Silver Solder Brass to Stainless?

NordicDave

NordicDave

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NordicDave
I'm getting conflicting views on fabricating new primer lines for an O-470R. Consensus seems to be 1/8" copper line works OK and was originally used, but current standard is to use Stainless Steel.

I'd prefer to use 1/8" stainless tubing. Firewall side of the line is doubled flared with AN4D couplings... no problem.

The manifold side is terminated with AN800 ball fitting which requires low temp welding. Many are recommending silver solder. Looking at a Lycoming fuel injection line of same type looks like a brazed-on fitting.

QUESTION: What type of low temp welding is required for brass fitting to stainless steel tubing. Brazing or silver solder?

Any knowledgable guidance is much appreciated.
 
https://www.muggyweld.com/video/brazing-stainless-steel/

https://www.wikihow.com/Solder-Stainless-Steel

Brazing (with common brazing rod) will melt the brass fitting. Silver solder is commonly used. There are, however, some "silver" solders that don't contain much silver. No good for this.

Cessna didn't use copper much at all, even a long time ago. Copper work-hardens with vibration and cracks and breaks. I had a copper oil pressure line in a Champ fail on me and blow most of the oil overboard. I know of another airplane that it happened to, as well, but that one lost all the oil and seized.

Cessna's "copper" lines were a steel tubing electroplated with copper for corrosion protection and solder adhesion. Everdur, they called it, IIRC. It didn't work-harden and crack nearly as easily as copper. If I were you, I'd get some from Cessna, and silver solder it. Much better chance of success. Watch that you don't oversolder it, as the surplus will wick down and into the tubing itself, and now you're trying to find a tiny drill bit to get it out.
 
@Dan Thomas Thanks for the help. Everywhere I've gone the chief MX usually groans then offers vague advice of what to try next. Amazing there isn't a more ready solution for primer tube replacement.

I was also able to find the AN800 tube end in both stainless and Brass. Brass was $2.50 and Stainless $32.

Wonder if it makes more sense to solder stainless to stainless or stainless to brass?
 
Is there a replacement manifold available that uses flare fittings?
 
Dana said:
Is there a replacement manifold available that uses flare fittings?
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Flared tubing of that size is often in trouble with the vibration. The flares crack at their roots. The primer lines use that long sleeve to stabilize the tubing, and the ball end on the fitting makes the seal with the primer nozzle, which is designed for that fitting.

upload_2022-5-4_18-9-0.jpeg

upload_2022-5-4_18-8-29.jpeg
 
Disclaimer - not an aircraft mechanic. But familiar with solder a bit. I've done carbide to carbon steel with silver solder. Never tried stainless silver solder. Have done tig stainless and mild steel.

In the old days, they called it "silver solder" or "hard solder". Now they call it "silver brazing" but it's using the same silver alloys, and some newer ones that don't have cadmium. One of the uses is to silver solder carbide teeth to blades. Flow temps are around 1200-1400F, depending on the alloy. You don't want the BCuP alloys, those are silver copper phosphorous alloys that are for brazing copper based products to each other only. And as others have mentioned, you don't want the "soft solder" plumbing solders that have temps below 1000F, as they have little strength and melt sometimes below 500F.

I think it'll work, but you need the right alloy, the right flux, and a torch. Maybe BAg-1 or BAg-3, and Superior 601 or 601B flux. Mapp gas might work for the torch, don't think propane will, acetylene will.

If it were stainless to stainless it would be easy, you could just have someone tig weld it. But you can't tig brass, way too hot, it'll vaporize the zinc out.

Three warnings - if you braze brass and maybe stainless tubing without a purge gas, you'll get oxides inside. Other than cleaning out the black stuff, no idea if there's any harm to that. Second, I've heard of some report of brazing stainless with alloys containing zinc (brass) can maybe result in corrosion of the stainless. Finally, I've seen some reports that the cadmium containing alloys like BAg-1 or Bag-3 are less brittle than the non-cadmium alloys...but zinc may make the stainless more brittle.

some info: https://bssa.org.uk/bssa_articles/brazing-stainless-steels/

So i think it's possible, but I'm not sure I'd do it in an airplane. To me it would depend on the stress on the joint, and the type of joint. Probably if the tubing slid over the fitting, so it didn't have much stress, I'd do it.
 
Albany Tom said:
Oh that looks good. That should last forever. Nice work!
Click to expand...

It's hard to see in the above pic, but at the 2 o'clock position just under the base of the bulb is a dark spot. This was a 1/16" witness hole which demonstrated the braze flowed fully through the union.


Really appreciated your guys help! And of course the IA will inspect the work.

close-up Witness mark.png
 
Albany Tom said:
Disclaimer - not an aircraft mechanic. But familiar with solder a bit. I've done carbide to carbon steel with silver solder. Never tried stainless silver solder. Have done tig stainless and mild steel.

In the old days, they called it "silver solder" or "hard solder". Now they call it "silver brazing" but it's using the same silver alloys, and some newer ones that don't have cadmium. One of the uses is to silver solder carbide teeth to blades. Flow temps are around 1200-1400F, depending on the alloy. You don't want the BCuP alloys, those are silver copper phosphorous alloys that are for brazing copper based products to each other only. And as others have mentioned, you don't want the "soft solder" plumbing solders that have temps below 1000F, as they have little strength and melt sometimes below 500F.

I think it'll work, but you need the right alloy, the right flux, and a torch. Maybe BAg-1 or BAg-3, and Superior 601 or 601B flux. Mapp gas might work for the torch, don't think propane will, acetylene will.

If it were stainless to stainless it would be easy, you could just have someone tig weld it. But you can't tig brass, way too hot, it'll vaporize the zinc out.

Three warnings - if you braze brass and maybe stainless tubing without a purge gas, you'll get oxides inside. Other than cleaning out the black stuff, no idea if there's any harm to that. Second, I've heard of some report of brazing stainless with alloys containing zinc (brass) can maybe result in corrosion of the stainless. Finally, I've seen some reports that the cadmium containing alloys like BAg-1 or Bag-3 are less brittle than the non-cadmium alloys...but zinc may make the stainless more brittle.

some info: https://bssa.org.uk/bssa_articles/brazing-stainless-steels/

So i think it's possible, but I'm not sure I'd do it in an airplane. To me it would depend on the stress on the joint, and the type of joint. Probably if the tubing slid over the fitting, so it didn't have much stress, I'd do it.
Click to expand...
You CAN Tig brass. It doesn't look good and pops like crazy. But I've had to do on some non critical (not aircraft related) fittings. Silicon bronze. But for this application, the silver solder is how I would have done it. Nicely done.
 
Google Solderit. I can't post links yet.

This solders pretty much any metal to any metal. Including stainless steel.
 
Silver solder/brazing is one of my favorite ways of joining metals. However, it won't build up much like brass but it loves to flow in to tiny gaps. The thinner the stronger. I think narrow gaps approach 200,000 PSI in tensile strength!
 
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Yeah, based on the design of that joint, silver solder was the perfect fix, wicking into the space.
 
Snowmass said:
Silver solder/brazing is one of my favorite was of joining metals. However, it won't build up much like brass but it loves to flow in to tiny gaps. The thinner the stronger. I think narrow gaps approach 200,000 PIS in tensile strength!
Click to expand...

If any of you tour a WWII submarine like the USS Pampanito (in SFO), check out the construction and silver soldering of the 3,000 PSI compressed air piping. There's an overhead manifold on the starboard side of the control room that's visible. It's some nice work.
 
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