The common quality defects of bearing parts after heat treatment include: quenched microstructures are overheated, underheated, quenched cracks, insufficient hardness, heat-treated deformation, surface decarburization, and soft spots.
Overheated
The overheating of the microstructure after quenching can be observed from the rough mouth of the bearing part. However, to determine the degree of overheating, the microstructure must be observed. If acicular martensite appears in the quenched structure of GCr15 steel, the quenched superheated structure is quenched. The cause may be the overheating caused by too high quenching heating temperature or too long heating and holding time; or it may be due to the severe banding carbides of the original organization, and the formation of local martensite needle-like coarseness in the low-carbon zone between the two bands. Local overheating caused. The amount of retained austenite in the overheated tissue increases, and the dimensional stability decreases. Due to the overheating of the quenched structure, the crystals of the steel are coarse, resulting in a reduction in the toughness of the parts, a reduction in the impact resistance, and a reduction in the life of the bearings. Severe overheating can even cause quench cracks.
2. Underheating
Quenching at low temperatures or poor cooling results in a microstructure in the microstructure that exceeds the normative requirement for torqite, known as under-heating, which reduces hardness and wear resistance, and affects bearing life.
3. Quenching cracks
The crack formed by the internal stress of a bearing component during quenching and cooling is called a quench crack. The causes of such cracks are: due to the quenching heating temperature is too high or too rapid cooling, the thermal stress and the metal mass volume change when the organizational stress is greater than the steel's fracture resistance; the original surface of the work defect (such as surface micro cracks or scratches Traces) or internal defects in the steel (such as slag inclusions, severe non-metallic inclusions, white spots, shrinkage remnants, etc.) that form stress concentrations during quenching; severe surface decarburization and carbide segregation; insufficient tempering after parts quenching Or not tempered in time; the previous process caused by excessive cold punching stress, forging folding, deep turning tool marks, sharp edges and other oil groove. In short, the cause of quench cracking may be one or more of the above factors, and the presence of internal stress is the main reason for the formation of quench cracks. The quenched cracks are deep and slender, the fractures are straight, and the broken section has no oxidation. It tends to be longitudinal cracks or toroidal cracks in the bearing ring; S-shaped, T-shaped or ring-shaped on the bearing steel ball. The microstructure of quenched cracks is that there is no decarburization on both sides of the crack, which is obviously different from forging cracks and material cracks.
4. Heat treatment deformation
Bearing parts in the heat treatment, there is thermal stress and tissue stress, this internal stress can be superimposed on each other or partially offset, is complex and variable, because it can follow the heating temperature, heating rate, cooling method, cooling rate, part shape As the size changes, the heat-treatment distortion is inevitable. Understanding and mastering its changing laws can make the deformation of bearing parts (such as the ellipse of the ferrule, size increase, etc.) be placed in a controllable range, which is conducive to the production. Of course, mechanical collisions during heat treatment can also deform the part, but this deformation can be reduced and avoided with improved operation.
5. Surface decarburization
Bearing parts in the heat treatment process, if it is heated in the oxidizing medium, the surface oxidation occurs so that the part of the surface of the carbon mass fraction decreases, resulting in surface decarburization. Depth of the surface decarburized layer beyond the final machining allowance will scrap the part. The determination of the depth of the surface decarburized layer can be used in the metallographic examination of the metallographic method and microhardness method. The criterion of surface microhardness distribution curve shall prevail, and can be used as an arbitration criterion.
6. Soft points
Due to insufficient heating, poor cooling, improper quenching operations, etc., the phenomenon that the surface hardness of the bearing parts is not enough is called a quenching soft point. Like surface decarburization, it can cause a serious reduction in surface wear resistance and fatigue strength.
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