This is a brief guide for anyone contemplating silver soldering for the first time.
If you want to convert stainless sankey kegs for homebrew use, unless you are a competent welder, the best diy option is to silver solder the fittings to the keg. It is also the best option when fixing cheap brass fittings to stainless steel as there are no dissimilar metal/corrosion problems. If you have ever done a bit of diy plumbing with copper tube and fittings then you already have the basic knowledge required for silver soldering.
Silver soldering is a half way house in-between soft soldering and brazing. Although silver soldering uses brazing techniques and equipment, it is still soldering not brazing.
Equipment: You will need a special torch for silver soldering, normal propane type gas torches just do not deliver enough heat to solder sheet metal, oxy-acetylene rigs are quite the opposite and are dangerous in the wrong hands. All is not lost though, brazing torches using a special gas (MAPP Gas) are readily available from the better D.I.Y. stores and welding suppliers. Pictured left is a Bernzomatic brazing torch, it uses mapp gas canisters for brazing and silver soldering, some models can also use normal propane gas canisters for soft soldering. This is a nice piece of kit and can picked up usually for less than 50 quid. Other types of torches that use mixtures of propane/butane and oxygen are also available, but I like the Bernzo!
Flux: Using the right kind of flux is very important in silver soldering. Briefly, flux is a substance that allows the solder to flow or "wet" the surfaces of the work, flux also cleans and protects the work surfaces. For silver soldering, the use of "high temperature flux", HT5 is used, this is usually in powder form and is mixed with water and a couple of drops of washing-up liquid (true!) to form a paste. The flux becomes useable when heated to the 500-800oC range.
Solder: There are many types of silver solder alloy but the ones to look out for are alloys with a melting point of around 650-700oC and Cadmium free. (cadmium fumes are very bad for the health) With these kinds of temperatures you can see why ordinary propane torches are no good. The solder I use is 440 alloy available from CUP Alloys Ltd as is the flux and lots more useful information about soldering and brazing.
Techniques: Silver soldering follows virtually the same procedure as normal soft soldering. First make sure the work is scrupulously clean, then apply flux to the area to be soldered, if you don't want solder to spread all over the work surface then using a pencil, just rub around the areas you wish to keep solder free, solder and flux will not flow over graphite. To Heat the work apply heat indirectly to the side of the work or from behind if possible, in other words don't directly heat the joint and the flux. When the flux becomes clear, this is a good sign that the flux is at or near working temperature, gently apply the solder to the work as you would for soft soldering, allowing capillary action to do it's job. Do not flood the joint with solder, it not only looks very shoddy, more importantly it weakens the joint, too much solder is as bad as not enough.
Conclusion: Although I am a proficient soft solderer with many years experience in the trade and as a hobbyist, I am by no means an expert when it comes to silver soldering or brazing. The correct choice of alloy and flux is very important when it comes down to soldering stainless steel as is the application of heat. Stainless steel can become brittle if overheated, not desirable when the vessel is full of boiling wort. If in any doubt check the many resources available of the web.
This is only a basic primer About Silver Soldering, there are lots of good documents available on the web which go further into the process. If you are silver soldering for the first time, I recommend you do some research before diving in.
Another informative Silver Soldering Primer is Here (PDF Reader Required)