Published: 07 November 2014
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Cite this document
Much effort has been spent developing microscope non-metallic inclusion rating systems and refining them over the years. The initial Jernkontoret rating standard was further developed and refined, and today, the ASTM A295/A295M-09 standard and its related ASTM E45-13 rating is the vastly dominating world-wide standard used for evaluating the cleanliness of bearing steel casts. The ISO 4967 standard, also frequently used, is similar to ASTM E45 in most aspects. Attempts to relate standard inclusion ratings to bearing steel fatigue generally have shown weak correlation. To compare inclusions actually initiating fatigue and microscope ratings, three sets of data have been compiled for a high cleanliness steel variant where the vastly dominating inclusion type had been manipulated into D-type oxides. Samples from bearings that had seen service in field applications, and laboratory tested bearing rings were examined. All inclusions that had initiated micro-crack associated butterfly formations were recorded as regards size and inclusion type, and their size distributions were compared to the corresponding standard inclusion size range limits. In addition, rotating beam samples from the same steel variant were used to evaluate initiating inclusion size distributions. The results indicate that the inclusions that actually initiate fatigue damage are significantly larger than the inclusions that are detected and recorded in microscopic inclusion rating standards. Only a minor share of the actually initiating inclusions is in a size range corresponding to the limits in the ASTM A295 rating standard requirements for D-type inclusions. Ultimately, D-type inclusions are not more harmful to bearing steel fatigue than other hard oxide types at the same total oxygen/oxide content if their size distribution is well controlled and monitored.
non-metallic inclusions, fatigue, rolling contact fatigue, bearing steel
Lund, Thore B.
Project Manager, AB SKF, Gothenburg,
Materials Engineer, AB SKF, Gothenburg,