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There are four main forms of damage or f...

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Edit time : 2020-08-03

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

In case of damage or failure of high-temperature bearing, the machine and equipment will stop running and its function will be damaged. Therefore, it is necessary to investigate and deal with the causes in a short time and take corresponding measures.

The manifestation of damage or failure of high temperature bearing

High temperature bearing damage or failure of many forms, the reason is complex, high temperature bearing damage in addition to the bearing design and manufacturing of internal factors, mostly due to improper use, such as: improper selection, support design is unreasonable, improper installation, poor lubrication, poor sealing and other external factors caused.

Four types of damage or failure of high temperature bearing

It is of great significance to study the formation and cause of damage or failure of high-temperature bearing. On the one hand, it can improve the use method, correctly use high-temperature bearing, and give full play to the performance of high-temperature bearing. On the other hand, it is helpful to develop new products with better performance. This paper not only describes the matters needing attention, installation methods and operation supervision in the use of rolling bearings, but also focuses on the damage forms and causes of high-temperature bearing and the countermeasures that should be taken.

1. Grinding heat of bearing

In the grinding of stainless steel bearing, the contact area between the grinding wheel and the workpiece consumes a lot of energy and generates a lot of grinding heat, resulting in a local instantaneous high temperature in the grinding area. It can be found that the instantaneous temperature in grinding zone within 0.1 ~ 0.001ms can be as high as 1000 ~ 1500℃ by deducing and calculating the theoretical formula of heat transfer with linear motion heat source or measuring the instantaneous temperature under experimental conditions by using infrared method and thermocouple method. Such a transient high temperature is sufficient to produce high-temperature oxidation, amorphous structure, high-temperature tempering, secondary quenching, and even burn cracking at a certain depth of the surface layer.

(1) Surface oxide layer

The surface of the steel under the action of instantaneous high temperature interacts with the oxygen in the air and rises to a very thin (20 ~ 30nm) layer of iron oxide. It is worth noting that there is a corresponding relationship between the oxide thickness and the total thickness of surface grinding metamorphic layer. This indicates that the oxide thickness is directly related to the grinding process and is an important marker of grinding quality.

(2) Amorphous tissue layer

When the surface of the workpiece reaches the melting state due to the instantaneous high temperature in the grinding zone, the molten metal molecular flow is evenly coated on the working surface and cooled by the matrix metal at an extremely fast speed, forming an extremely thin layer of amorphous structure. It has high hardness and toughness, but it is only about 10nm and can be easily removed in precision grinding.

(3) High-temperature tempering layer

The instantaneous high temperature of the grinding zone can heat the surface to a certain depth (10 ~ 100nm) above the tempering temperature of the workpiece. Under the condition that the austenitizing temperature is not reached, with the increase of the heating temperature, the surface layer by layer will produce the retempering or high-temperature tempering tissue transformation corresponding to the heating temperature, and the hardness will also decrease. The higher the heating temperature, the more the hardness drops.

(4) Two hardening layers

When the surface layer of the workpiece is heated to above the austenitizing temperature (Ac1) by the instantaneous high temperature of the grinding zone, the austenitized structure of the layer is then re-hardened into martensite structure in the subsequent cooling process. Where there is a second quenching burn of the workpiece, the second quenching layer must be a very low hardness of the high-temperature tempering layer.

(5) Grinding cracks

The secondary quenching burn will change the surface stress of the workpiece. The secondary quenching zone is in the state of compression, and the materials in the high-temperature return fire zone below it have large tensile stress, which is the place where crack core is likely to occur. The crack propagates easily along the original austenite grain boundary. Severe burns can lead to cracks (often cracking) on the entire grinding surface, resulting in the workpiece being scrapped.

2. Metamorphic layer caused by grinding force on high temperature bearing

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. In particular, the latter two role, the workpiece surface layer formed a strong directional plastic deformation layer and the work hardening layer. These metamorphic strata inevitably affect the change of residual stress in the surface layer.

(1) Cold plastic deformation layer

In the grinding process, each moment of grinding grain is equivalent to a cutting edge. However, in many cases, the cutting edge of the front Angle is negative, in addition to the cutting effect, grinding grain is to make the workpiece surface under the pressure (plough action), so that the workpiece surface leaves a significant plastic deformation layer. The deformation degree of the deformation layer will increase with the increase of grinding wheel blunt degree and grinding feed.

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

The instantaneous temperature formed by grinding heat on the working surface makes the elastic limit of the surface layer of a certain depth of workpiece drop sharply, and even reaches the degree of elastic disappearance. At this time, the working surface layer under the action of grinding force, especially compression force and friction force, caused by free extension, subject to the restriction of the matrix metal, the surface is compressed (more plough), in the surface layer caused by plastic deformation. The high temperature plastic deformation increases with the increase of the surface temperature of the workpiece under the condition that the grinding process remains unchanged.

(3) Work hardening layer

Sometimes microhardness and metallographic method can be used to find that the surface hardness increases due to machining deformation.

In addition to grinding, if the surface decarburization layer caused by casting and heat treatment is not completely removed in the subsequent processing, the residual surface on the workpiece will also cause surface softening and metamorphism, contributing to the early failure of the bearing.

3. Fracture failure

The fracture failure of stainless steel bearing is mainly caused by defect and overload. When the applied load exceeds the material strength limit and causes the part to break, it is called the overload fracture. The main reason of overload is the sudden failure of the main engine or improper installation. The defects of bearing parts, such as microcracks, shrinkage holes, bubbles, large foreign debris, overheating tissues and local burns, can also cause fracture at the defect when impact overload or severe vibration occurs, which is called defect fracture. It should be pointed out that in the manufacturing process of bearing, the presence of the above defects can be correctly analyzed by means of instruments in the reinspection of raw materials, the quality control of forging and heat treatment, and the control of processing process, and control must be strengthened in the future. But generally speaking, the most common bearing fracture failure is overload failure.

4. Clearance change failure

Stainless steel bearings in the work, due to the external or internal factors, the original fit clearance change, precision reduction, and even cause "bite dead" known as clearance change failure. External factors such as excessive interference amount, inadequate installation, expansion amount caused by temperature rise, instantaneous overload, and internal factors such as residual austenite and residual stress in an unstable state are the main reasons for the failure of clearance variation.

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