stainless parts "locking up" - high temp/pressure... any ideas?

[USAeng]

Everything is 304SS. We test the pieces at 1750F for about 3 hours. We have a male piece with 1 1/4-12 class 2 threads and then a nozzle sits on top of the male piece. On top of that is a cap nut that screws down around the nozzle on the 1 1/4 threads and then clamps the nozzle down so that the nozzle forms a mechanical seal to the male piece. What is happening seems to be that the pieces are getting fused together where they make contact with pressure created from torquing the capnut down. There is no problems with the threads... it seems more like it is some kind of binding of the material under pressure...

I do not know how to overcome this. Any suggestions? I cannot use antiseize as the owner does not like the idea because our customers will not use it in the field.

 

[USAeng]

one more thing- once we break the pieces apart with some force or maybe a small amount of heat the threads come apart very easily.

 

[btrueblood]

You are seeing a process known as diffusion bonding at work.

You can try heavily oxidizing the surfaces before assembling (high temp. treatment in air).

You could try spraying the parts with boron nitride spray prior to assembly.

You can just live with the fact that stuff made from 304ss assembled and subjected to 1750F is gonna stick to itself.

 

[mrfailure]

Maybe I'm missing something here, but antiseize in commonly used in such high temperature applications and so specified. Is it really such a problem to educate customers on the reasons for using this?

Aaron Tanzer

http://www.lehightesting.com

 

[USAeng]

I agree about the antiseize... the owner just has it in his mind that our customer will not use it... so we must find a way to to make this work without it

I will look into the boron nitride spray and oxidizing

 

[looslib]

Put the use of antisieze in the assembly procedures.

Are you being hit with warranty claims because the parts are bonding?


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[USAeng]

no... just happening in testing here at the shop... I said to do that with the antiseize but he is against using it so my hands are tied...

 

[EdStainless]

Use a small amount of wash with Boron Nitride. This material is inert and will help prevent bonding. Call it a surface conditioner, not an anti-seize.
Pre-oxidizing the surfaces would help also. Heat first and then wash with BN.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[USAeng]

So is the boron nitride a lasting coating or does it need to be applied every time the pieces are taken apart and reassembled?

I see you can buy cans of boron nitride spray on mcmaster.com can I just buy a can of spray and heat the parts and then spray the chemical on?

Thanks.

 

[JamesCKelly]

304 stainless, 18Cr 9Ni, is not suited for long term service at 1750F. It will get thin and go away, disappear, from oxidation.

You might try 310 (nominal 25Cr 20Ni, actually available stuff is 310S UNS S31008)

It will still gall.

In Ancient Times General Electric's jet engine guys in Cincinnati used white Philips Milk of Magnesia as anti-seize. It is just magnesium hydroxide, entirely harmless to the alloy, and will not break down as will boron nitride at your 1750F temperatures.

However, the Boss or the Customer will laugh at this. To look more sophisticated, use a stop-off available from Wall Colmonoy. I disremember which is best but they have one for such applications. One just paints it on.

Also, regardless of alloy, you cannot maintain a clamping load at 1750F. The metal will relax and whatever clamping load you started with will disappear. Sorry, that's just the way it works.

 

[USAeng]

but I am imagining the boron nitride and magnesium hydroxide to be applications that must be applied each time the pieces are disassembled/reassembled??

 

[mrfailure]

JamesCKelly raises several interesting and valid points.

USAeng, is the 1750 deg.F test applied only for a 3-hour test or is this the service temperature? Also, if this is only for a test, then why do you care if you can unfasten, as the coupling cannot be reused anyway?

Please clarify.

Aaron Tanzer

http://www.lehightesting.com

 

[USAeng]

the pieces are taken apart for cleaning of the nozzle from time to time... then reassembled

the actual temperature that the pieces will see may vary from 1200F - 1600F this is on a daily basis

I will keep trying to push the antiseize unless there is something like silver plating or other more permanent solution I should try... but I think silver plating may not be good for these temps...it melts at around 1750 so at least for our testing it would be no good

 

[EdStainless]

Actually you may not need to reapply the oxides. Once it has seen a few cycles the natural oxide patina should minimize the risk of fusion.
I believe that Saint Gobain has the BN business. the trade name might be Combat, but it has been years.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[mrfailure]

One other question: Do you reuse the nuts? I have the impression that new nuts should be applied whenever you unscrew for cleaning due to the high temperature degradation JamesCKelly mentioned but I defer to others experience.

Aaron Tanzer

http://www.lehightesting.com

 

[btrueblood]

MgO+water was what we used to use on a critical bolted joint in the rocket engines I helped design and build. Particular joint held Moly/Rhenium alloy joint together, bolting was Waspalloy, and the joint saw 1850 to 2100 F in vacuum. MgO powder worked poorly at best, mostly because it wets surfaces poorly, and squeezes out of bolted joint and thread interfaces. We switched to BN spray, and/or powdered BN with water, and had much better results. Boron nitride with water forms a film with lubricating properties similar to graphite, and sticks well to metal surfaces. Yes, it will decompose in air at the OP's 1750F, but not in a reducing environment or vacuum - OP will need to test, likely it will hold up okay in the thin interface, which is where one cares about it.

Stop-off is useful for blocking braze alloy migration, but it too becomes brittle when it dries, and I don't believe it will form a film and stay put in interfaces. Probably worth a test.

Better solution would be to move the nozzle joint to a lower temperature zone?

 

[MikeHalloran]

If you're relying on a natively developed oxide coating for anti-seize, then preemptively replacing the nuts makes no sense.



Mike Halloran
Pembroke Pines, FL, USA

 

[USAeng]

the nut does not get replaced... they will always use the same nut... when it deteriorates they will have to get new parts.

I think the boss is coming to same conclusion that me and the rest of my coworkers came to - that is we are testing at too high of temperatures... I think the diffusion bonding may not be as much of a concern at the temps of 1300-1400F but we will see... I like the idea of the boron nitride spray so I will look into that today


thanks a lot

 

[USAeng]

Just fyi - I did try to get a thermocouple in the area where the parts go... boss said maybe in the future...for now they wanted to go on what the customer's estimate.