Global Metallurgy & Chemistry Laboratory–Plymouth-(GTD&Q)

Report N° AT13A020 Project N° Date 19 February 2013 9 (17)

800X

2300X

Figure 8. The inner ring surface also shows areas of microcorrosion pitting in the wear track. These pits are found chiefly where the oxidized grease is present.

Global Metallurgy & Chemistry Laboratory–Plymouth-(GTD&Q)

Report N° AT13A020 Project N° Date 19 February 2013 10 (17)

Figure 9. Etched cross section of the inner ring shows microstructure decomposition from Hertzian loading at the center of the wear path. Nital etch.

100X

Figure 10. Same as Figure 9, except at higher magnification. Nital etch.

Global Metallurgy & Chemistry Laboratory–Plymouth-(GTD&Q)

Report N° AT13A020 Project N° Date 19 February 2013 11 (17)

200X

1000X

Figure 11. Micrographs of the inner ring cross section also show cracks at the surface and white etching material. This white etch material is a product of microstructure decomposition that forms during Hertzian loading. Nital etch.

Global Metallurgy & Chemistry Laboratory–Plymouth-(GTD&Q)

Report N° AT13A020 Project N° Date 19 February 2013 12 (17)

1000X

1000X

Figure 12. Micrographs of the inner rings showing white etch material (arrows) just under the wear track surface. Nital etch.

Global Metallurgy & Chemistry Laboratory–Plymouth-(GTD&Q)

Report N° AT13A020 Project N° Date 19 February 2013 13 (17)

Figure 13. All balls have dark bands of oxidized grease on the surface.

Global Metallurgy & Chemistry Laboratory–Plymouth-(GTD&Q)

Report N° AT13A020 Project N° Date 19 February 2013 14 (17)

400X

1000X Figure 14. Ball surface showing areas of microcorrosion pitting on the surface. These pits were found chiefly at the bands of oxidized grease.

Global Metallurgy & Chemistry Laboratory–Plymouth-(GTD&Q)

Report N° AT13A020 Project N° Date 19 February 2013 15 (17)

Figure 15. FTIR spectrum of the dark band on the ball surface indicates the presence of oxidized grease.

Global Metallurgy & Chemistry Laboratory–Plymouth-(GTD&Q)

Report N° AT13A020 Project N° Date 19 February 2013 16 (17)

Conclusions

The inner ring spall was surface initiated. This is supported by visual and SEM evidence, which shows cold work, material flow, material wastage, and microcracking on the inner ring surface. Spalling was the result of heavy loading and lubrication that was compromised by heat.

A mirror finish on the wear track - as seen on the inner ring - typically denotes lubrication failure, which can come from overloading, from low viscosity lubricant (either overheated or improperly specified), or lubricant that is contaminated (e.g., water).

The inner ring overload condition is also supported by the presence of microstructure decomposition, which is unusual for a bearing that is expected to last more than ten years in service. This decomposition is best illustrated in Figures 9 and 10. Microstructure decomposition occurs during heavy Hertzian loading, which is a repetitive stress on material. The combination of high cycles and high loading produces this condition. The white etch material presented in Figures 11 and 12 also denote another (and more serious form) of microstructure decomposition, where subsurface cracking is present. This condition is deemed to have not matured enough to be the primary reason for spalling.

Micropitting was found associated with the dark bands found on the raceways (Figure 8) and balls (Figure 14). This pitting denotes corrosive attack of the bearing metal by the grease, which occurs when grease is overheated.

The contact angle of balls on the inner ring is 14°. This may indicate overloading by axial force. The wear surface was even around the circumference for both the inner and outer rings.

The fretting on the bearing mount surfaces noted during post test analysis at the customer site did not provide any significant input to the bearing failure.