Bearings are a world-renowned brand that has provided absolute support for the development of world industrialization. Due to their high prices, they should be given more attention to daily maintenance and upkeep. The main task of bearing service life analysis is to find out the main factors causing bearing failure based on a large amount of background materials, analysis data and failure forms, so as to propose targeted improvement measures, extend the service life of bearings, and avoid sudden early failure of cylindrical roller bearings.
Overheating of the microstructure after quenching can be observed on the rough surface of the bearing parts. However, the microstructure must be observed to accurately determine the degree of overheating. If coarse needle-shaped martensite appears in the quenched structure of GCr15 steel, it is a quenched overheated structure. The cause of formation may be comprehensive overheating caused by excessive quenching heating temperature or too long heating and insulation time; it may also be due to serious banded carbides in the original structure, resulting in local coarse martensite needles in the low-carbon zone between the two bands, causing local overheating. The amount of residual austenite in the overheated structure increases, and the dimensional stability decreases. Due to the overheating of the quenched structure, the crystals of the steel are coarse, which will lead to a decrease in the toughness of the parts, a decrease in impact resistance, and a decrease in the life of the bearing. Severe overheating may even cause quenching cracks.
If the quenching temperature is too low or the cooling is poor, the troostite structure exceeding the standard will be produced in the microstructure, which is called underheated structure. It will reduce the hardness and wear resistance sharply, affecting the life of the cylindrical roller bearing.
The cracks formed by internal stress in the quenching and cooling process of bearing parts are called quenching cracks. The reasons for this kind of cracks are: due to the quenching heating temperature being too high or the cooling being too rapid, the thermal stress and the organizational stress during the change of metal mass volume are greater than the fracture strength of the steel; the original defects of the working surface ( such as surface fine cracks or scratches ) or the internal defects of the steel ( such as slag inclusions, serious non-metallic inclusions, white spots, shrinkage cavity residues, etc. ) form stress concentration during quenching; severe surface decarburization and carbide segregation; insufficient tempering of parts after quenching or untimely tempering; excessive cold stamping stress caused by the previous process, forging folding, deep turning tool marks, sharp edges and corners of oil grooves, etc. In short, the cause of quenching cracks 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. Quenching cracks are deep and slender, with straight fractures and no oxidation color on the broken section. It is often a longitudinal straight crack or annular crack on the cylindrical roller bearing ring; the shape on the bearing steel ball is S -shaped, T- shaped or ring-shaped. The structural characteristic of quenching cracks is that there is no decarburization on both sides of the cracks, which is obviously different from forging cracks and material cracks.
When bearing parts are heat treated, there are thermal stress and structural stress. This internal stress can be superimposed or partially offset each other, and is complex and changeable, because it can change with the heating temperature, heating speed, cooling method, cooling speed, part shape and size, so heat treatment deformation is inevitable. Understanding and mastering its changing rules can put the deformation of cylindrical roller bearing parts ( such as the ellipse of the ring, the size expansion, etc. ) within a controllable range, which is conducive to production. Of course, mechanical collisions during the heat treatment process will also cause deformation of parts, but this deformation can be reduced and avoided by improving operations.
Internal factors mainly refer to the three major factors that determine bearing quality, including structural design, manufacturing process and material quality.
The metallurgical quality of imported bearing materials used to be the main factor affecting the early failure of rolling bearings. With the advancement of metallurgical technology (such as vacuum degassing of bearing steel, etc.), the quality of raw materials has improved. The proportion of raw material quality factors in bearing failure analysis has decreased significantly, but it is still one of the main factors affecting bearing failure. Whether the material selection is appropriate is still a factor that must be considered in bearing failure analysis.
The manufacture of cylindrical roller bearings generally requires multiple processing steps such as forging, heat treatment, turning, grinding and assembly. The rationality, advancement and stability of each processing technology will also affect the life of the bearing. Among them, the heat treatment and grinding processes that affect the quality of the finished bearings are often more directly related to the failure of the bearings. In recent years, research on the deteriorated layer on the working surface of the bearing has shown that the grinding process is closely related to the surface quality of the bearing.
