Boiler welding, i.e. more TIG...

The CuNi tube I had was too large for my boiler, so I removed a 120 mm wide strip, and rolled the tube to the correct curvature in the workshop at the workplace of a friend (thanks, Kusti... ;-)

CuNi needs "root protection", either by filling the entire enclosure with argon gas, or, as shown here, with a length of "backing tape", which has a glass-fiber strip that will prevent oxidation and melt-through. Without protection, the weld would become very porous.

After removing the strip I still had to grind a few places where the penetration wasn't perfect, and welded them from the reverse side - this shows the final result.

Here's the outside of the seam, after it has been wirebrushed clean.

This, again, is the inside of the logitudinal seam - hard to reach, which explains the wavy appearance... Just to be sure (since I'm a pretty inexperienced welder), I made the welds substantially thicker than the base material, in order to have a strong joint. However, it is important that penetration is sufficient (which can be seen from the fact that the edge of the filler metal bead "flows" into the base metal, not leaving a sharp edge.

Now, I've come this far. Next, the firebox and all its stays. Somehow, it feels a bit Deja vu... :-(

UPDATE 2006-08-16

Since the longitudinal seam of a boiler is subject to the most stress of all the seams, I decided to wear both belt and suspenders, and X-rayed the seam! Thanks to my friend Kusti (of backyard foundry fame), who is a software designer at a company making medical X-ray equipment, we shot a series of small digital X-rays of the entire seam, thus:

The machine is a Planmeca "Dixi" intra-oral dental X-ray unit. We taped the sensor (which is normally in the mouth of a patient) to a wooden slat, and had the X-ray source outside the boiler. By shooting several small images (the sensor is only about 25x40 mm, 1x1.5") and moving the boiler 30 mm between shots, I could assemble them all to a long strip (left), showing the entire seam.

Here is the setup we used. For those technically inclined, or anyone wishing to do something similar, here are the technical data of the "shoot":

- X-ray tube power: 70 kV, 8 mA
- Beam shield equiv. to 2 mm Al
- Exposure time: 1.6 sec.
- Distance from focus to sensor approx 300 mm

Of course, we stayed the proscribed distance away from the machine during exposures...

As you can see on the assembled strip at left (boiler front is at top), I was rather satisfied with the result. In fact, it looks like the seam fulfills ISO 6520 requirements for demanding welds, with only a few errors of type 2011, i.e. single porosities (at arrows). The fact that the wavy, cosmetically ugly appearance of the seam isn't very professional, doesn't bother me at all - after all, I'm only an amateur welder with a new TIG toy... ;-)

Update 2006-09-06:

Now, I finally got around to welding in the tubes and staybolts. I made a test, experimenting with different protrusion of the tubes from the a piece of scrap tube plate (holes were countersunk 1 to 1.5 mm into the 3 mm CuNi plate) before welding, and here are the results (marked numbers are in millimeters):

As can be seen, 0.5 mm is far too little, the gap didn't close, while the welds are looking good with a protrusion of 1 to 3 mm. To be safe, I chose 2.5 mm for the final welds. Splitting up a weld showed good penetration into the countersink:

I also checked the integrity of the weld, by pinching the tube with pliers:

Should be quite OK! This is how the firebox tubeplate looks after all the tubes were welded in, and the plate into the firebox:

Then the staybolts, quite a few of them... Here's a shot "before":

... and one "after":

As you can see, what I have basically done, is to melt the protruding part of the bolt into a "rivet head" that also hermetically seals to the boiler. I made a test of this also, trying to get the bolt out of the plate by brute force. The bolt broke before the weld...

This is how the boiler looks on the frame, at the moment:

The backhead and the foundation ring are the only missing parts, can't be made before all the innards in the boiler are in place...

Update 2006-11-19, Hydrotest:

My method of testing may seem a bit unorthodox, but here's how I did it:

First, I filled the boiler to the brim with water, shaking up all bubbles. Steam dome top back on. I let in a few bubbles of compressed air through a ball valve - that was all that was needed to bring the pressure up to 8 bars, 120 psi, a cubic inch of air, or so. No leaks anywhere. (A soap water test had confirmed that earlier.)

Then, I took a hand-help propane torch, and gently warmed the inside of the firebox (if it had been a green winter here, not the white, kind, I'd just have left the boiler in the sun for a while...)

Keeping my hand on the boiler, it slowly got lukewarm (no more than 40°C, 100°F or so), and the pressure rose to 20 bars, 2 MPa, or 280 psi, almost three times the working pressure, which will be 7 bars, 100 psi:

Then I left the boiler alone to cool. An hour and a half later, it was still slightly warm to the touch, and now had a pressure of 7 bars, 100 psi, due to the cooling.

So, I'd say it passed hydro with plenty to spare...

If you take care not to heat the boiler to more than 100°F or so, and heat it evenly, not in one spot only (which may cause localized boiling), I consider this procedure a safe one. In fact, when finishing, I loosened the steam dome top bolts and got only a little squirt of water, no more - so there was very little energy in the boiler even with its internal pressure at 100 psi.

IMPORTANT: You MUST fill the boiler TO THE BRIM with COLD water for this method to be safe. More than a few cubic inches of air in the boiler may make this test risky, because more heat is then needed to get the pressure up - air is compressible, water is not.