Thus, you are seeing only a tiny fraction of the possible cuts that could have been made around the joint, and to assume that the portion of the brazed-joint you are looking at will be almost identical to what you would see all around the entire circumference of the joint is not wise! I’ve seen photos of progressively-polished brazed joints in which the visible void content in the exposed length of joint changed significantly as the polishing progressed further and further around the periphery of the joint.
Am I saying that I would expect to see significant variations inside the joint as I progressively moved through the joint around the periphery of the tubular fitting? Would I discover that what appears to be a perfectly fine joint in one polished mount might actually reveal a poor-quality joint with a large number of voids in it if I were to polish much further down into the joint, or if I were to make cross-sectional mounts at different locations in the joint? No, not at all. Such an occurrence would be highly unlikely, since, based on my experience, I’ve never seen (in all my years of brazing) an “acceptable joint microstructure” suddenly become a “rejectable joint microstructure” merely by polishing further down into the joint.
My point is, don’t “assume” that what you are seeing in one cross-sectional view is the same as you would see in all cross-sectional views were you to take cuts all around the full periphery of the joint. If you have doubts about the quality of a joint, take additional cross-sectional cuts and examine them carefully both macroscopically and microscopically, before making any judgements about the quality of the rest of the joint around the full circumference.
Cross-sectional analysis of braze joints is an interesting process–verification procedure that lets your shop see how well your brazing processes are working and whether or not they are in control. Good braze joints will show complete fill of the joint by the molten BFM, and very few voids in the joint microstructure.
However, if such cross-sectional analysis reveals braze-joint microstructures and/or void-content that is less than desirable, then it is my strong recommendation that you make additional cuts through that same joint to see if that first microstructure is an anomaly or if it represents what you find in other parts of the same joint. And if it is found that the poor quality of that cross-section is indeed represented in other sections of the same joint, then repeat those analyses on another brazed assembly to verify if it’s indicative of other parts that were brazed in that same load of parts.
And if a lot is rejected for any cross-sectional analysis results, then the full lot, in my opinion, should be subjected to extensive service tests (pressure testing, leak-testing, etc.) to see whether or not the parts will fully meet the expected service conditions.
Then, and only then, in my opinion, should you do any rejection or acceptance of full lots of brazed assemblies.