Hi Again Doug,
My experience for silver soldering.
A really nice silver solder for flow and building-up beads is the 55% solder. But to have a couple of different solders for different parts of the assembly I also have stock of 2 others. That way I can make first sub-assemblies in the "hotter grade", build them into "second level" assemblies with the "middle temperature grade" and final assembly with the "coolest" grade.
A quote from a welder: "
The most widely used silver solder melting points are 1328° Fahrenheit at 33 percent silver volume, 1247° Fahrenheit at 40 percent silver content, and 1202° Fahrenheit at 55 percent " - I have used Johnson Matthey silver solders if I want something "special",
Silver-flo-filler-metals - matthey.com
(I ordered some "flux-less" silver solder for a Works job for joining wires once - contains Phosphorus - Not the best for a boiler, but good for gas fittings.). But generally, the "cheapest" market quality 55% is OK for most of my final assembly work, and regular "cheapest" is a 33%~35% grade. I guess you'll get ASTM grades in Yuba?
In theory, you should only use certified material for Boiler-work. Talk to your BOILER TESTER/INSPECTOR, in case he wants to know what solder grades you are using, and he may want to see prepared but unassembled parts, and drawings for the boiler. I have never needed to present calculations to the Boiler Tester (they just do a visual exam to be sure all joints look OK, before doing the pressure testing), but (now) always do those FIRST, as it is easier to increase a material thickness or change a design before making anything! But I have had to De-Rate some boilers (even one I designed and made) following a more comprehensive study of ASME, some university papers and "industrial practice" papers. But your Yarrow is relatively simple and we have already discussed sizes and strengths of tubes.
There is a curious effect when silver soldering: (A metallurgist should be able to explain?). When the pool of silver solder is molten, some of the adjacent copper diffuses into the solder, changing the metal composition, so it moves to a different place of the "liquidus" curve, and later needs a higher temperature to melt the joint. Too much heat (prolonged or too high?) can increase this diffusion, but what it does for joint strength I cannot say.
Cheers!
K2