It happens every time a US member asks about silver soldering.
Confusion arises because of a name.
We have no such problems in the UK.
In the engineering circles, we have three (or four if you break brazing down into two) methods of soldering.
Soft soldering, this is used mainly by the plumbing and electrical trades, and is usually a lead based (or more modern, I think, antimony based) product with a low temperature melt and is used with a free flowing either paste or liquid flux (or in the case of electricians, it is in the solder itself in the form of a core). This is only used in engineering where a low temp, non load bearing joint is required. It is of no use for making up cranks or stressed parts.
Silver soldering (this should not be confused with jewellers silver soldering, that has a lot more silver in it) is classed as hard soldering, and go into any engineering supplies, ask for silver solder and you will invariably be given a stick of easiflo, about 3 ft long and 1/16" in diameter, it has a slight silvery/golden colour to it. If you buy it from a model engineering suppliers, then you will usually be asked for temperature range and diameter. The temp range is so that you can start a fabrication off with a higher range, than as you reheat, you use lower temp silver solder so that you don't melt the previously made joints. But for most applications, easiflo (or its equivalent) will do just fine. Most of my work is done with 0.5mm thick easiflo wire, and only use the larger 1.5mm for large assemblies (I used to use that when I produced boilers).
As for fluxes. It seems that the US market is devoid of any flux standards, and I have even seen people being told to use Borax. That was OK maybe 60 or 70 years ago, but we have come out of the dark ages now. The generally accepted flux is again Easyflo flux, and that will cope with most materials, but not the higher ranges of heat required. I always swore by Tenacity #4a flux, but have now found an updated one called Tenacity #5. This is designed for silver soldering stainless steel, and will take prolonged overheating before the flux becomes overwhelmed and gives up the ghost. I have found this perfect for model engineering use, and will allow soldering of all the normal metals and will allow over abuse by the blowlamp.
The third method is brazing, which can be divided into two. Brazing and bronze welding. Both use the same rods and flux (1/8" diameter brass rods and Sifbronze flux), The difference between the two methods is that with brazing, the rod and flux is used in a similar way to silver soldering and is usually carried out with a large propane torch, bronze welding is a different technique where the metal is fillet prepared like for normal welding, and the operation is normally carried out with an oxy/ acet setup with a carburising flame, and the rod is used as a filler like normal gas welding, but instead of melting the parent metal, flux is used to assist the bond of parent metal to filler rod. This is usually the method used for joining cast iron.
Just a note about what to use for the heat source. Propane or butane gas torches are ideal for silver soldering, oxy/acet is NOT. The oxy setup is just too hot a flame, and will usually kill your flux off instantly, unless you are working on a large fabrication such as a loco boiler, in which case it can be used for localised heating, in conjunction with a propane burner that keeps the main fabrication hot.
If you follow a few basic rules, silver soldering is a doddle.
Silver solder works by capillary action, so it needs a gap to flow into to make the joint. Don't force fit a pin into say a crank then silver solder it afterwards. You are wasting your time, the solder is only sitting on the surface, just more cleaning up to do. Make the part of the pin undersized slightly or the hole it is going into very slightly oversized (a 0.002" clearance is ideal). Wack the bit that is going down into the hole with a hammer, to deform it, so that part of it grips on the inside of the hole, a straight knurl does a very good job of leaving enough gap. If the pin has a clearance of 0.002", just as you see the knurl being formed is about 3 thou, so a nice tap in fit into the hole. The silver solder will flow down the hole and make a perfect joint.
Cleanliness is paramount to a good joint, any contaminants will stop the joint being formed.
Solder and flux will always flow towards the heat. If you look at the mess of silver solder around a hole on one of the previous posts, that looks just like the heat was directed straight at the silver solder and flux. The flux was exhausted and the solder was melted before the main part had heated up enough to join the two parts. The correct way to heat up and make a joint like that would be to dip the pin into flux and put it down into the hole, paint around the hole and pin with flux (not too much, wherever the flux goes, the silver solder will go as well), put a pallion (please look it up, it will explain it a lot better than I can) of solder around the joint, then heat from the underneath of the disc. As the disc gets hot (Cherry red) you should see the solder run into the hole and form a perfect fillet around the pin. It just cannot be explained any easier than that.
If you are in any doubts as to what the supplier is giving you to carry out your silver soldering, then I would suggest you bite the bullet and order direct from the UK, where every model engineering supplier will be able to give you exactly what you require.
I showed this on another post on here the other day, it shows a little of what I have tried to explain above. There are bits about silver soldering all thru the article, which isn't quite finished yet.
http://madmodder.net/index.php?topic=1370.0
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