Advice on Brazing the Barrel Joint - 7" OD Copper

[joeburgard540]

All,

I recently purchased copper material for a loco boiler (see first pic) and the tapered barrel has the castellated joint already cut. Problem is that the teeth are not that exact fitting (depth is mostly okay), with some gaps approaching 1/16". Obviously, this gap is too large for most brazing alloys. The barrel is tapered from about 7-7/16" OD to 7" OD with a flat top, thickness of 0.157" (4mm).

Reckon there are the following options -

Castellated Joint:

1. Add V-bevels (like welding) to the teeth and 'sifbronze' the joint via braze welding. Problem is that the RBCuZn-C (low fuming bronze) brazing alloy has a zinc content of about 40% and dezincification becomes a serious consideration.

2. Use a alloy like BCuP-5 (Sil-Fos 15) that has a good paste range to bridge large gaps and is flexible. This alloy is widely used in the HVAC industry. Problem is the phosphorus content and the dangers of joint embrittlement due to sulphur gases (although I plan on firing this boiler with propane, I would like the option of using coal or other fuels).

Add a Butt-Strap:

3. Silver solder the joint with a internal butt-strap about 1" wide to cover all the teeth. Heat from the outside to pull the solder from the inside so proper penetration of the solder is visible.

3. a) Use BAg-1 (45% Silver w/ Cadmium - Easy Flo 45)

3. b) Use Silver Flo 24 (24% Silver, 43% Copper, 33% Zinc) to solder the barrel joint via step brazing since this has a higher brazing temperature (1364 - 1470 F) ensuring the next soldering operations with BAg-1 will have less affect.

TIG welding is out since I don't know what alloy the copper is (joint area embrittlement if the copper is not the de-oxidized).

What's your advice?

Thanks,
Joe

[Michel Richard]

My .02

I would definitely go the butt strap route. The challenge will be to attach it effectively. I would use a small copper rivet in the middle of each "tooth" or castellation, plus a few more outside. Make sure the strap overlaps the tube outside the castellated portion by at least twice the thickness of the tube.

That way, you know nothing will move when you heat it up, and joints will remain tight. Super clean is No 1 priority, tight clearances is not far behind.

It may take a bit of time for preparation: installing all the rivets, but that is time well spent. The actual welding should then be fairly straightforward.

Others may have more relevant advice.

Michel Richard
(with a Kozo A3 that could see first steam within a couple of weeks !)

[stenella]

I would add another option - why not an external butt strap ? It should be as strong as an internal one, and you do not run the risk of constricting the water flow around the lowest row of tubes.

Cris

[steamin10]

The entire purpose of using the tooth joint is to provide strength without a dificult lap or strap joint. When the seal is made it is clearly visible. Not so with any lap joint where voids may hide out. The precise fitting of the strap, and the practical matter of expansion while heating is what makes for the problem of voids and burnt flux trapped in the joint. The tooth joint is regarded as superior in those respects. Small gaps are objectionable, but not world ending. Any of the compounds mentioned will do the job. with an open joint like you dispay, I would use the bronze rod even with the high zinc content, as the de-zinc problem is only relevent to the water exposure, and should be quite slow. It would provide the best strength initially given the bridging abilities for the relatively small gaps. The Sil-phos 15 has proven to show cracking with age and is the least desirable of those mentioned, with the 45% silver probably the most prefered. The silver is removed in boilers where the sulfer in the fuel can impinge on the hot joints, over time. But that is not a real concern as the process is very slow.

Personally I would use the Bronze rod, followed a close second by the very expensive 45% Silver. Just my opinion for what it is worth.

[JohnHudak]

I'm wondering if you can just file some more off of the side of the worse fitting "T" and just insert a small piece of copper the same thickness (and curvature) as the rest of the shell, it will all be silver soldered together anyway.. I've seen this done before with mud rings, and there didn't seem to be any problems. Harry, would this be OK?
Also, when my two boilers were done this way, I used large hose clamps to pull everything together. If they don't sell them that big (7+"), you can un-hook two and join them together as one..
John

[JohnHudak]

Here's a pic of one of the boilers, you can see the "T's" and also the hose clamps. Note that the hose clamps were only used to pull everything together so that small holes could be drilled along the line of the joints and small sections of bronze rod (3/16") was placed in the hole to hold everything together... When the clamps were removed, the shell didn't "spring" open.. Silver Soldering was then done to the joint, and the bronze rods were filed down...
John

[joeburgard540]

I reread the section on how to make the castellated joint of Alec Farmers "Model Locomotive Boilermaking" and confirmed an idea I had of planishing the teeth (squishing the metal to expand it) for the final fit, then braze it with silver solder. Will need to file the teeth clean, straight & square first though.

