Effect of the Al 2 O 3 Content in the Slag on the Chemical Reactions and Nonmetallic Inclusions during Electroslag Remel
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Effect of the Al2O3 Content in the Slag on the Chemical Reactions and Nonmetallic Inclusions during Electroslag Remelting REINHOLD S.E. SCHNEIDER, MANUEL MOLNAR, GERALD KLO¨SCH, and CHRISTOPHER SCHU¨LLER The service life of roller bearings is extremely sensitive to large and hard nonmetallic inclusions (NMIs), requesting the highest standards in their production. To determine the metallurgical possibilities, the effect of Al2O3 contents between 0 and 33 pct in the remelting slag was investigated by remelting a roller bearing steel in an experimental electroslag remelting (ESR) plant. Thereby, changes in the chemical composition of the materials (electrode vs ingots) and in the remelting slag during remelting, as well as the amount and composition of the NMIs prior to and after remelting, were investigated. Changes in the chemical composition can largely be explained by equilibrium reactions between the slag and the metal, thereby low Al contents in the remelted materials could only be achieved with the lowest Al2O3 contents in the slag. Furthermore, higher Al2O3 contents in the slag also lead to higher oxygen and sulfur contents in the steel as well as higher amounts of NMIs after remelting. The composition of the NMIs changed from alumina type for high Al2O3 contents to spinel type and other mixed MgO-SiO2 oxides for low Al2O3 contents. The results indicate solutions for the production of bearing steels with the lowest amounts of undesired large and hard NMIs. https://doi.org/10.1007/s11663-020-01896-8 The Author(s) 2020
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INTRODUCTION
BEARING steels, which are exposed to extreme service environments, such as high contact pressure, high rotational speed, and elevated temperatures, suffer rolling contact fatigue (RCF). This RCF is a key factor affecting bearing life. Many authors[1–12] describe that nonmetallic inclusions (NMI) besides other factors, e.g., hardness, brittleness, and possible residual stresses, have significant influence on fatigue properties. NMIs occur typically in low or extremely low volume fractions.[1,2,9] Thereby, at low oxygen contents of E-modulus of the matrix) supports the matrix because it attracts the flow of force. This leads to a higher stress level not only inside the NMI but also inside the matrix zone, where the force flows into the NMI. This ‘‘high stressed’’ area is called the pole zone. At the pertinent equator, the matrix is relieved form load. Conversely, with a ‘‘soft’’ inclusion (E-Modulus NMI < E-Modulus of the matrix), some of the forces pass by the NMI, and the matrix in terms of load is relieved at the pole zone, with more load being found at the equator area. Therefore, an important objective in terms of material optimization is to reduce the number and size of NMIs, which act as ‘‘internal notches,’’ or to gain a positive effect on their distribution and to minimize their risk potential by changing their chemical composition.[4] Furthermore, as described in Reference 3, the crack propagation under RCF follows the prior austenite grain boundaries, which can also be affecte
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