LAUSR

Bearing steels can be exposed to extreme service environments such as high contact pressure, high rotational speed, and/or elevated temperatures. Rolling contact fatigue (RCF) is therefore a key factor affecting the bearings life. Many authors in previous studies have pointed out that nonmetallic inclusions (NMIs), in addition to other factors such as hardness, toughness, and residual stresses, have a significant influence on these fatigue properties. The maximum inclusion size is considered to be the most relevant factor. SiO2–Al2O3 inclusions are regarded less harmful than Al2O3 or Al2O3–CaO inclusions and the sulfur content, resulting in different amounts of sulfides, has also a strong effect on RCF. Microcracks are preferentially initiated at complex oxisulfides rather than plain sulfides, but no simple relationships can be established. Compared with many other steel grades, NMI in bearing steels occur typically in low or very low volume fractions. However, at oxygen contents below 10 ppm, the particle size distribution of oxide inclusions can differ quite significantly at almost same oxygen contents. Recent investigations on super clean bearing steels have shown, that MnS-type inclusions are not that harmful, whereas Al2O3, TiN as well as silicates act as favorable sites for crack nucleation. According to the study by Ebert, the elastic properties, hardness, and brittleness of NMI play a decisive role. The NMI size distribution, orientation as well as the hot forming process can also influence RCF conditions but published works on the effect of hot deformation is still sparse. The inclusion–matrix interface, in correlation with the chemical composition of the NMI, plays also an important role for RCF, thereby hot isostatic pressing (HIP) can increase RCF life by closing cavities between inclusions and metal matrix. As a consequence, the production of high-purity bearing steels is still one of the most demanding challenges in steel metallurgy. Electroslag remelting was developed and is still mainly used to reduce inhomogeneity such as segregations and shrink holes and to improve the cleanliness level, especially regarding large NMI. The cleanliness level and the type of inclusions depend significantly on the remelting slag. The composition and basicity of the slag also have a major effect on silicon losses. However, detailed investigations on the effect of various slag compositions on the formation and composition of NMI during remelting are still limited, but this topic has gained recent interest to produce steels with lowest amounts of oxygen and NMI. Prof. R. S. E. Schneider, M. Molnar School of Engineering University of Applied Sciences Upper Austria – Research and Development Roseggerstraße 15, 4600 Wels, Austria E-mail: [email protected] M. Molnar K1-MET GmbH Stahlstraße 14, 4020 Linz, Austria Dr. G. Klösch, C. Schüller, Dr. J. Fasching voestalpine Stahl Donawitz GmbH Kerpelystrasse 199, 8700 Leoben, Austria