Galling in stainless steel plates

[rogerpw]

Can someone advise: we are using 2 machined stainless steel 316 surfaces to transmit loading of a subsea unit (not ideal I know!). These surfaces will rotate relative to each other at a high contact load, albeit at low speed. In assembling the unit the surfaces have become damaged (scored). They have been repaired by locally buffing/ grinding any protruding metal. The concern is that the surfaces are no longer perfectly flat and that galling (metal pick-up) will occur. My approach would be to hone the local damaged areas using an oil stone. An alternative would be to 'scrape away' any proud anomolies (identified by hi-spot blue). My concern with this is a scraper would generate tiny furrows which would allow build-up of hard fines (present where this is used) between the surfaces and consequently promote galling. I'd be grateful for any advice on this!

 

[Tmoose]

http://www.nickelinstitute.org/~/Me...lingCharacteristicsofStainlessSteel_9006_.pdf

Table XII on page 25 has some representative galling pressures for various materials, unlubricated. I do not know if seawater is a significant lubricant.
Dry some recipes of Stainless-on-stainless can be quite prone to galling. Like, they can only handle a few ksi without galling.

Is this a proven successful design, except for the new surface damage?
I'd think the application of some kind of kind-to-the-fish dry lube or grease would help the plates wear-in and buff down significant high spots.

 

[rogerpw]

Thanks Tmoose, the design has been bought in from a reputable third party, however I think it's dodgy. We're stuck with this as to change it would be major. I have to decide whether to just hone off hi-spots or blue first and scrape. I cannot convince myself to use a scraper, if it were white metal yes, but stainless I may make things worse! No space or possibility of coating, this is a large components weighing around 60Te and the gap between the SS plates is only 0.3mm.

 

[shamgarbrett]

If the plates must be flat, I would be reluctant to scrape any surface. Take a stone to the area to knock down high spots and lightly buff the area. The lower the roughness of the surface, the less likely it will be to react. Tmoose's suggestion of grease would also aid in lowering the friction.

 

[iainuts]

I would suggest using a course valve lapping compound between the two plates to bring them back to a smooth surface finish. Depending on how smooth you'd like to get them, proceed to ever finer grades of lapping compound. Lapping like that should produce a smooth and flat surface finish without taking much material off.

You might then consider having one or both of the parts coated using Melonite which is generally used for guns but is an excellent coating to protect stainless steel and prevent galling.

 

[Tuckabag]

I'd suggest PTFE/Nickel plating (NicoPTFE) on each of the plates. We use this coating a lot on S/S lock collars used on S/S shafts to prevent galling if the lock collars happen to slide (when the collar is not tightened to spec).
Works a treat.

 

[boo1]

Could you use HVOF tunstine carbide face one of the plates?

 

[rogerpw]

These surfaces are clad welded stainless steel around 2mm thick. We could flatten anomalies by stoning etc. but to coat them i.e. PTFE/ Nickel would mean major disassembly work and sending typ. 50Te of fabs for coating. A radical solution would be to remove the SS clad (and some of the base material) and bolt on thrust faces (not SS). Even then we may hit problems with clearances/ alignment etc. being so tight (typ. 0.3mm nominal clearance over 1.8m). My gut feel is we remove the high spots by stoning and blue the surfaces to see how much surface we have in contact. The manufacturer is asking for 95%+ but this isn't achievable for surfaces which haven't been lapped in. The issue of galling I think has gone away but has been replaced by that of ware. I think we'll be lucky if we achieve 60% surface contact which will result in 2MPa max. pressure well below the theoretical 7MPa galling threshold for austenitic SS. A mess I know but perhaps we're making too much of an issue given the actual (as opposed to design) thrust loads during operation will be pretty low. Back to the original issue: given that we need to remove about 0.3mm from each surface do we scrape or stone to achieve flat & smooth surface finishes?