Damaged Bearing Race 1

[LanaCarrie]

I have a large bearing from a pump that has surface damage on both the inner and outer race but not to the rollers. Macroscopic discoloration of the undamaged areas and smooth "globular" regions in the damaged areas made me first think that the damage may have been caused by a stray welding arc. However, after mounting and etching the sample I found no evidence of melting (cast-like surface structure I would have expected). The material is tempered martensite with a carburized layer. Could this still be due to a stray arc despite the lack of melted material in the micro and the lack of damage to the rollers or is it more likely that this is fatigue spalling?

 

[metman]

Likey to be fatifue spalling possibly aggaravated by heating from stray arc which could cause overtempered martensite. Does your cross section of the discolored race show subsurface cracks? Hertzian (compression or contact)stresses result in subsurface shear stresses which initiate cracks that fatigue and propagate to the surface resulting in spalled out divots.

Overtempered martensite would be less resistant to initiation of cracks from Hertzian stresses.

Jesus is THE life,
Leonard

 

[LanaCarrie]

Thank you for your reply. I suspected subsurface fatigue as I did indeed find subsurface cracking, one of which appeared to have initiated from an inclusion (which would make sense). The thing that threw me off was the branched nature of some of the cracking. Most fatigue cracking I've seen hasn't been branched like that (but I've never done a failure analysis on a bearing before). I have digital micros I could email you if you're interested. Either way, thanks for your help - much appreciated.

 

[metman]

Am interested to see your micro's. lsanders@packeraircraft.com

Jesus is THE life,
Leonard

 

[TVP]

Did you perform a hardness traverse around the cracks? If the hardness decreases around them, compared to the nominal hardness away from the cracks, then overtempering due to a stray arc may be part of the problem.

 

[Carburize]

If it was a stray arc I would expect that you would find at least one of the rollers also showing surface damage. If the damage only affects the races it strongly indicates fatigue initiated pitting.

 

[metman]

TVP I aske Lana to perform a very similar hardness test for the same reason via eamil after observing her micro's. I think you might do better at suggestions than me after looking at them.

Carburize...You make an excellent point. This thought was bothering me but would not quite surface. If there is evidence on few rollers it would tend to corroborate the arcing theory.

Jesus is THE life,
Leonard

 

[LanaCarrie]

I'll try the transverse hardness testing tomorrow. Just to clarify, the discoloration I was talking about was the macroscopic blue and red/orange macro indications on the surface.

Thanks for everyone's help. I only have about a year and a half of experience doing failure analysis so it's great to be able to receive the advice of more experienced people.