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Common factors causing magnetic mark defects in bearing outer ring cross-section

time:2023-09-13   pv:0

Defects are more likely to occur during metal thermal processing. For example, in forging processing, cavities and microcracks caused by overheating of forging are likely to become fatigue sources of bearing contact fatigue failure.

1. Magnetic marks on the bearing outer ring cross-section

1) Visually inspect the magnetic marks on the cross section of the outer ring of the bearing

The magnetic marks on the outer ring of a bearing on the railway are located on the end face of the ferrule, and are densely distributed along the circumferential direction in the form of short lines or points, with the distribution area being about 1/3 of the circumference.

2) Microscopic observation of magnetic marks on the bearing outer ring cross-section

Prepare a metallographic sample at the magnetic marks on the end face of the bearing outer ring. After lightly polishing the sample, observe it under a magnifying glass. It can be seen that there are a large number of small pit-like defects densely distributed on the end face of the bearing outer ring. The defect shines under light due to reflection from its bottom or side walls. The pit-like defects vary in size, but it can be roughly seen that their shape distribution direction is along the circumferential direction.

Observed under a microscope, it can be seen that small defects are holes, which are strip-shaped. No inclusions are found inside. The length is about 0.3mm and has a certain depth.

2. Some common defects on bearing surfaces

During the bearing production process, pit-like defects may appear on the bearing surface. There are several possibilities: mechanical damage, corrosion pits, outcrops of forging burnt holes, and raw material defects, etc. Among them, mechanical damage is caused by the impact or bump on the surface of the part, and corrosion pits are caused by the contact of the surface of the part with the corrosive medium, resulting in pit-like corrosion marks on the surface. These two defects determine from the generation mechanism that they will not appear on the part. internal. Raw material defects are metallurgical defects, and their distribution on the final finished parts is relatively irregular; forging over-burned holes are thermal processing defects, which are distributed in areas with severe segregation within the organization. These two defects will not be distributed only on the surface of the part.

3. Bearing processing process

As we all know, during bearing processing, bearing steel only undergoes three higher temperature (above 800°C) processing processes: hot rolling forming, forging forming and heat treatment bainite quenching process. Generally speaking, among the three heating processes, the heat treatment and quenching heating temperature is lower than the forging temperature and hot rolling forming temperature. The structural rating of the magnetic bearing ring is qualified, which proves that there is no overheating during the heat treatment process, so the holes in the bearing ring should not be generated during the heat treatment process.

Too little forging will cause the bearing steel grains to grow. In severe cases, not only the surface metal grain boundaries will be oxidized and cracked, but also in areas where the internal components of the metal are segregated more seriously, the grain boundaries will also begin to melt, forming sharp angular holes. If the bearing steel is forged too little and causes holes in the structure of the finished ring, there are two situations:

1) Within the temperature range specified by the process, the local structure on the ferrule is severely segregated, resulting in local overburning;

2) The forging temperature is too high, causing the grains of the entire ferrule to grow, the grain boundaries to melt, and holes to form in the structure.

In the above two cases, it is inevitable for the grains to grow in the area where the holes are located. Therefore, when observing the fractures in the over-forged areas, they generally show sparkling grain edges, which are called stone-like fractures.

Observation of the fracture of the smashed magnetic ferrule shows that the fracture is in the shape of fine porcelain, indicating that the metal grains have not grown significantly; through scanning observation, there are holes and defects on the fracture surface. This situation is different from the typical fracture state of over-forging, and metallographic examination found that most of the holes are distributed on the carbide belt, which makes it doubtful that the holes in the ferrule structure are generated during the forging process.

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