Stainles against Stainless threaded materials

[SpaciouS]

Hi,

I'm looking for a good combination of stainless to stainless threaded materials that that will hold up well against wear and galling (Martensitic against Austenitic).

-High RPM's is not a factor.
-Lubrication is not allowed.
-Nitronic 60 has been considered for one element (Austenitic).
-15-5PH has been considered for the other element (Martensitic).

Though I believe Nitronic 60 against 15-5PH (let's say hardened to H900) will work well, I'm looking for advice on whether or not that's a good combination or what other good options I have.


Thanks,
Sean

 

[TVP]

Can you provide some additional details such as other joint materials, environment, loads, etc? Is this for small quantities, meaning that the threads will likely be cut for both the externally and internally threaded members? Carpenter has some data on stress to cause galling in their handbook. I'm not in the office to consult on your stated combination, but will look it up.

 

[SpaciouS]

-It is small quantities.
-Very low load conditions.
-Hex adjuster set screw/bushing set-up.
-M3 X 0.20 "extremely fine threads".

Design is for a kinematic optical fold mirror (small assy).

Very fine adjustments are made to adjusters by hand.

Thanks,
Sean

 

[TVP]

Thanks, that was helpful, but why do they both have to be stainless? Using a brass or bronze alloy for the nut/bushing against a rolled (not cut) stainless external thread would be much more resistant to galling, etc.

 

[KENAT]

For what little it's worth...

Your application sounds vaguely similar to the type of stuff I deal with and galling doesn't seem to come up often on the smaller threads though I can't think of any stainless on stainless that fine a pitch. However for some of our larger structural joints we will use anti galling compound.

That said for something that's routinely adjusted we tend to use a bushing not stainless on stainless so you're probably correct to be concerned.

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[SpaciouS]

We having issues using brass due to wear. Over time and a few adjustments we are getting a "wobbly screw" affect (micron levels), and it’s falling out of spec. It does not have to be stainless against stainless necessarily, however it is desired in our vacuum environment, we can accept certain non-stainless steels.

Thanks,
Sean

 

[KENAT]

May not directly addressed the wear over time issue but have you considered having the screws & bushings as matched pairs? That way you can have a tighter 'wobbly' screw control at the sub assy level.

Although it will still wear over time, it starts off better.

Also, rather than making the bushing harder, is there any scope to make the screw softer?

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[SpaciouS]

We are using a "matched-assy" (which comes in-spec), and you're right, it's only better at first. It seems wearing is the inevitable with the softer material (brass/bronze).

I think we want to look into harder materials at this point.

Thanks,
Sean

 

[tomwalz]

Try HP alloys (High Performance Alloys). Used them for years. Great advice and nice, honest folks.

Just sold me some Nitronic 60.

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[bill1898]

Is any kind of plating acceptable on either of the mating parts?

 

[TVP]

I checked the Cartech recommendations, and your proposal of Nitronic 60 against 15-5 PH is a reasonable one. I recommend that you have the external thread rolled, and the internal thread tapped using a cold forming tap instead of a cutting tap. The smooth surfaces are more resistant to galling.

 

[SpaciouS]

Thanks everyone for your help.

Thanks,
Sean

 

[johnchrc]

One way to mitigate galling is large dissimilar material hardnesses. Keep them as far apart as possible. In our experience, mating austenitic stainlesses (300 series) in a threaded joint are notorious for galling, about the only worse case is 300-series on 17-4. So getting hardness of mating threads as far apart as possible is important.

Shot peening is something that would be helpful in this respect where material similarities or combinations are a worry. The pin thread must by peened.

What type of thread are you using. What size? Keep pitch as large as possible. If premium tapered thread go to vendor.

You have a potentially serious galling problem. If these threads are heavily loaded...

You have to keep joints clean and lubricated. Otherwise pitting or crevice corrosion will become the issue (or cementing of parts along the threads of whatever the dried out mud may be present).

Recommned Nedox-SF2 or CR+ on Box thread.

Good Luck...


- CJ

 

[johnchrc]

Have you looked at Tougmet Spinodal Cu-Ni-Sn Alloy, Tube, Bar and Rod – UNS C72900 for bushing?

It is a high strength copper alloy capable of yield strength
exceeding 100 ksi. Its high strength and low coefficient of friction make it well suited for use as a bushing and bearing material. It also out-performs many copper alloys and
nickel alloys in corrosive environments.

Search by UNSC72900

- CJ