scope:

The Metallurgical Specifications for Steel Gearing information sheet (AGMA 923-B05) [1] details non-metallic inclusion limits to meet AGMA grade expectations for a range of gear metallurgical processing methods (case carburized, through hardened, etc.). These limits allow gear designers and producers to select material suppliers that will meet the minimum expectations for material fatigue performance, but do not provide the data needed by designers to meet ever-increasing demands for high power density gearing applications faced today. Modern air melted and vacuum refined steel making processes, when optimized to provide a minimum frequency of harmful non-metallic inclusions, can now produce steel cleanness that is on par with Vacuum Arc Re-melted (VAR) steels at 1/3^rd to 1/5^th the cost. Similarly, modern Scanning Electron Microscopy / Image Analysis (SEM-IA) technologies can be employed to characterize the non-metallic inclusion population in a way that allows gear designers to consider inclusions when assessing the risk of failure using linear elastic fracture mechanics. This paper describes the methods used to characterize premium quality clean steels through the use of statistics of extreme values (SEV) and quantitative stereology. These data can be used to perform gear design relevant engineering analysis of the potential for a gear failure due to bending fatigue in the root or flank or rolling/sliding contact fatigue of the gear tooth face. Literature evaluation, modeling results, and experimental results are presented in order to validate the approach. Potential improvement using this approach might include selecting a leaner alloy, switching from case carburized to through hardened or light weighting through increased power density.