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A large number of failure analysis and Research on rolling bearings show that the short life or premature loss of accuracy of bearings are caused by material defects or improper manufacturing, but to a considerable extent, they are not installed and maintained in strict accordance with the requirements of bearing use,
Or the abnormal damage of the Good Use bearing is caused by improper bearing selection or the actual load exceeds the rated load of the bearing itself
For example, the fatigue spalling of bearing parts is largely caused by impurities in the lubricating oil. It can be seen that in order to realize that the rolling bearing has a longer Good Use service life and precision retention period,
in addition to requiring the bearing manufacturer to improve the product quality, the bearing users must also use the bearing with scientific Good Use methods and procedures, otherwise, the best bearing will die prematurely under the harsh and arbitrary use conditions.
Rolling bearing is a kind of precision mechanical support element. Bearing users deeply hope that the bearing installed on the main engine can not be damaged and maintain its dynamic performance within the predetermined service life, but the objective facts are sometimes not satisfactory. Sudden bearing failure accidents will cause great losses to users.
The common quality defects of bearing parts after heat treatment are: quenching microstructure Good Use overheating, underheating, quenching cracks, insufficient hardness, heat treatment deformation, surface decarburization, soft spots and so on.
1. Overheating
The microstructure overheating after quenching can be observed from the rough mouth of bearing parts. Good Use However, the microstructure must be observed to determine the degree of overheating. If coarse acicular martensite appears in the quenched structure of GCr15 steel, it is quenched superheated structure.
The cause may be the overall Good Use overheating caused by too high quenching heating temperature or too long heating and holding time;
It may also be due to the serious banded carbide in the original Good Use structure and the formation of local martensite acicular coarseness in the low-carbon region between the two bands, resulting in local overheating. The retained austenite in the superheated structure increases and the dimensional stability decreases.
Due to the overheating of quenching structure and the coarseness of steel crystal, the toughness of parts will be reduced, the impact resistance will be reduced, and the service life of Good Use bearings will also be reduced. Serious overheating may even cause quenching cracks.
Low quenching temperature or poor cooling will produce troostite Good Use structure exceeding the standard in the microstructure, which is called underheated structure. It will reduce the hardness, sharply reduce the wear resistance and affect the service life of the bearing.
3. Quenching crack
The crack formed by internal stress during quenching and cooling of bearing parts is called quenching crack. The reasons for this kind of crack are as follows: because the Good Use quenching heating temperature is too high or the cooling is too urgent,
the structural stress when the thermal stress and metal mass volume change is greater than the fracture strength of the steel; The original defects on the working surface (such as surface micro cracks or scratches) or internal defects of steel
(such as slag inclusion, serious non-metallic inclusions, white spots, shrinkage cavity residue, etc.) form stress concentration during quenching; Severe surface decarburization and carbide segregation; The tempering of parts after quenching is insufficient or not tempered in time;
Excessive cold punching stress caused by the previous process, forging folding, deep turning tool marks, sharp edges and corners of oil groove, etc. In a word, the cause of quenching crack may be one or more of the above factors, and the existence of internal stress is the main cause of quenching crack.
The quenching crack is deep and slender, the fracture is straight, and the fracture 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 can be S-shaped,
T-shaped or ring-shaped. The microstructure characteristic of quenching crack is that there is no decarburization on both sides of the crack, which is obviously different from forging crack and material crack.
4. Heat treatment deformation
During the heat treatment of bearing parts, there are thermal stress and structural stress. This internal stress can be superimposed or partially offset each other, which is complex and changeable.
Because it can change with the changes of heating temperature, heating speed, cooling mode, cooling speed, part shape and size, heat treatment deformation is inevitable. Understanding and mastering its change law can put the deformation of bearing parts (such as the ellipse of ferrule, size increase, etc.)
in a controllable range, which is conducive to production. Of course, mechanical collision during heat treatment will also cause deformation of parts, but this deformation can be reduced and avoided by improved operation.
5. Surface decarburization
During the heat treatment of bearing parts, if they are heated in oxidizing medium, the surface will be oxidized, reducing the mass fraction of carbon on the surface of parts and causing surface decarburization.
If the depth of surface decarburization layer exceeds the allowance of post-processing, the parts will be scrapped. The depth of surface decarburization layer can be measured by metallographic method and microhardness method.
The measurement method of microhardness distribution curve of surface layer shall prevail and can be used as arbitration criterion.
6. Soft spot
The phenomenon of insufficient local hardness on the surface of bearing parts caused by insufficient heating, poor cooling and improper quenching operation is called quenching soft spot. Like surface decarburization, it can cause a serious decline in surface wear resistance and fatigue strength.