Arecent participant on the www.weldingweb.com website described trying to soldering a 7-in. diameter copper steam boiler for a model train. Over the joint, he riveted a 1.5-in. wide strap running the length of the boiler for extra strength. He then placed 3 propane burners under the boiler (for heating the strap), with an oxy-acetylene torch for extra heat. Solder then was fed from the inside of the boiler, so the burners were not directly heating the solder, until the solder flowed out from the sides of the strap. His concern was over solder pitting , possibly from over-heating.

Over the course of several replies, another participant provided this thoughtful answer:

Porosity or bubbling of the silver solder indicates that you have boiled off some of the components of the alloy by overheating – not a good thing....

How did you select your alloy? "Silver solder" covers a very wide range of alloys with a wide range of melting temperatures and gap-filling capabilities. You have three major considerations in your alloy selection: melting temperature, gap-filling, and capillary penetration. The latter two are not necessarily optimized in the same alloy. Advice from a pro or careful study of the numerous handbooks out there will be worthwhile.

I would recommend not using an oxy-acetylene torch. It is much too hot for the task, and a moment's inattention, too long a dwell at any location, or less than perfect torch control, will easily boil your alloy.

Using a torch that has plenty of heat output, but a temperature closer to that required to flow the alloy, like a MAPP torch, will help you avoid overheating the alloy or the copper.

I'd make sure the three propane burners are set up in such a manner that they cannot overheat the joint by themselves, no matter how long they heat it, then use a MAPP torch to get the joint up to flowing temperature locally and lead the flowing alloy along the length of the joint. It sounds like you have appropriate preheating with your present propane burner setup.

Your rivets may be weakening, rather than strengthening your joint, by both causing stress concentrations around the rivet holes, and by causing a non-uniform gap along the length of the joint. Each alloy has a specific range of gap thicknesses, for which it is optimum — by squeezing the joint intimately together at the rivets and allowing it to possibly bulge apart between rivets, you may not be able to get optimum capillary penetration with any single alloy. You may also find that no alloy can penetrate into the tightly squeezed regions A properly silver soldered joint with adequate overlap has tremendous strength and I don't think your rivets are helping, except to hold things in place before soldering. Proper clamping and no rivets — not easy to do — would, I think, result in a stronger joint after proper soldering.

Your rivets are along the outer edge of the strap. This results in having a fairly narrow soldered region between the rivets and the edge of the strap. I don't believe that the rivets provide anywhere near the strength of a properly silver soldered lap joint, so I would want to maximize the width of the undisturbed seam in from the edge of the strap by omitting the rivets.

I'd also recommend getting advice from a pressure testing firm about test pressure. A steam boiler is much more dangerous than an air tank at the same pressure due to the energy it contains. I was advised by one testing firm to test a second-hand air tank at three times operating pressure.

Ideally, you would also test at operating temperature – probably a practical impossibility, since you would lose all the safety aspects of hydrostatic testing by heating the water up to your operating temperature. If you look closely at some ASME pressure vessels, they give the operating pressure at a specific temperature, and it is lower at elevated temperature than at room temperature.

I was a little surprised at your statement (if I understood it correctly) that boilers have been made with silver-soldered butt joints that pass 600 psi hydro testing.

A quick calculation of hoop stress at 600 psi for a 7-in. boiler with 3 mm wall shows that the tensile stress on the joint would be 17,800 psi. Superficially, for silver brazing alloys offering 35,000 psi joint strength, that sounds OK, depending upon the quality of the soldered joint and the appropriate safety factor.

I do not know the appropriate safety factor and I do not know if the 35,000 psi cited is for shear stress or tensile stress. I, personally, would not feel good about a silver-soldered butt joint on a homemade boiler.

Are you sure they weren't talking about a scarf joint?

In searching for the strength of silver brazed joints, I ran across an excellent source of information at www.cupalloys.com/content.php?categoryId=100