There are just about too many types of solder calling themselves "silver solder" these days. What you want is what jewelers use. It will often be called "silver brazing" rather than soldering because getting the silver braze to flow requires that the steel get to at least a dull cherry heat. Mapp or propane won't get hot enough to flow good high temperature silver braze wire onto steel. Well, maybe they will with a master, but for use mortals we use acetylene/air or acetylene/oxy.
At that point where the steel begins to glow, the silver braze will suddenly flow everywhere on the joint in a matter of seconds. Your task is just to hold the heat exactly at that temperature while it flows. Too much and it will burn the metal and ball up. Too little and it stops flowing. If you mess up, just clean and start over Cleanliness is everything. Flux is very important. Going slow is the right way. Nothing cleans a joint better than filing to good metal and then cleaning with acetone wipes. I never grind or sand. Too many particles.
People who do a lot of silver brazing I would often clean a joint, apply flux, and then cut some pieces of silver brazing wire and stick them into the flux about where you want them to flow. Let the whole thing dry. Now begin to apply heat - not to the joint but nearby and gradually move the heat into the joint. The flux will melt first into a protective liquid glass. Sometimes it will bubble and move the silver pieces away from where you want it, but just push it back using the tip of the remaining silver brazing wire.
Keep the heat on and the silver in position....the bubbling will stop shortly and that is when you know the silver is about ready to flow. Let it flow. Overheating will cause blackening, although it is common for the edges of the flux pool to turn black as it cools. Most of the flux will simply be glassy.
If you got a good copper patch on, I would expect that to hold. I've even seen some joints on bent tubing repaired by a method of making a flexible patch with copper wire wound around the tubing bend. Do the super cleaning followed by winding a coil of #14 clean copper wire onto the steel tubing for an inch or more and then silver brazing the whole copper coil turns together and to the steel tubing.
Always go slow when adding heat and use plenty of flux. You can add flux even as it heats up.
rScotty
