Rolling Contact Fatigue Life and Spall Propagation of AISI M50, M50NiL, and AISI 52100, Part I: Experimental Results

Abstract

This article is the first part of a three-part series that investigates the rolling contact fatigue (RCF) initiation and spall propagation characteristics of three bearing materials, namely, AISI 52100, VIM-VAR AISI M50, and VIM-VAR M50NiL steels. Although there is substantial prior work published on the rolling contact fatigue initiation of these materials, little has been published on their spall propagation characteristics after spall initiation. It is recognized that rapid spall growth can lead to catastrophic bearing failure. Hence, understanding the spall growth phase and factors that may cause accelerated growth rates is key to achieving a reliable and robust bearing design. The end goal is to identify control parameters for optimizing bearing materials for improved spall growth resistance. This first part study features the experimental results from 208-size (40 mm bore) angular-contact ball bearings endurance life tested at maximum Hertzian contact stress levels of 3.10 GPa and bearing outer race temperatures up to 131°C. Spall propagation experiments were conducted on new and life tested bearings at 2.10 and 2.41 GPa maximum contact stress. Spall propagation experiments show that all materials exhibit a rapid or critical spall growth rate after undergoing an initial low-rate spall growth period. The time-to-critical growth rate is dependent on contact stress and was swiftest in AISI 52100 steel. To better understand the underlying physics, the driving factors, and failure mechanisms, the state of stress is modeled using finite element analysis in Part II and an in-depth microstructural analysis of selected bearings is presented in Part III.

KEY WORDS:

• Rolling Element Bearings
• Gas Turbine Engines
• Rolling Contact Fatigue
• Fatigue Crack Propagation
• Hybrid Ceramic Bearings
• Oil Debris Monitoring

ACKNOWLEDGMENTS

The authors wish to express their gratitude to Dr. Elizabeth Cooke and John Imundo of the Timken Company for providing the M50NiL bearings and microstructural and case profile results. Thanks also to Hitesh Trivedi of UES, Inc., for conducting the fatigue life rig tests, data acquisition, and photomicrographs and to Chris Klenke of AFRL for the SEM micrographs. This work was funded by the Propulsion Directorate, Air Force Research Laboratory.

Review led by Mike Kotzalasd

Notes

*Aero Material Specification.

*From Hamrock and Dowson ( 41 ).