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Furnace Silver Soldering

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JA01/04/2019 19:34:34
723 forum posts
38 photos

Has anyone tried furnace silver soldering on a model engineering scale? I would like to know if it is feasible and, if so, what are the pit-falls.

I see it as a good alternative to the conventional propane torch technique. It seems to offer advantages when soldering a complex assembly with step soldering where accurate control of temperature is required. Obviously the equipment cost is higher but pottery kilns are readily available and can easily reach silver soldering temperatures. However the furnace/kiln would need an inert gas purge system.

In industry furnace silver soldering is the preferred technique. A major reason is that with an inert atmosphere very little or no flux is required thus reducing cleaning time and costs.

I have done a very quick look at the internet and have found a very good introduction to furnace brazing by Air Products. It is obviously aimed at industry.

Andrew Tinsley01/04/2019 19:44:52
870 forum posts

It works very well indeed. I was lucky enough to use a vacuum furnace with a very large hot zone that could be stabilised to a few degrees Centigrade. It also had the ability to quench rapidly with Argon.

The only drawback was that the furnace cost around £15,000 around 25 years ago!

John Ockleshaw 102/04/2019 07:19:34
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41 forum posts
7 photos

Hello JA, I agree with Andrew, furnace silver soldering works well at home. In my case I put a heat resistant stainless steel muuffle in the kiln, this has a sand seal to retain the gas which has a continuous very small flow.

If you use a mixture of 10 % hydrogen in nitrogen the hydrogen replaces any flux and gives a superb job. There is no fear of an explosion with this mixture.

This same set up allows you to copper braze at 1080 degrees centigrade with 2 different grades of silver solder as well for sub assemblies.

My kiln is built inside part a part of a 44 gallon drum ,the muffle is 9 inches diameter by 9 inches high .

If you let me have your email address, as a PM, I could send you some photos of the setup.

Regards, John

CuP Alloys 102/04/2019 11:30:01
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186 forum posts

The biggest pitfall will be money.

If you have vast numbers of self jigging components then consider a furnace. If not, then do what the majority of industry does and use gas torches, induction heating, resistance heating (direct or indirect).

I offer you the same advice as I did an F1 racing company who had a similar thought process, but they ignored it because they simply said "money is no object if we get the right result". Result not known!

There are two basic types of furnace - vacuum and atmosphere. But they have to achieve the same result ie that of producing oxide free surfaces into which a filler metal will flow by capillary action into a joint gap

The vacuum furnace involves creating conditions of temperature and pressure where the surface oxides break down and the the oxygen is pumped away. The pumps will also remove any metal vapour produced when the alloy melts. Expect a coating of zinc everywhere!

An atmosphere furnace uses an atmosphere that removes oxides to produce the same effect that a flux does.

Common atmospheres are natural gas, forming gas (5% hydrogen nitrogen) and cracked ammonia. The choice depends on how dry the atmosphere has to be for the oxides to be removed. The dew point of the atmosphere is a measure of the atmosphere dryness. For a silver solder to work and create a joint at about 700 deg C, the dew point has to be about minus 50 deg C! If you cannot achieve that then braze at a higher temperature, in which case why use a silver solder when copper or brass will do.

You aren't brazing copper or brass are you?

Avoid the risk of liquation caused by slow heating, ie the alloy splits as it enters the semi-molten stage. Use a filler metal with a narrow melting range- ideally less than 25 deg C.

Another problem is maintaining the integrity of the atmosphere.

Now I suggest that you talk to Kepston, who, I believe are market leaders in this technology. **LINK**

I have no connection with them other than appreciating they are a sound, reliable company and who have been around in this industry for over 50 years.

I believe that in the event of your not having an F1 budget, they can offer a sub-contract brazing facility.

Wow!

What a project!

Regards

Keith

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