heat treatment of deformed bearing parts during heat treatment; there are thermal stresses and arrangement stresses; such internal stresses can be superimposed or partially offset; they are complex and variable; because they can follow heating temperature, heating rate, cooling method, cooling rate, parts The shape and the size change; therefore, the heat treatment deformation is inevitable. Knowing and grasping the changing rules can make the deformation of the bearing parts (such as the ellipse of the ferrule, the scale increase, etc.) on a controllable scale; Of course, the mechanical bump during the heat treatment will also deform the part; but this deformation can be reduced and prevented by the improved operation.
4. The soft spot is lack of heating; the poor cooling; the hardening operation is not suitable. The appearance of the bearing part is not the hardness of the hard part. It is called quenching soft point. It can form the same wear resistance and fatigue strength as the outer surface decarburization.
5. Overheating can be investigated from the rough mouth of the bearing part to the overheating of the microscopic arrangement after quenching. However, it is necessary to investigate the microscopic arrangement when discriminating the degree of overheating. If coarse acicular martensite is present in the quenching arrangement of GCr15 steel, then Arrangement for quenching and overheating. The cause of the formation may be that the quenching heating temperature is too high or the heating and holding time is too long to form all the overheating, or the original arrangement of the banded carbide is severe; the low carbon region between the two bands constitutes part of Markov The needle is coarse; the part is overheated. The retained austenite increases in the overheating arrangement; the dimensional stability decreases. The quenching arrangement is overheated; the steel crystal is coarse; the resistance of the part is reduced; the impact resistance is reduced; the life of the bearing is reduced Also falling. Overheating is severe and may even constitute quenching cracks.
6. Quenching cracked bearing parts are called quenching cracks due to internal stress during quenching and cooling. The causes of such cracks are: because the quenching heating temperature is too high or the cooling is too fast; the thermal stress and the metal mass volume change the arrangement stress It is greater than the fracture strength of the steel, the original defects of the work appearance (such as fine cracks or scratches on the surface) or the internal defects of the steel (such as slag inclusions, severe non-metallic inclusions, white spots, shrinkage holes, etc.) during quenching Constitutive stress gathering, severe surface decarburization and carbide segregation, lack of tempering after quenching of parts or tempering in time, the cold pressing stress of the previous process is too large, casting folding, deep turning tool marks, oil groove sharp edge The reason for the quenching crack may be one or more of the above factors; the existence of internal stress is the main reason for the quenching crack. The quenching crack is deep and slender; the fracture is straight; the fracture has no oxidation color. On the bearing ring, there are often longitudinal straight cracks or ring cracks. The shape of the bearing steel ball is S-shaped, T-shaped or ring-shaped. The quenching crack is arranged on both sides of the crack. Carbon appearance; significantly different from the casting crack and crack data.
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