Its all about the flux and heat
What you may have already discovered is a very tedious and time consuming art...
Soldering anything to stainless steel required a judicious application of high heat. Also, the flux must be correct. One cannot use the paste flux a plumber uses...
Here are some tips:
Both surfaces must be immaculate...sterile would be an understatement. The best way to ensure this is a violent scrub with a Scotch brite green pad and acetone.
Allow the acetone to evaporate (can be accelerated with a heat gun)
Apply flux to both the filler and the surface.
Now this may seem contradictory...but you will want to gradually heat the surface to be soldered, but you also want to get in and out quickly with the torch. Watch the colors of the SS when heating...it will go brown, blue then red quickly...the soldering needs to occur between the blue and red stage...and it happens fast. Also remember that if the flux gets too hot it will loose its binding properties, the metal will oxidize and the solder will just pill.
Two type of solder we use is a pure silver which bonds around 1120*F but only has a 85K psi rating. The one we use most often is a Nickle Bronze alloy which bonds at 1400*F but has a 125K psi rating.
The silver alloy specification is 1210fc and is sold by the ounce
The nickle bronze specification is 528fc and is sold by the pound
The flux is the most important though. The best is Type B1. We get this flux from Rio Grande-a jewelry supply house in New Mexico.
The equipment you use also makes a difference. To get to the heat layer you need quickly enough you should use an oxy/acetylene setup with a slight carborizing flame (no yellow, just a hint of white, mostly blue). You could use a oxy/MAPP gas setup, but it will take considerably longer to get everything properly heated. Remember, the longer it takes to heat, the smaller window of opportunity you will have because the flux will roll off. A propane trigger torch just does not carry enough btu loading to be effective for this application.
We generally avoid soldering when at all possible...GTAW is the far preferred method both in speed and finish.
Hope this helps,
Tom
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