Bearing Failure SEM Image

[NJAus1]

Hi All,

I would like to seek some clarification on observations in an SEM image obtained from a bearing outer raceway that appears to be spalled in one section.

The spherical balls are Iron as confirmed by SEM analysis.

What phenomena would cause the steel in the raceway to become spheres?

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1580944936/tips/20191113-02190027_ubnc29.tiff[/url]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1580944958/tips/20191113-02190026_u74z2w.tiff[/url]

Thanks

 

[IRstuff]

fascinating. Iron, but not steel? Where did the rest of the steel go? Did the SEM detect the other metals that should be part of the ball bearings or the raceway?

Are the balls bearing themselves missing metal?

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[NJAus1]

Apologies, yes the balls are steel (iron and carbon is present). See attached

 

[IRstuff]

Whoa! that's nearly 50% oxygen, so those are actually rust balls? Fe2O3?

Is the steel supposed to be standard ball bearing steel, i.e., SAE 52100, which is supposed to have significant Cr, and some Mg and Si

Not seeing anything standard that has that much carbon:

https://www.astbearings.com/bearing-materials.html

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[swall]

Melting could cause the formation of spheres.The melting would come from electrical arcing. Was this bearing from a motor?

 

[AidanMc]

None of the common bearing steels that I'm aware of would have a composition with that much carbon in them, especially without any other alloying elements. My first guess would've been similar to IRstuff, in that 52100 typically has spherical shaped chromium carbides distributed throughout it but I you aren't showing any chromium there and the oxygen is extremely high.

I've done some work on failed bearings but haven't seen anything that looks like that before. Can you give us the history of the failure? What is the original composition of the raceway and the rolling elements? What environment and service life did it see? Do you know what design the bearing was, i.e. ball bearing, roller bearing, full complement, retainer/cage?

 

[swall]

The carbides in 52100 range from blocky to globular.They are not generally spherical, except for some of the tiny ones. The spheroids shown in the image are not part of the microstructure; they are laying on the surface.

 

[mrfailure]

Those types of globular balls are material that melted and resolidified. As swall indicated, arcing would be the first thing to think about here.

As for measured carbon - many people make the mistake of giving credence to the carbon semi-quant listed in SEM EDS analysis. In fact, EDS cannot provide a measurement of amount of carbon because it is too light. This is defined in the governing standard ASTM E1508. Usually, carbon amounts given in EDS are misleading and totally inaccurate. Your peak may include carbon from the grease used. You should look at the spectrum generated to get a good idea of relative amount of carbon qualitatively.

 

[IRstuff]

The ratios shown might suggest CO2, and possibly, that's from whatever lube there might have been, if any.

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You should submit that into a contest for 'weirdest SEM image'; it would be a contender to the top prize.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

The balls could also be agglomerations of wear debris.
The wear debris in a bearing is nearly 100% rust.
What does the race look like overall? How about a macro shot of it.
What do the balls (or rollers) look like?

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P.E. Metallurgy

 

[NJAus1]

Below is the macro image of the outer raceway, from an AC Generator that is connected to a diesel engine. The bearing is a ball bearing and is lubricated by grease. The balls have no significant damage. This is a low hour failure.

Below is an image of the inner raceway, the damage is consistent all the way around. This could be damage due to the metal contamination in the grease or electrical pitting potentially.

Another image, that shows the presence of Chromium,

I am theorizing that a stray current through the motor shaft has gone through the bearings, has melted/vaporized and cooled the a section of the outer raceway.

The SEM Image "balls" surface appear porous somewhat...

I think the surface may have appeared similar to bearing electro fluting with some pitting that has spalled out with continued use of the bearing.

 

[IRstuff]

One would think that "melted/vaporized and cooled the a section of the outer raceway" would have required the stray current to have clobbered one or more ball bearings as well as part of the inner raceway with that much damage to the outer raceway. Yet, there's essentially only smashed up debris on the inner raceway.

I would think that melting would show smoother appearance than the picture shows. It looks more like a pothole developed in the outer raceway and the broken case hardening pieces got dragged along and gouged out more and more of the raceway

Was the direction of rotation of the shaft up, or down, in the first picture?

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[NJAus1]

I believe it has rotated downward.

Below are some images related to electro fluting, fluting would create a smooth appearance typically, the other image shows a smooth appearance with pitting/spalling