Tips on Brazing Copper and Stainless Steel
Copper & stainless steel can be brazed, but it's challenging and costly.
Brazing is the process of joining two pieces of metal using a filler metal. It's very similar to soldering; the difference, as far as I can tell, is that soldering uses a lower melting point filler metal and results in a weaker joint. Brazing can result in a very strong joint — not as strong as proper welding, but still strong. (The advantage vs. welding is that brazing can be used to join dissimilar metals; welding generally shouldn't.)
Brazing stainless steel and copper is challenging but can be done. Here are some tips on this process based on what I've learned from the Internet and tried so far.
Don't: my first tip is just "don't." If you can change your design such that this particular combination of metals doesn't need to be brazed, you should avoid this process altogether.
Heat: you're going to need an oxyacetylene torch. But be careful; copper has a much lower melting point than stainless steel, so it's very easy to turn your copper part into a puddle. Controlling your heat carefully is critical.
Flux: use an aggressive flux. People online recommended Harris Stay-Clean SCPF4, and it seems to work well. Note that, particularly with an aggressive flux, you'll want to clean the flux off the joint and surrounding areas quickly after brazing.
Silver filler: you need brazing rods with a shockingly high silver content; these 45% Ag rods work well for me.
Allegedly, some people have successfully joined copper and stainless steel with relatively standard silver-bearing lead-free plumbing solder. I did not have success with that, and I wasted about $25 of solder trying.
It's important to note that this is costly. Three, 1/16" 45% Ag brazing rods go for $50 online, and especially when first practicing this, you'll go through them quickly. Which brings me back to my first point: avoid this if possible.