John,

Yes I have five pipe clamps for the barrel to hold it as in your picture. They're the largest I can find locally at Home Depot, 7-1/2" OD max. The barrel is out of round, and one of the methods Kozo shows is to use pipe clamps to hold the seam tight while pushing a wood former sized for the barrel ID through for final shaping. Currently the barrel is still rather oval, so aquired some scrap 4" and 6" pipe for bending from my welding class.

If understand correctly what you did, the small bronze rods placed at the joints were used to tack braze the joint?

Joe

[JohnHudak]

If understand correctly what you did, the small bronze rods placed at the joints were used to tack braze the joint?

Joe

No, the holes are drilled on a perpendicular (to the length) line of the "T's" Here's a pic of what I mean....
Once the boiler is all clamped up, and everything is ready for soldering drill one every few spaces and tap in some short round bronze pieces.. When you pull off the clamps, the boiler wont "spring" apart, and you'll be ready for soldering...
Harry Wade told me about this trick..
BTW, are you Army at Ft Hood?
John

[Fender]

I'm not a copper boiler guy, but I see some silver brazing alloys, like Harris Safety-Silv® 30, are designed to be able to fill wider gaps.

[Andy R]

Joe,

You mentioned use of Safety-silv 45. It contains cadmium.

Cadmium and its compounds are toxic even in small amounts, so you don't want to breathe even a small amount. That's why the precautionary information for brazing filler metals containing cadmium includes "FUMES ARE POISONOUS AND CAN KILL." (ANSI Z-49.1)

Cadmium is a stable element that cannot be broken down or otherwise destroyed. It accumulates in your body. It doesn't go away.

The lungs absorb cadmium more efficiently than the stomach, so no, you aren't eating it, you are ingesting it in an efficient manner, when breathing the fumes.

There are cadmium-free silver brazing alloys available from Harris and others. Use them. (I prefer Safety-silv 56.)

[Bill Shields]

you could always TIG weld it....

[gwrdriver]

What's your advice?

Joe,
Here is how I would approach the job. First I would not push the envelope with regard to bridging gaps. I don't care how much of a bridge a particular solder alloy claims it will make I would draw the line at .005" with a max of .008". With gaps of more than that I would add copper not solder. A butt strap is (obviously) adding copper. But does the situation call for a full length strap? It sounds to me like you only have a few places where the gap is a problem, so why not add a few short transverse (annular) straps which would be little more than the width of a tooth? 4-5 of these along the length of the butt would do two things, they would hold the entire joint securely together while soldering was taking place, and it would cover the offending gaps without creating a condition where penetration was any more than an ordinary concern. This would be easiest if applied externally and everything is accessible from outside. Something else to put in you mental note file, . . . in certain situations, where solder capillary flow has the potential to be a problem, I put very small solder feed holes in or near a joint. For instance when I need solder to run between two plates (such as a flange or a strap) and I suspect there might be a feed problem I will put a feed hole or holes here and there so that I can feed a bit of solder directly in to the lap and be certain. The solder easily pools up in the feed hole so sealing the hole. Something to keep in mind.

I would file-relieve the outside edges of the teeth slightly to allow better solder entry, a shallow chamfer on one side would be sufficient, and those rivets which John Hudak has illustrated are easy to do and add a bit of protection. Make the rivets a tap fit, just enough to hold them in place during soldering.

Harris Safety-silv 45, or any quality brand 45% non-cad bearing silver solder, is the optimum product to use, but definitely NOT brazing rod or any phos-copper spelter. Anyone who checked the current Harris Safety-silv on-line specifications will see that product line no longer contains cadmium, and hasn't for some time. However NOS (old-stock) Safety-silv (or other brands) can indeed contain cadmium and the only way you can know is the cannister label. Under the right conditions and with enough experience someone could probably tell the difference from the way it flowed. I personally use only non-Cad solder because even with good ventilation the Cad-bearing does cause me mild respiratory distress.

I wouldn't be concerned about using a higher or lower melting point silver-solder on this particular joint. It will be one of the joints least affected by subsequent heats and will be well tied down at both ends and the middle so if a little solder does remelt it won't be going anywhere. Although I have no experience with it TIGging the joint would be interesting, if you had the expertise and equipment to do it.

Please feel free to ask more questions.