HIGH temperature thread sealant.

Has anyone any ideas about a suitable thread sealant to make an M8 thread on a boron-nitride tube gas tight in a stainless steel block (to
roughly 10" of water), whilst the stainless is cooking at at least 400 Centigrade ? I am at a complete loss ! I have tried Zypcoat boron nitride paint, but once the carrier has dispersed, the thing still leaks.
Thanks
Steve
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If I could help it, I wouldn't use a thread sealant at 400C (or a thread seal even at low temperature). Hard solder another fitting to it, like Swagelok?
TorrSeal is pretty tough stuff. I don't know what temperature it's rated for, but it could surely handle 400C for brief periods.
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I used TorrSeal once to repair a auto radiator in a pinch. It worked for a short while enough to get me home, but under a 400C environment it is not going to cut it. The ceramic cements typically just dont get dense enough to form hermetic seals. Youre best best is a low Tm glass or low temperature braze. Wetting of the BN may be a problem with the braze, but there are many brazes you can work with. You might try water glass cements ( I have had good sucess with sodium silicate + aluminum hydroxide cements for special applications)
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I'd assumed he's going to 400C for an hour or so to bake out a vacuum system, and then would run at room temperature.
If it's 400C for sustained use, forget the thread sealants, lose the thread, and replace it with conflat, VCR, or some other all-metal bakeable fitting. Or replace it with a hard solder joint and plan to take a propane torch to it every time the tube goes in or out, but forget threads and sealants.
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Gregory L. Hansen wrote:

Nope, the device runs (intermittently) at upto 1750 C, about 5.5 inches from the stainless steel parts. The stainless bit is a block around 6" long and 1" diamteter, with a water cooling system for about 1/3rd of the length. The temperature of the cooled part is almost ambient.

Still have to join airtight to Boron Nitride,and what will let me do that ? Thanks Steve
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Pretty heavy-duty.

It's easy to come up with possibilities since I won't be the one implementing them. I don't know where you're getting your boron nitride parts but you could think of a glass-to-metal seal giving a steel tube that can be sealed with Swagelok, have a VCR gland welded to it, or just be brazed or hard soldered into place. Add a few $$$ with each buzzword. Does it have to be boron nitride? It would be more convenient if you could solder directly to the ceramic, then you could use solder as the thread locker. And sure you'd need a propane torch to replace it, but what the heck. But I don't know anything about soldering or brazing to ceramics, except that I think they usually vacuum deposit a metal base coat.
High, sustained temperatures make this a hard problem.
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Gregory L. Hansen wrote:

Hi Gregory, Back to my work persona, instead of posting from the field from a lab in Egypt where I was trying to commission the equipment, and needed some quick advice....
I will post a couple of pictures of the rig mounted over the furnace system on the company website for any interested folks.
It has to be boron nitride, Molybdenum would work, but we could't get lengths of tubing for anything like a sensible price, and Mo becomes incredibly fragile once it has been fired at these temperatures. I doubt BN can be metal coated, since its not very hard.
No other ceramic is compatible with the melt. It dissolves pure alumina in a fraction of a second.
We fabricate the BN tubes from BN bar stock in our own workshops. The tubes are nominally 3/8 OD, we bore down 1/8" almost all the way through, and then pierce the ends 0.5 and 1.0 mm, for the bubbles of gas to escape...
Thanks once again.
Steve
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Anything can be metal coated. But possibly at considerable effort and expense for in-house production of consumeable parts. It would be different if you just need one. There are deposition businesses that would coat parts for you, but I don't know what they would charge. And get more expert advice than mine before you decide it would solve the problem. My advice is worth everything you paid for it.

Harsh. But I don't understand why they can be mounted on a stainless steel block. Can the tubes be made longer and the block cooler?

Glass frit is another possibility, again requiring a torch. But high temperature seals are not my strong point. Good luck with that.
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Gregory L. Hansen wrote:

;-) It is nonetheless much appreciated.

The BN stock is only available in relatively short lengths, and I don't fancy the chances of survival of long, fragile pieces of BN out of the furnace !
I think it would help greatly if we had more, well any, radiation shields on the front of the probe. The radiation shielding is a royal P.I.A.
Thanks
Steve
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On Tue, 15 Feb 2005 17:14:21 +0000, Steve Taylor

IMO the thread sealing problem will be very difficult due to differential thermal expansion and low compliance. How about a packing gland with Grafoil packing, possibly with bellville or other spring loading? You might be able to eliminate the threads on your tube entirely and retain the tube with the packing if forces tending to pull the tube out are small.
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Glen Walpert wrote:

Its an interesting idea, I'll try and explore it.
Thanks
Steve
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That makes me think of a compression seal, or test port seal, or whatever you want to call it. A port with a close-fitting o-ring capped by a female nut. A smooth tube is inserted, and the nut tightened down to squish the o-ring against the sides.
I can't think of any o-ring materials that can handle sustained 400C, but cork gaskets are used in engines. I don't know how well cork squishes out. It might be tightened down into a cone-shaped cup, as long as it doesn't wrinkle. Or if you have a squish seal you might just use Viton or Teflon or something and plan to replace the gaskets often.
I don't know the thread you have, NPT or straight thread. But maybe a straight thread with a gasket against a flat-bottomed fitting, e.g. cork or felt. I don't know how tight you can screw down a BN tube.
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snipped-for-privacy@steel.ucs.indiana.edu (Gregory L. Hansen) wrote:

How about a soft copper gasket like that found in an Scanning Auger?
Al
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Gregory L. Hansen wrote:

Apart from my diameter constraints, I'd agree with you, that, and the finding the seal material.

The consistency of this BN is rather like compressed talculm powder. Its depressingly easy to cut.
Steve
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Like dear Uncle Al said, an interesting problem. And not just because of the temperature. I'm not sure what to say, except better you than me!
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Gregory L. Hansen wrote:

Despite us not reaching any breathtaking insights you and Al have both helped me think clearly about the problem, and let me bitch about it with folks who can also feel the pain.
I appreciate all your comments.
Steve
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Gregory L. Hansen wrote:

The trouble is I don't think they realised that what they want is not routine off the shelf engineering. Push the button - poof, out comes an answer, the instruments are for much more fundamental research .
We asked the customer for their origianl comments on the materials of construction, to be told that Alumina was quite ok and there would be no problem. On site of course, they announce that there is no way that we can use the alumina parts.
The rest of my thoughts had better remain private on a public forum.
Steve
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wrote:

Not knowing any better I use Copper Anti-Seize This is available in automotive stores and is dirt cheap. I use it on my spark plugs, manifold and turbocharger parts
It is good to 980C Since it is untended for use on metals I do not know what it will do to ceramics.
http://www.caswellplating.com/permatex/permlubricants.html
There is also Nickel Anti-Seize that is good to 2400F
Regards,
Boris Mohar
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