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Common quality defects of bearing parts after heat treatment
Source: Bearing network time: 2014-01-27
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Common quality defects of bearing parts after heat treatment include: quenching micro-arrangement overheating, underheating, quenching cracks, hardness failure, heat treatment deformation, surface decarburization, soft spots, etc.
6. The soft point is lack of heating; the cooling is poor; the quenching operation is not appropriate. The appearance of the bearing parts is somewhat hard. The appearance is called quenching soft point. It can form the same wear resistance and fatigue strength as the outer surface decarburization.
The cracks formed by the internal stress of the bearing parts during the quenching and cooling process are called quenching cracks. The reasons for the formation of such cracks are: because the quenching heating temperature is too high or the cooling is too fast; the arrangement stress when the thermal stress and the metal mass volume change is greater than that of the steel The fracture resistance, 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.) constitute stress during quenching. 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 and so on. The quenching crack is deep and slender; the fracture is straight; the fractured surface has no oxidation color. The arrangement of the quenching crack is characterized by no decarburization on both sides of the crack; significant difference and casting crack and data crack. The reason for quenching crack may be the above One or more of the elements; the presence of internal stress is the main cause of the quenching crack. It tends to be a longitudinal straight crack or ring crack on the bearing ring, on the shaft. Shape of the ball there is an S-shaped, T-shaped or ring.
2. Underheated 1. Overheated 3. The quenching crack 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 surface decarburization is formed. The determination of the depth of the outer decarburization layer is available in the metallographic examination. Metallographic method and microhardness method. The depth of the outer decarburization layer exceeds the final processing allowance, which will make the parts invalid. The external surface microhardness dispersion curve measurement method shall prevail; the ruling criterion can be made.
5. Decarburization on the surface 4. Heat treatment deformation quenching temperature is low or poor cooling will occur in the micro-arrangement of the tortite arrangement beyond the normative rules; called underheating arrangement; it reduces hardness; wear resistance sharply reduced; affect bearing life.
Bearing parts are heat treated; 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, part shape and The change of the size changes; so the heat treatment deformation is inevitable. Knowing and grasping its 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.
From the rough mouth of the bearing part, it can be investigated that the micro-arrangement after quenching is overheated. However, it is necessary to investigate the microscopic arrangement when discriminating the degree of overheating. If thick acicular martensite is present in the quenching arrangement of GCr15 steel, then Quenching and overheating arrangement. Overheating is severe and may even constitute quenching cracks. Because quenching arrangement is overheated; steel crystals are coarse; the resistance of parts will be reduced; the impact resistance is reduced; the life of the bearings is also reduced. The retained austenite is increased in the overheating arrangement; The dimensional stability is reduced. 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 area between the two bands constitutes some Markov The body is needle-shaped and thick; the composition is somewhat overheated.
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