bearing balls and rollers in 100Cr6

[Gerry45]

Most rolling element bearing manufacturers set limits for the maximum section thickness of rings made from 100cr6 material. This is usually around 15~20mm. For section thicknesses above this, they tend to make their rings from a higher hardenability steel, such as 100CrMn6.

Somewhat strangely though, when it comes to the rolling elements themselves, the rules seem far less consistent. Whilst I know of some that insist on 100CrMn6 for larger rolling elements, I know of some bearing manufacturers who will confidently use 100Cr6 for balls of 60~70mm diameter.

I use large brgs and the majority of the balls fitted into them are 100Cr6 material. However, I do occasionally get broken balls and investigation by the supplier usually suggests poor/defective heat treatment (as shown by soft cores) as the most likely cause.

My personal feeling is that with 100Cr6 material, 'soft cores' are probably inevitable in balls of 45mm and above, with the residual stresses resulting from this are probably the root cause of my catastrophic fractures. The solution may be 100CrMn6 material ?

I'd welcome anyones thoughts on whether 100Cr6 can be successfully heat treated for large ball or roller applications, and if so, what sort of surface hardness would I expect to see ?

Gerry.

 

[Goahead]

Hi Gerry,
the maximum diameter for obtaining full martensitic structure at the center of a round bar of 100Cr 6 is about 32 mm. The american equivalent is 52100. The fact that you have only occasional failures in large rolling elements, could be interpreted to signify that the typical heat treatment is adequate. A tough core might be preferred for certain applications. I am not sure that the quoted reports on failures are reliable, because all the balls of those diameters (even if not failed) would have "soft" cores. It is also strange that a supplier finds exceptions with his own heat treatment because it is an acceptance of liability.
If the catastrophic character of those failures warrants a move to a higher hardenability material, bearing manufacturers would probably suggest their preferred solutions.

http://www.welding-advisers.com/

 

[diamondjim]

I am surprised that you do not see excessive
spalling of the raceways with these large
diameter balls. How deep are you able to
harden the raceways? I would think a soft
core would help make the balls more ductile.
How deep are the balls hardened to at least
48 Rc? I assume they are 63 Rc at the surfaces.
We have seen balls from China fail in the
manner that you are speaking. Where are you
purchasing these large diameter balls and what
grade quality? Is the material vacuum degassed?
What cleanliness level are they in respect to
grain size?

 

[Gerry45]

Many thanks for your comments.

Goahead - you say "a tough core might be prefered for some applications" ...I think this is the line that some suppliers take too. But does a tough core not also create a 'notch sensitive' surface layer due to residual tensile stresses ? If so, perhaps in my application where there is a risk of surface damage (from say metal chip debris indenting, corrosion etc), a tough core is not a desirable attribute if this gives the ball an inherent risk of spliting ?

Diamondjim - I did have some hardness profiles for my failed and unfailed balls and although I cant find the exact figures at the moment, I can recall that there were around the figures you quote. I dont know the source I'm afraid, but as regards the cleanliness, they would have been vacuum degased but not to any specific purity level. Occasionally, there is some spalling of the ball surfaces but this may also be exacerbated by surface tensile stresses, rather than unaceptable purity levels. What do you think ?

Gerry.

 

[metman]

It appears there is a question of the actual mode of failure and understandibly some guessing. A thorough metallurgical failure anlysis would seem in order.

For one thing, a microhardness traverse might eliminate a case/core anomalie and a falure initiation site needs to be established. Meanwhile, the other observations are possibly relevant but with a failure-mode, one could tie in the theroy(ies).

eh what?