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There are four main forms of damage or failure of high temperature bearing

Release time: May 31, 2018 16:21:00 reading: second

In the use of high temperature bearing, due to its own quality and external conditions, its bearing capacity, rotation accuracy and friction reduction performance will change. When the performance index of high temperature bearing is lower than the use requirements and can not work normally, it is called high temperature bearing damage or failure.

Once the high temperature bearing is damaged or invalid, the machine and equipment will stop running and the function will be damaged. Therefore, it is necessary to investigate and deal with the causes in a short time and take corresponding measures.

Manifestation of damage or failure of high temperature bearing

There are many forms of damage or failure of high temperature bearing, and the reasons are complex. In addition to the internal factors of bearing design and manufacturing, the damage of high temperature bearing is mostly caused by improper use, such as unsuitable selection of type, unreasonable support design, improper installation, poor lubrication, poor sealing and other external factors.

Four cases of damage or failure of high temperature bearing

It is of great significance to study the causes of damage or failure of high temperature bearing. On the one hand, it can improve the use method, correctly use the high temperature bearing, give full play to the performance of high temperature bearing, on the other hand, it is helpful to develop new products with better performance. In this paper, in addition to the matters needing attention, installation methods and operation supervision of rolling bearings, the damage forms and causes of high temperature resistant bearings and the countermeasures to be taken are also introduced.

1. Grinding heat of bearing

In the grinding process of NTN bearing, a large amount of energy is consumed in the contact area between the grinding wheel and the workpiece, resulting in a large amount of grinding heat, resulting in local instantaneous high temperature in the grinding area. It is found that the instantaneous temperature in the grinding zone can be as high as 1000-1500 ℃ within 0.1-0.001ms. Such instantaneous high temperature is enough to cause high temperature oxidation, amorphous structure, high temperature tempering, secondary quenching and even burn cracking of the surface layer on the working surface.

(1) Surface oxide layer

Under the action of instantaneous high temperature, the steel surface reacts with oxygen in the air to form a very thin layer of iron oxide (20-30nm). It should be noted that there is a corresponding relationship between the thickness of oxide layer and the total thickness of surface grinding metamorphic layer. This shows that the oxide layer thickness is directly related to the grinding process and is an important indicator of grinding quality.

(2) Amorphous structure layer

When the instantaneous high temperature in the grinding area makes the workpiece surface reach the melting state, the molten metal molecular flow is evenly coated on the working surface and cooled by the base metal at a very fast speed, forming an extremely thin amorphous structure layer. It has high hardness and toughness, but it is only about 10nm, so it can be easily removed in precision grinding.

(3) High temperature tempering layer

The instantaneous high temperature in the grinding zone can make the surface heated to a temperature higher than the tempering heating temperature of the workpiece in a certain depth (10-100nm). When the austenitizing temperature is not reached, with the increase of the heating temperature, the microstructure transformation of re tempering or high temperature tempering corresponding to the heating temperature will be produced layer by layer, and the hardness will also decrease. The higher the heating temperature, the more severe the hardness decrease.

(4) Two layer quenching layer

When the instantaneous high temperature in the grinding zone heats the workpiece surface layer above the austenitizing temperature (AC1), the austenitized structure of the layer will be re quenched to martensite structure in the subsequent cooling process. For the workpieces with secondary quenching burn, there must be a very low hardness high temperature tempering layer under the secondary quenching layer.

(5) Grinding crack

The stress of the surface layer of the workpiece will change due to the secondary quenching burn. The secondary quenching zone is in compression state, and there is a large tensile stress in the high temperature tempering zone under the secondary quenching zone, where the crack core may occur. The crack propagates easily along the original austenite grain boundary. Severe burn will lead to cracks (mostly cracks) on the grinding surface, resulting in workpiece scrapping.

2. Deterioration layer of high temperature bearing due to grinding force

In the grinding process, the surface layer of the workpiece will be affected by the cutting force, compression force and friction force of the grinding wheel. Especially the effect of the latter two makes the plastic deformation layer and work hardening layer with strong directivity formed on the surface layer of the workpiece. These metamorphic layers will inevitably affect the change of residual stress in the surface layer.

(1) Cold plastic deformation layer

In the grinding process, every moment of the abrasive particle is equivalent to a cutting edge. However, in many cases, the front angle of the cutting edge is negative. Besides the cutting effect, the abrasive particles make the surface of the workpiece bear the extrusion effect (ploughing effect), leaving an obvious plastic deformation layer on the surface of the workpiece. The deformation degree of this deformation layer will increase with the increase of grinding wheel blunt degree and grinding feed rate.

(2) Thermoplastic deformation (or high temperature deformation) layer

The instantaneous temperature formed by grinding heat on the working surface makes the elastic limit of workpiece surface layer decrease sharply, even reach the degree of elastic disappearance. Under the action of grinding force, especially compression force and friction force, the free extension of the working surface layer is limited by the base metal, and the surface is compressed (ploughed), resulting in plastic deformation on the surface layer. High temperature plastic deformation increases with the increase of workpiece surface temperature under the condition of constant grinding process.

(3) Work hardening layer

Sometimes the microhardness and metallographic methods can be used to find that the hardness of the surface layer increases due to machining deformation.
In addition to grinding, the surface decarburization layer caused by casting and heat treatment heating, if not completely removed in the subsequent processing, will also cause surface softening and deterioration, and promote the early failure of bearings.

3. Fracture failure

The main reason of NTN bearing fracture failure is defect and overload. When the external load exceeds the material strength limit, the part fracture is called overload fracture. The main cause of overload is the sudden failure of the host or improper installation. The defects of bearing parts such as microcracks, shrinkage cavities, bubbles, large pieces of foreign debris, overheated tissue and local burns will also cause fracture at the defects when the impact overload or severe vibration occurs, which is called defect fracture. It should be pointed out that the above defects can be correctly analyzed by instruments in the process of bearing manufacturing, such as re inspection of raw materials, quality control of forging and heat treatment, and control of processing process, and the control must be strengthened in the future. But generally speaking, the most common bearing fracture failure is overload failure.

4. Clearance change failure

In the operation of NTN bearing, due to the influence of external or internal factors, the original fit clearance is changed, the accuracy is reduced, and even "bite" is called clearance change failure. External factors such as excessive interference, improper installation, expansion caused by temperature rise, instantaneous overload, and internal factors such as residual austenite and residual stress in unstable state are the main reasons for the failure of clearance change.

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