Defects are relatively prone to occur during metal hot working processes. For instance, in forging operations, cavities and microcracks generated by forging overheating may readily become fatigue sources leading to contact fatigue failure in bearings.

1. Magnetic trace condition on bearing outer ring cross-section

1) Visual inspection of magnetic trace condition on bearing outer ring cross-section

2) Microscopic observation of magnetic trace condition on bearing outer ring cross-section

Prepare metallographic specimens from the magnetic trace area on the bearing outer ring end face. After light grinding and polishing, examination under a magnifying glass reveals numerous densely distributed fine pitted defects on the end face. These defects exhibit a bright appearance under illumination due to reflection from their bases or side walls.

2. Common surface defects in bearings

During bearing production, pit-like surface defects may arise from several causes: mechanical damage, corrosion pitting, exposed forging burn-through holes, or raw material defects. Mechanical damage stems from impacts or collisions, while corrosion pitting results from exposure to corrosive media. Both mechanisms preclude internal occurrence. Raw material defects, being metallurgical in nature, exhibit relatively random distribution on the finished component. Forging burn-through holes, as thermal processing defects, occur within areas of severe segregation within the microstructure. Neither of these defects is confined solely to the component surface.

It is well established that during bearing manufacturing, bearing steel undergoes only three high-temperature processing stages (above 800°C): hot rolling forming, forging forming, and heat treatment bainite quenching. Generally, among these three heating processes, the heat treatment quenching temperature is lower than both the forging temperature and the hot rolling forming temperature.

The microstructure grading of the bearing race with magnetic traces is qualified, indicating that no overheating occurred during heat treatment. Therefore, the formation of cavities within the bearing race bore should not have originated from the heat treatment process.

Overheating during forging causes grain growth in bearing steel. In severe cases, not only do surface metal grain boundaries oxidise and fracture, but grain boundaries within regions of significant internal composition segregation also begin to melt, forming angular cavities. Should overheating during bearing steel forging cause cavities within the finished ring structure, two scenarios may occur:

1) Localised severe segregation within the ring structure leads to localised overheating within the specified process temperature range;

2) Excessively high forging temperatures causing grain growth throughout the ring, with grain boundaries beginning to melt and voids forming within the structure.

In both scenarios, grain growth in the affected areas is unavoidable. Consequently, fracture surfaces of overheated regions typically exhibit sparkling grain edges, termed a stony fracture.

In contrast, fracture analysis of impact-formed magnetic-trace rings reveals an overall fine porcelain-like fracture surface, indicating no significant grain growth. Scanning reveals pitting defects on the fracture surface. This condition differs from the typical fracture state of forging overheating. Furthermore, metallographic examination indicates that most pits are distributed along carbide zones, casting doubt on the hypothesis that the pitting originated during the forging process.

4. Causes of Magnetic Traces on the Bearing Outer Ring Cross-Section

Analysis indicates that the porosity in the bearing ring originated in the billet. According to the steel supplier's analysis, excessive temperatures during billet rolling caused low-melting-point substances in segregated areas of the steel structure to melt, resulting in localised overheating—the fundamental cause of micro-porosity formation. As these minute pores are distributed at carbide aggregation sites, it indicates that low-melting-point substances within carbide bands melted first, creating voids that formed the pores. Based on the above analysis, the magnetic marks on the bearing ring end face are related to micro-pores in the raw material.

5. Common Factors Contributing to Magnetic Marking Defects on Bearing Outer Ring Cross-Sections

1) One prevalent factor for magnetic marking on bearing outer ring end faces stems from the banded distribution of micro-pores and carbides within the raw material.

2) A second common cause of micro-pores leading to magnetic marking is associated with the hot rolling process of bearing steel.