Supplier cleanness in vacuum‑remelted bearing steels: electron microscopy and extreme‑value inclusion analysis

The aerospace industry increasingly demands higher bearing performance across applications such as jet turbine engines and helicopter rotors. When contamination, lubricant loss, and mechanical damage are not dominant, rolling contact fatigue (RCF) governs bearing life. This emphasizes the importance of as-manufactured steel cleanness, and for some applications, ultra-clean steels from a vacuum-remelted process are required. Conventional optical microscopy techniques and metrics for inclusion content offer limited discrimination when evaluating these ultra-clean steels. This study applies electron microscopy combined with semi-automated image and compositional analysis to characterize cleanness in remelted bearing steels. Multiple suppliers were sampled for each of the following steel grades: M50 VIM-VAR, M50NiL VIM-VAR, and 52100 VAR. Extreme value analysis of the inclusion populations effectively distinguished supplier differences and informed supply-chain decisions that helped mitigate recent long lead times for these materials. Torsional fatigue testing is in process to establish a laboratory-scale linkage between measured cleanness and RCF performance.