Using acetylene with silver solder

Currently building a 7 1/4 Terrier Boiler.

I had some issues getting the firebox door , foundation ring and firebox stays soldered due to the large mass of copper involved. I eventually used propane assisted by acetylene to get the heat where I though I needed it.

I am not happy with the visual look of some of the soldering where it appears as if tiny bubbles were on the surface and have left small craters and am concerned it may go or be porous.

What am I doing wrong ?

Ianmac

Hello Ianmac,

Acetylene is far hotter than propane, I think you have overheated the job and the silver solder has bubbled, due to too much heat,

Saying that, I always use oxy acetylene for silver soldering and brazing on model and full size steam engine jobs.

You've just got to be awake and have your mind on what you are doing, There's not much time between the silver solder not being hot enough, and then being too hot.

Richard.

Hi Ianmac,

I agree with Richard, I also use Oxy/Acetylene for those last little bit of additional heat areas, and its really important not to touch the silver solder with the O/A flame.

Normally once the flux has run properly and the heat is well into the job, the ss will flash into the joint without any directed heat at all, but as Richard says, you really do have to keep your wits about you.

How about getting a couple of bits of copper and having a practice?

For your current job, a re clean and acid pickle, reflux and reheat will more than likely help to give a better joint.

Lofty

Many thanks all for the feedback.

Looks like Ive scored an own goal by being slow and /or allowing the Acetylene flame to hit the SS.

I also used Cadmium SS ! Bought a batch at the right price just before it became illegal to sell it.

I'll revisit the affected areas after a clean / acid pickle

again many thanks

Ianmac

I has used acetylene to silversolder the copperboiler without problem. Use the smaller size of welding torch and keep the distance between welding torch and silversolder without bursting the silversolder.

Ianmac,

It seems from your description that you have, or very nearly, finished the boiler. If so, you may be lucky in that the blistered SS joints may not leak and I would be tempted to carry out a low pressure test to check this. I say this because I think Michael Williams` post is probably correct - remaining SS could now only melt at a much higher temperature. You could possibly have difficulty. I would therefore do a low - press. test to see if its OK rather than risk trouble. Let the club boiler tester see it and and accept his opinion as the last word on this. Good luck, hope all turns out OK.

John.

Hi Ian,

A word of comfort - should your joints not be sound, you could make them so by using the same alloy as used first time.

The re-melt temperature of all silver solders is higher than the original due to alloying of the silver solder by the parent materials. With care you should be able to melt fresh alloy without melting the first run. Be aware that you have a complex assembly in which it is very difficult to prevent thermal stresses caused by expansion. Keep the variation in temperature to a minimum. You need more general heating not intense localized heating.

Get the whole joint area upto temperature. Use propane to get the work hot - burners can generate upto 80+kw of heat. Insulate the work to keep heat in. Use a long life flux. Use the oxy-acetylene only at the end to finally lift the joint temperature for silver soldering.

Regretably your experience is not uncommon when oxy-acetylene is used.

Finally - ensure that any future work is done in a well ventilated area.

Regards

keith

Using Oxy-Acetylene can locally overheat small areas where you can have cadmium boil off, this can me very bad for your health, even fatal if you keep it up. I would only use propane or propane-air with this old stock of ss.