The bearing rig tests performed in this study, demonstrate superior bearing performance of Cronidur 30 steel over conventional bearing steels. In these tests the L10 life of Cronidur 30 steel as calculated by the DIN/ISO 281 method was 80 times the unfactored L10 life under full lubrication conditions. In boundary lubrication conditions, the Cronidur 30 steel demonstrated the L10 life capability typical of EHD lubrication conditions, whereas the other steels showed a drastically reduced lives. In tests with predamaged races and boundary lubrication conditions, Cronidur 30 demonstrated 8 times the calculated L10 life, whereas the conventional steels exhibited further debit in lives as compared to the boundary lubrication testing whithout predamage. The improved performance of Cronidur 30 steel over conventional bearing steels is attributed to its unique compositional formulation and microstructure that results in provision of balanced properties in the alloy – hardness, toughness and corrosion resistance.
Results are reported for a project sponsored by the United States Air Force Wright Laboratories entitled “High Temperature Bearing / Lubricant System Development.” The major emphasis of this project was the evaluation of bearing materials with improved corrosion resistance, high hot hardness, and high fracture toughness, intended to meet the requirements of the Integrated High Performance Turbine Engine Technologies (IHPTET) Phase II engine. The project included material property studies on candidate bearing materials and lubricants which formed the selection basis for subscale and full-scale bearing rig verification tests. The carburizing stainless steel alloy Pyrowear 675 demonstrated significant fatigue life, fracture toughness, and corrosion resistance improvements relative to the M50 NiL baseline bearing material. The new Skylube II (MCS-2482) lubricant provided significant thermal degradation improvements with respect to the Skylube 600 (PWA-524, MIL-L-87100) lubricant. Two 130 mm bore Pyrowear 675 hybrid ball bearings with silicon nitride balls were run successfully for 231 hours with Skylube II lubricant at temperatures consistent with IHPTET II requirements.
Hybrid bearings containing large silicon nitride balls are considered a critical technology for high speed turbine engine bearing applications. High costs of the balls as well as the lack of a reliable life prediction methodology have hindered extensive use of hybrid bearings in aerospace applications. The presence of surface cracks on silicon nitride balls necessitates the development of a fracture mechanics based approach for life prediction. The key element of the fracture mechanics based approach is the identification of a critical flaw size in silicon nitride balls. Finite element analysis was performed to parametrically vary the crack geometry and to determine the worst case crack geometry conditions. Stress intensity factors were computed for the worst case crack under Hertzian contact loading and in the presence of traction stresses. Failure maps were created that provide a prediction of the maximum permissible surface flaw in silicon nitride bearing balls. Single ball rig tests were performed with induced C-cracks to validate the predictions. Results from the single ball rig test were in good agreement with the results of the analysis for spontaneous spallation. The results of the analysis indicate that 100 μm deep cracks should not cause failure under nominal bearing operation conditions.
The corrosion behavior of bearing steels was screened using potentiodynamic scans in seawater. The results of electrochemical testing provided a relative ranking of the bearing steels when tested in aqueous chloride-containing solution. The corrosion behavior of bearing steels in the lubricant environment has been observed to be quite different than in aqueous solution. Both the amount of water contamination in oil and chloride content of the water impact the observed corrosion rates in oil-water mixtures. All steel compositions tested demonstrated localized corrosion damage when exposed to oil with water added; however, inherently less corrosion-resistant alloys had more widespread shallow attack, whereas higher Cr-containing alloys displayed more localized severe attack. The mechanism of corrosion in the two-phase, oil plus aqueous phase system appears to be controlled by an aqueous corrosion process dependent on steel microstructure and emulsified or free water and likely limited by oxygen availability and amount of water (with aggressive ions) in the oil
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