Common quality defects in bearing parts after heat treatment

Common quality defects of bearing parts after heat treatment include: quenching microstructure overheating, underheating, quenching cracks, insufficient hardness, heat treatment deformation, surface decarburization, soft spots, etc.
Overheat
Overheating of the microstructure after quenching can be observed from the rough mouth of the bearing part. However, the exact microstructure must be observed to determine the degree of overheating. If coarse needle-shaped martensite appears in the quenched structure of GCr15 steel, it is quenched and superheated. The formation may be caused by excessive overheating of the quenching heating temperature or excessive heating and holding time; or it may be due to the serious banding of the original structure, forming a local martensite needle-like coarseness in the low carbon zone between the two zones. Local overheating caused. The retained austenite increases in the superheated structure, and the dimensional stability decreases. Due to the overheating of the quenched structure, the coarse crystal of the steel causes the toughness of the part to decrease, the impact resistance is lowered, and the life of the bearing is also lowered. Excessive heat can even cause quenching cracks.
2. Underheat
If the quenching temperature is low or the cooling is poor, the tortite structure exceeding the standard is formed in the microstructure, which is called the underheated structure, which lowers the hardness and sharply reduces the wear resistance, which affects the bearing life.
3. Quenching crack
The crack formed by the internal stress of the bearing part during the quenching and cooling process is called quenching crack. The causes of such cracks are: due to excessive heating temperature of quenching or too rapid cooling, the microstructure stress when the thermal stress and metal mass volume change is greater than the fracture strength of the steel; the original defects of the working surface (such as surface micro cracks or scratches) Trace) or steel internal defects (such as slag inclusions, severe non-metallic inclusions, white spots, shrinkage residuals, etc.) form stress concentration during quenching; severe surface decarburization and carbide segregation; insufficient tempering after quenching of parts Or not tempered in time; the cold punching stress caused by the previous process is too large, forging and folding, deep turning tool marks, sharp edges and corners of the oil groove. In short, the cause of quenching crack may be one or more of the above factors, and the existence of internal stress is the main reason for the formation of quenching cracks. The quenching crack is deep and slender, the fracture is straight, and the fractured surface has no oxidation color. It is often a longitudinal straight crack or annular crack on the bearing ring; the shape on the bearing steel ball is S-shaped, T-shaped or ring-shaped. The microstructure of the quenching crack is that there is no decarburization on both sides of the crack, and the difference is obvious with forging cracks and material cracks.

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What aspects should be considered when choosing a bearing clearance?

 When choosing a bearing clearance, the following aspects should be considered:

      1, the working conditions of the bearing, such as load, temperature, speed, etc.;

      2. Requirements for bearing performance (rotational accuracy, friction torque, vibration, noise);

      3. When the bearing and the shaft and the housing hole are in an interference fit, the bearing clearance is reduced;

      4. When the bearing is working, the temperature difference of the inner jacket ring causes the bearing clearance to decrease;

      5. The bearing clearance is reduced or increased due to the different expansion coefficients of the shaft and the outer casing material.

      6, according to the experience of use, the most suitable working clearance of the ball bearing is close to zero; roller bearings should maintain a small amount of working clearance. In components requiring good bearing rigidity, the bearings allow a certain amount of preload. It is specifically pointed out here that the so-called working clearance refers to the clearance of the bearing under actual operating conditions. There is also a kind of play called the original play, which refers to the play before the bearing is not installed. The original clearance is larger than the installation clearance. Our choice of clearance is mainly to choose the right working clearance. Valve inlet pump

      7. The clearance values ​​specified by national standards are divided into three groups: basic group (0 group), small clearance auxiliary group (1, 2 group) and large clearance auxiliary group (3, 4, 5 groups). When selecting, under normal working conditions, the basic group should be preferred, so that the bearing can get the proper working clearance. When the basic group does not meet the usage requirements, the auxiliary group clearance should be used. The large clearance auxiliary group is suitable for interference fit between the bearing and the shaft and the housing hole. The temperature difference between the inner and outer rings of the bearing is large. The deep groove ball bearing needs to bear a large axial load or needs to improve the centering performance. Reduce the bearing friction torque and other occasions; the small clearance auxiliary group is suitable for the requirements of higher rotation accuracy, strict control of the axial displacement of the housing hole, and the need to reduce vibration and noise.

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