Hopefully this should be a real easy one to answer. The sketch is of a 5/8" O.D. .035" wall copper tube brazed to a carbon steel boss. My question is about the thickness that would need to be qualified in a PQR to be able to braze this setup in production.  In QB-451 the max thickness qualified is always 2T so what thickness am I looking at qualifying for this boss.  Is there a rule of thumb that for situations like these do you always go with the length of overlap as the max thickness that would need to be qualified?  So in this case the length of overlap is .5"  Can I qualify a piece of .035" to a piece that's .25" thick to get me to .5" thickness?  Or do I need to look at it the other way in that I have 1" thick one way and over 4 inches the other way?  Thanks and let me know if you need some more clarification.

I don't have Section IX in front of me, but brazing is not like welding where the welder can work progressively around the joint heating only the immediate area to be welded. Preheat is a function of thickness and carbon content when welding steel to prevent cracking. That isn't the issue with brazing where the important consideration is heating the entire joint to the brazing temperature before introducing the filler metal. That being the case, the volume of heat is a major consideration. Thus, the size of the heating torch (or other heat source) must be considered. Nearly as important is the time taken to heat the joint and complete the braze. If too much time is required because the heat source is too limited, you can get "skull.” Skull is where the constituents in the filler metal separate and solidify at a higher temperature than the filler metal. It looks like cubes (crystals) rising up and out of the braze wetting the surface being joined.

With regards to the code rquirements, have you reviewed Section IX for clarification?

Best regards - Al