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The production of high-quality bearing steels requires tight control by the steel producer of a range of factors including composition, segregation, and inclusion content. The latter, in particular, is critical in determining resistance to rolling contact fatigue, in which premature failures have been attributed to crack nucleation at sub-surface inclusions. Over several decades, improvements in rolling contact fatigue performance have been obtained by changes in steelmaking practice, which reduced the number and size of inclusions. To assess rolling contact fatigue performance, several tests were developed; this paper describes experience with one of these, the Unisteel test, and charts the change in fatigue performance arising from improved steelmaking practices. The test could be viewed as a very effective tool for assessing steel cleanness, and for many years proved extremely successful in quantifying improvements in performance. However, as steel cleanness improved, the test became less effective as a method of ranking material. The influence of inclusion content on rolling contact fatigue performance depends on the size, spatial distribution, and chemistry of the inclusions. As steel quality improved, the Unisteel test suffered from two serious deficiencies. The very low numbers of inclusions in modern production material meant that the test sampled insufficient volume to provide a statistically meaningful result, and it provided no information on inclusion chemistry. To characterise inclusion content and its effect on performance, better methods of measuring inclusion contents are required. Several possible options are discussed, including scanning electron microscopy, high-frequency ultrasonic testing, and laser-induced breakdown spectroscopy (LIBS).
rolling contact fatigue testing, inclusions, cleanness assessment, bearing steels, LIBS, ultrasonics
Morris, P. F.
Tata Steel, Swindem Technology Centre, Research, Development &Technology, South Yorkshire,
Tata Steel Speciality, Sheffield,