Critical factors for bearing failures by localized severe plastic deformation in the 100Cr6 and X30CrMoN15-1 steels
Srikakulapu, Kiranbabu; Raabe, Dierk (Thesis advisor); Svendsen, Bob (Thesis advisor)
Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2022
Wind turbine gearbox bearings often fail due to premature rolling contact fatigue (RCF). These failures are associated with sub-surface crack initiation and propagation resulting in extended crack networks. These cracks are called white etching cracks (WECs). WECs are associated with local microstructure alterations known as white etching areas (WEAs), which initially form as butterflies around non-metallic inclusions and later encompass propagating WECs. The extended crack networks lead to ‘spalling’ failures which dramatically reduce the bearing lifetime from about twenty years to less than four years. Although such RCF failures are known for more than a century, the critical factors influencing crack initiation and propagation are still unclear and require systematic investigation. Identifying the predominant factors influencing WEC initiation and propagation helps in designing pathways to prevent premature RCF failures, which is the focus of the current thesis. To do so, two types of bearing steels, namely 100Cr6 high carbon steel and X30CrMoN15-1 high nitrogen steel, are investigated. Heat-treatments and processing methods are further modified as per the aim of each chapter in the thesis. Microstructure characterization is carried out using scanning electron microscopy, transmission electron microscopy and atom probe tomography. RCF loading conditions are employed to understand the role of MnS inclusions in initiating WECs. During RCF loading, the 100Cr6 steel with smaller ‘cigar’ shaped MnS inclusions that are oriented perpendicular to over-rolling direction showed no signs of WECs in both, the presence and absence of electric currents. Whereas the 100Cr6 steel cut from an industrial scale wind turbine gearbox bearing with larger ‘pancake’ shaped MnS inclusions that are oriented parallel to the over-rolling direction resulted in WEC/WEA formation. Therefore, reducing the inclusion size and number density, or even eliminating them, offers a way to prevent premature RCF failures. However, inclusion refining processes such as electroslag remelting add significant costs to the end product (e.g., X30CrMoN15 high nitrogen steel). Thus, other ways of preventing WEC/WEA formation and propagation are called-for.As WEC/WEA formation in bearings occurs through localized severe plastic deformation at cracks below the surface, systematic or even ‘in-situ’ investigations are challenging. Therefore, the high-pressure torsion is employed to simulate a comparable ingress of severe plastic deformation in both steels. To this end, the precipitate decomposition also plays a critical role in WEA formation. Investigations on soft-annealed 100Cr6 steel, with M3C precipitates of different composition, size and morphology showed that the precipitate size distribution and surrounding matrix hardness significantly affect the extent of decomposition upon severe plastic deformation. On the other hand, investigations on through hardened X30CrMoN15-1 bearing steel showed that carbonitrides are far more resistant to decomposition when compared with M3C precipitates in 100Cr6 bearing steel under equivalent loading conditions. The investigations conclude that mechanically hard and thermodynamically stable precipitates (such as carbonitrides in X30CrMoN15-1) with smaller size distribution in the presence of a softer matrix (e.g., bainite instead of martensite) is the most optimal condition for preventing microstructure degradation and precipitate decomposition upon severe plastic deformation.
- Division of Materials Science and Engineering 
- Chair of Materials Physics and Institute for Physical Metallurgy and Materials Physics