Figure 1. Tubular brazed joint cross-sectioned along its length.

All brazed joints should be inspected after brazing to verify that the parts being brazed will be acceptable to the end-use customer. If the brazed-assemblies are complex, then 100% of the assemblies should be inspected to verify that the parts have been brazed properly and will meet end-use requirements. This may include visual inspection of the exterior of all surfaces that have been brazed, and it may also involve one or more non-destructive testing (NDT) procedures to verify that each assembly will properly handle the end-use service conditions to which it will be subjected, such as fluid pressure, thermal cycling, or mechanical shock. Sometimes some destructive testing (DT) procedures are mandated, via an appropriate sampling plan, in order to physically examine the internal structure of some of the brazed joints. Is there any danger in using the examination of the cross-sectional microstructure of a brazed joint as an accept/reject criterion for the brazed tubular assembly? Yes, there is! Let’s see why…..

Fig.1 shows a drawing of what a typical cross-sectioning of a brazed tubular assembly might look like. Notice that the brazed tubular assembly is cut in half down the longitudinal length of the brazement, thus exposing for visual analysis (either macroscopically or microscopically, or both) two portions of the brazed joint, about 180-degrees apart, labeled in the drawing as “Joint Length A” and “Joint Length B”.

An actual photo of a cross-section of a tubular assembly is shown in Fig. 2, showing those two exposed joint-lengths. A metallurgical mount of another cross-sectioned brazement of a flat-sheet assembly is shown in Fig. 3.

Cross-section of actual tube-in-fitting brazed joint, showing two portions of brazed joint, approximately 180-degrees apart.

Figure 2. Cross-section of actual tube-in-fitting brazed joint, showing two portions of brazed joint, approximately 180-degrees apart.

As can be seen in each of the figures, a portion of the internal structure of the brazed joint is opened up for viewing. As many readers know, these exposed surfaces are often polished and then etched to clearly reveal the internal microstructure of the brazed joint — obviously, at that very specific location in the joint.

The danger comes when either the brazing shop personnel, or the end-use customer, wants to use that one specific cross-sectional view as an accept/reject criterion for the entire brazed joint, and perhaps for the entire lot of parts brazed in that specific furnace load! I have seen some customers’ requirements in which the voids along that one cross-sectional view are to be counted, and if the amount of voids along that length exceeds 20% then the entire lot of parts is to be rejected, and either re-worked or scrapped. This is not wise for a couple of reasons:

1. 20% is merely a number pulled out of the air.

This number has no real meaning, and certainly can not tell you whether or not a part will actually fail in service.

I have seen parts come back from successful field service (the parts were scheduled for regular replacement every few years), which were then cross-sectioned to see what, if anything, happened internally to the parts while being in service, and I was surprised to see that the void content of that particular sectioned brazed joint was over 30%. Yet the part had performed well in robust field-service for several years with no problems. This was not a unique situation, in that I’ve seen similar situations with other parts over the years. In one of my previous articles on this website (entitled “Braze Joint Design: Percentage of Voids in a Joint can Increase with Overlap Distance”) I talked about void-content criteria for brazed joints, and refer you to that article for more information on this topic.

2. The cross-section represents only one of many possible cuts through the joint.

In a tubular joint, the brazing filler metal (BFM) is expected to flow around the full 360-degree circumference of that tubular joint. A single cross-sectional cut through that joint exposes two (2) joint surfaces, perhaps 180-degrees apart from each other (as shown in Fig. 1), but is only one of many possible cross-sectional cuts that could have been made at varying locations around the entire circumference of the joint.

Cross-section of an overlapped sheet assembly, mounted for polishing and metallurgical examination under a microscope

Figure 3. Cross-section of an overlapped sheet assembly, mounted for polishing and metallurgical examination under a microscope.

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.

Conclusions

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.

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