This is a really appropriate question for SD, as jewlers, welders, refrideration workers,
and plumbers all use “silver solder” in thier trades, and in general are
all talking about different processes, or at least different alloys.
The 2% silver bearing solder sold for plumbing use AKA Sta-brite, isn’t
much stronger than old 50-50 (Sn-Pb) solder. Not fatigue resistant
at all. it really needs a substantil lap joint, as provided by
copper sweat fittings. Collectivly, all this low temperature stuff is known
as “soft” solder…fairly discriptive, as most can be dented with your
thumbnail.
To distinguish from these, I’ll refer to silver brazing. I don’t consider “Silver solder” incorrect usage however, just likely to cause confusion now that silver bearing soft solder is widely available.
This uses filler with much higher melting poing. Though you can still sometimes use a air propane torch for small jobs, Mapp gas, and air-acetylene work really well. OXy-Acetylene requires some care to avoid overheating, but is certainly usable. It depends on the alloy, but you need to get the metal showing some color, and perhaps to a cherry red for some. Orange is definatly too hot.
Silver brazing is MUCH stronger than soft solder, though not quite as strong as bronze brazing. (which can be stronger than welding)
There are several different silver brazing alloys. You can find a discriptive table in the McMaster-Carr catalog. (available on-line) I’ve only used the two most common, which I discribe below. In my experience, the guys at the welding stores know squat about this part of thier product line. If you have the McM-C table, you are an expert. Unfortunatly the prices are quite high. Yes I know it has silver in it. Silver is not THAT expensive. Jewlers suppliers might be good source.
Most alloys will work well on a variety of metals, with higher melting points, including stainless steel, Except as noted below.
Flux will be required for use on steel. McMaster-Carr 7693A1 should work well for you.
One alloy typically used in refrigeration work is called “sil-phos” due to its phosphorous content. This is an exeption to the above. This makes it self fluxing when joining brass or copper. It also will fill gaps fairly well, and leaves a substantial fillet. It usually come in long sticks with a rectangular or square cross section. It is NOT recommended for ferrous (iron based, what you probably have) or nickle (stainless) steel.
Another type is 45% silver. One trade name is “EasyFlow-45”. This stuff does NOT fill gaps,
and it leaves almost no fillet. When it melts, it has a lower viscosity, than water, and the flux kills any hint of surface tension. Ideally, joints should have a consistant gap of .0005 to .002". It will flow by capillary attraction long distances through these joints…several inches at least. It work great on mild steel, tool steel, stailnless, copper, brass. Carefully done, it will even stand up as a butt joint on a bandsaw blade, provided you scarf the joint to increase area. This is usually supplied as a coiled wire. It will be almost as stiff as piano wire…NOT ductile like soft-solder.
For your work, I would NOT use the 45% alloy, unless you can get the parts to fit really close. (maybe by expanding the tube some?). The 35% Ag alloy is said to fill gaps better, and that should give you some fillet. I would cautiously recommend this, and only hesitate because I have no first hand experience with that alloy.
You need to get the parts really clean…and that is a problem for you as they have soft solder on them. Abrasive (but not sand, leaves dirt embedded) basting or emory paper and lots of elbow grease will fix that. After you have the soft-solder off, use a CLEAN rag or Q-tip and either acetone or MEK to make sure there is no PS fluid near the joint. and don’t touch the area of the joint after that.
It will be very helpful if you can make the parts “self jigging”. Work the parts until they have a faily close fit. Then take an automatic center-punch, and prick the tube three places. Done correctly, this will raise three craters that will hold the part in position while you braze it. Three bumps are to hold the tube centered…ideally you’ll have an equal gap all the way around,
If the flux is a powder, of very thick, thin some flux using distilled water to the consistancy of very runny yogurt. Use an artist’s paintbrush, or a Q-tip, and apply flux to BOTH parts, every-where you want the filler to flow to. Try to be neat, because the filler WILL go there if there is flux and heat.
Since I haven’t used the 35% Ag filler, I’m not sure which technique below will work best.
SilPhos brazing is done like bronze brazing, or like a plumber does soft soldering: Heat the joint, then feed in some filler. Use the work, not the flame, to melt the filler. Move the flame around, and the filler will follow the heat. It may help to dip the rod in flux.
45%Ag brazing calls for a different techique. Very little filler is needed. One or more small pieces of solder (perhaps 3/8" long maximum) are placed on the joint prior to heating. The joint MUST either be self jigging, or a jig will be required to keep the parts in position. Use small enough pieces that the flux will hold them in place. The joint is heated, but try NOT to melt the filler with the flame. Heat the parts, and let the hot part melt the filler. It is best if you can apply heat from the opposite side of th joint from where the solder was placed. Once the filler melts, it may form drops that sit on the surface. If you poke or scratch at these drops with a sharpened wire dipped in flux, the flux will disrupt the surface tension and allow the filler to flow into the joint.