Defects are more likely to occur during metal hot processing, such as voids and microcracks caused by overheating during forging, which may become fatigue sources for bearing contact fatigue failure.

1. Magnetic trace condition of the outer ring section of the bearing

1) Appearance inspection of magnetic traces on the outer ring section of the bearing

 2) Microscopic observation of magnetic traces on the outer ring section of the bearing

Take a metallographic sample of the magnetic mark on the outer ring end face of the bearing, slightly grind and polish the sample, and 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 outer ring end face of the bearing. The defect shines under light due to reflection from its bottom or side walls.

2. Common defects on the surface of bearings

During the production process of bearings, there may be pit like defects on the surface of the bearing, which can include mechanical damage, corrosion pits, exposed forged burnt holes, and raw material defects. Among them, mechanical damage is caused by the marks on the surface of the part due to impact or collision, and corrosion pits are caused by the pit like corrosion marks on the surface of the part due to contact with corrosive media. These two types of defects determine from the mechanism of their occurrence that they will not appear inside the part. The raw material defects are metallurgical defects, and their distribution on the final product parts is relatively irregular; Forged over burned holes are thermal processing defects, which are distributed in areas with severe segregation within the structure. These two types of defects are not only distributed on the surface of the parts.

As is well known, in the process of bearing processing, bearing steel only undergoes three higher temperature processing steps (above 800 ℃): hot rolling forming, forging forming, and heat treatment bainitic quenching process. Generally speaking, in the three heating processes, the heating temperature for heat treatment quenching should be lower than the forging temperature and hot rolling forming temperature

Degree. The qualified organization rating of the magnetic trace bearing ring proves that there was no overheating during the heat treatment process, so the formation of holes in the bearing ring should not have occurred during the heat treatment process.

Overheating during forging can lead to grain growth in bearing steel. In severe cases, not only are the surface metal grain boundaries oxidized and cracked, but also the areas with severe segregation of the metal's internal components begin to melt, forming pointed holes. If the forging of bearing steel causes holes in the finished ring structure due to overheating, there are also two situations:

1) Within the specified temperature range of the process, severe segregation of local tissue on the ring leads to local overburning;

2) Excessive forging temperature leads to the growth of the entire ring grain, causing the grain boundaries to melt and the formation of pores within the structure.

In the above two situations, grain growth in the area where the hole is located is inevitable. Therefore, when observing the fracture surface of the forged burnt area, it generally presents shiny grain edges, known as stone shaped fracture surfaces.

Upon observation of the fracture surface of the smashed magnetic trace ring, it was found that the fracture surface was overall in the shape of fine porcelain, indicating that there was no significant growth of metal grains; Through scanning observation, there are hole defects on the fracture surface. This condition differs from the typical fracture state of forging and overburning, and metallographic examination found that most of the holes are distributed on the carbide band, which raises doubts about whether the holes in the ring structure were generated during the forging process.

4. The reason for the magnetic marks on the outer ring section of the bearing this time

After analysis, it was found that the holes in the bearing ring had already appeared in the bar material. According to the analysis of the steel supplier, the high temperature during the rolling of the steel ingot caused the melting of low melting point substances in the segregated parts of the steel structure, resulting in local overburning, which is a fundamental reason for the formation of micro pores. Due to the distribution of small pores at the location of carbide aggregation, it indicates that low melting point substances on the carbide band first melt and produce voids, forming pores. Based on the above analysis, the generation of magnetic traces on the end face of the ring is related to the microscopic pores of the raw material.

5. Common factors causing magnetic trace defects in the outer ring section of bearings

1) One of the common factors causing magnetic marks on the outer ring end face of bearings is the band like aggregation distribution of micro pores and carbides in the raw material.

2) The second common factor causing magnetic marks on the outer ring end face of bearings is the generation of micro holes, which is related to the hot rolling process of bearing steel.