The bearing process produces a common quality defect
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| Time：2016.11.10 News Sources：Shandong Xinkaite bearing Co.,Ltd. Views：
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First, the soft point
As the lack of heating, poor cooling, quenching improper operation and other reasons caused by the phenomenon of local bearing parts hardness is not enough hardened soft spots known as the phenomenon. It can cause the same as the surface decarburization wear resistance and fatigue strength of a serious decline.
Bearing parts in the quenching process due to internal stress caused by the crack, said quenching cracks. The reasons for this crack are as follows: the quenching temperature is too high or too fast cooling, thermal stress and metal mass volume changes in the stress when the steel is greater than the breaking strength of the original work surface defects (such as surface micro-cracks or (Such as slag, severe non-metallic inclusions, white spots, shrinkage porosity, etc.) in the formation of stress concentration during quenching; serious surface decarburization and carbide segregation; parts quenched after the lack of tempering Or not timely tempering; in front of the process caused by cold punch stress is too large, forging folding, deep turning marks, sharp edges and other oil groove. Quenching crack deep and slender, fracture straight, broken surface oxidation color. The microstructure of the quenched crack is that there is no decarburization on both sides of the crack, which is obviously different from forging crack and material crack. In short, the cause of quenching cracks may be due to one or more of the above factors, the existence of internal stress is the main reason for the formation of quenching cracks. It is often in the bearing ring on the vertical straight cracks or ring cracking; in the shape of the bearing steel ball S-shaped, T-shaped or ring.
Second, less heat
Third, overheating
Fourth, quenching cracks
Bearing parts in the heat treatment process, if it is heated in the oxidizing medium, the surface oxidation will occur on the surface of the carbon content of parts decreased, resulting in surface decarburization. Determination of surface decarburization depth in the metallographic examination can be metallographic and micro-hardness method. The depth of the decarburized layer on the surface exceeds the final machined residual and the part is discarded. The surface layer of microhardness distribution curve measurement method shall prevail, do arbitration criteria.
Fifth, the surface decarburization
Sixth, heat treatment deformation
Quenching temperature is low or poor cooling will produce more than the standard in the microstructure of the organization of the body of torsite, known as the less heat organization, which makes the hardness decreased abrasion resistance dramatically reduced bearing life.
Bearing parts in the heat treatment, there are thermal stress and tissue stress, the internal stress can be superimposed or partially offset each other, is complex and varied, because it can with the heating temperature, heating rate, cooling, cooling rate, part shape And the size of the change, so heat treatment deformation is inevitable. Understanding and mastering the law of its changes can make the deformation of bearing parts (such as ring oval, size, etc.) placed in a controllable range, is conducive to production. Of course, a mechanical impact during the heat treatment also causes deformation of the part, but such deformation can be reduced and avoided with improved operation.
From the bearing parts rough mouth can be observed after quenching the microstructure of overheating. However, to determine the extent of overheating, the microstructure must be observed. If GCr15 steel in the quenching of coarse needle-like martensite, compared with the quenched superheat organization. Overheating serious and even cause quenching cracks. As the quenched tissue overheating, the steel crystal thick, will lead to decreased toughness of parts, lower impact resistance, bearing life is reduced. The residual austenite in the superheated tissue increases, and the dimensional stability decreases. May be due to the quenching temperature is too high or too long heat insulation time caused by the overall overheating; may also be due to the original organization of serious banded carbide, in the two low-carbon zone between the formation of local martensite acicular thick, Resulting in local overheating
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