Spall Propagation Characteristics of As-Manufactured Aerospace Bearing Steels

Abstract

This study was conducted to investigate the spall propagation characteristics of several advanced aerospace bearing steels. Spall propagation experiments were conducted on new bearings at a 2.41 GPa (350 ksi) maximum Hertzian contact stress. Rockwell hardness indentations at 150 kg were used to initiate the spall. Spall initiation tests were conducted at a maximum Hertzian stress of 2.65 GPa (385 Ksi). Both spall initiation and propagation tests were conducted at 128 °C. All of the bearings were made to the same 208-size bearing geometry with a split inner race, silver plated 4340 cage, and silicon nitride rolling elements. Vacuum induction melted-vacuum arc remelted (VIM-VAR) American Iron and Steel Institute (AISI) M50 (with both M50 and Si3N4 rolling elements) and VIM-VAR M50 NiL bearing steels were used as the baseline materials. Pyrowear 675 in two different heat treatments and nitrided versions of M50, M50NiL, and Pyrowear 675 configurations were compared to the baseline materials. The effect of alloy and heat treatment on the spall propagation rate was examined. The propagation rates of the bearings were measured using an oil debris monitor (ODM). The ODM is a self-contained sensor with an inductive coil to continuously monitor metallic debris in the lubricant scavenge system. By counting the number and size of particles flowing through the coil, the spall propagation characteristics of the bearings can be determined in situ. The bearings were also investigated for alteration in microstructure.