Metallographic Analysis of Premature Bearing Failure
Source: China Bearing Network | Time: 2013-04-12
Bearings are essential components in many mechanical systems, but they can fail prematurely due to various factors. These include improper installation, poor lubrication, vibration, impact, and manufacturing defects. In some cases, bearings may appear intact during installation but still fail due to hidden flaws or incorrect handling. This article explores a case study involving premature failure of 6205 deep groove ball bearings in a diesel engine gearbox. In the case analyzed, four sets of 6205 bearings were installed in a single gearbox, with a total batch of 3,000 units. However, only 1,000 were used, while the remaining 2,000 were not installed due to user concerns. Upon inspection, the inner rings of the damaged bearings showed signs of cracking. The dimensions of the shaft met the design specifications, and the installation was performed by a skilled worker. Hardness tests confirmed that the inner and outer rings were within the acceptable range (61–65 HRC), as per JB1255 standards. To investigate further, physical and chemical tests were conducted on the damaged bearings. The inner rings were subjected to heat treatment at 840–860°C, followed by low-temperature oil tempering at 160°C for 2 hours. Microscopic analysis revealed that the cracks originated from the raceway area, extending inward in a straight, knife-like pattern. The fracture surface was smooth and brittle, indicating a porcelain-like, fine-grained structure. Acid etching experiments showed the presence of grinding burns and cracks along the raceway. The cracks were dendritic, aligned with the grinding direction, and connected to secondary longitudinal cracks. Additionally, black spots and pores were observed, suggesting internal defects. Micro-hardness testing confirmed a hardness of 64–65 HRC, with a quenching and tempering structure consisting of fine martensite and residual austenite. Further analysis identified a "white layer" caused by over-tempering, which significantly reduced the material's strength. This layer, combined with high tensile stress, contributed to crack propagation. The presence of banded carbides and microvoids also weakened the bearing’s structural integrity, making it more susceptible to failure under load. Grinding quality plays a critical role in bearing performance. Poor grinding techniques, such as excessive feed rates or untrimmed wheels, can lead to localized overheating and stress concentration. This results in grinding cracks and burns, which reduce the bearing’s fatigue life and increase the risk of premature failure. The study concludes that the early failure of the 6205 bearings was likely due to a combination of metallurgical defects, heat treatment issues, and grinding-related stresses. Proper manufacturing, careful installation, and regular maintenance are essential to prevent such failures and ensure long-term reliability. For more information on bearings and their applications, visit [China Bearing Network](http://www.chinabearing.net). Previous: Occupational Use of Cylindrical Roller Bearings and Product Features Next: Reasons for Rolling Bearing Heating and Solutions
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