Sherlock Holmes was a famous detective in some British spy stories written in the late 1800’s by British author Sir Arthur Conan Doyle. Sherlock was a great detective primarily because he was an expert observer. When he inspected a crime scene he saw things that others often overlooked. From these keen observations, and from his extensive experience, he was able to solve problems others seemingly could not.
In this month’s article, we wrap up our discussion of the seven (7) essential criteria for good brazing (see Table 1), by looking at what constitutes “knowledgeable inspection” of finished brazed assemblies. I often encourage people to “become a Sherlock Holmes” in their work when trying to solve brazing problems, and during final-inspection of brazed assemblies. Our goal should be to produce parts that have “zero-defects” (see Fig. 2), and to do this, each “inspector” needs to understand what a defect actually is, how to accurately find it, and what to do with the brazed assembly when an actual defect is found.
So, what then is a “defect”? Let’s take a look at how the word “defect” is defined by the American Society of Materials (ASM) in their Metals Reference Book:
Defect. (1) A discontinuity whose size, shape, orientation, or location makes it detrimental to the useful service of the part in which it occurs. (2) A discontinuity or discontinuities which by nature or accumulated effect (for example, total crack length) render a part or product unable to meet minimum applicable acceptance standards or specifications. This term designates rejectability.
Please notice the last sentence in that definition. The presence of a defect in a part means that the part must be rejected, and thus, either scrapped or repaired.
I hear people who are responsible for final-inspection of parts sometimes tell me that, because the brazed-assembly only had a couple of small defects in it, the assembly would be okay to release to the customer. NO – NO !
Do NOT use the word “defect” as being synonymous with such words as “imperfection”, “anomaly”, “void”, or another similar type term. A defect in a part means that the part is defective – which means it won’t work! When the word “defect” is applied to a brazed part, that part must be scrapped, or it must be repaired and then re-inspected before it can be released to the customer!
Shown in Fig. 3 is a drawing of what I consider to be a very poor inspection requirement. Unfortunately, this drawing comes from a brazing specification released by a large aero-company here in the US. Notice what this drawing is highlighting: it is asking some inspector of the part to measure the total accumulated length of voids along the surface of the external braze meniscus (fillet), and to then draw conclusions about the acceptability of that part for service, as long as the distance between each group of voids along the length or around the periphery of that joint meets additional criteria of soundness. This inspection requirement really makes no sense to me at all. As you can see in the drawing, the cumulative length of the voids should be no more than 0.100” (that’s to three decimal places)! Three decimal places! And this highly accurate measurement is supposed to take place over a distance of only 1” (no decimal places specified), and then there must be a minimum of at least 2” between such occurrences (again, no significant decimal places in that number). “Wow…” you must think, “..whoever came up with that inspection criterion must have spent hours developing such a specific accept/reject criterion, probably based on examination of a large number of parts, some of which had failed, etc., and from which he/she was able to come up with this specific inspection note. (Never mind the inconsistency of three-decimal places vs. none).
Actually, no, there was no such “field experience” upon which to draw for developing the drawing and inspection-notes in Fig. 3. I spoke at length with the person responsible for this drawing and notes, and his answer to me was: “I had to put something down.” So, this inspection note was actually just “pulled out of the air”, so to speak, based on that person’s “feelings”. Have you ever encountered something like this?
Where is the important part of any brazed joint – inside the joint, or in its external meniscus (fillet)? It’s inside the joint, between the faying surfaces. The external inspection requirement such as shown in Fig. 3 might possibly have some real meaning were it based on a lot of wind-tunnel tests showing perhaps the effects of such voids on airflow patterns through turbine blades, etc. But alas, it apparently was not.
By “knowledgeable inspection” I mean inspection that is based on real experience and knowledge of what the part will actually see in service, and then having an understanding of what a particular type of imperfection would actually have on the functionality of that part in service. It’s like having a new inspector counting such voids on a fillet (such as the one shown in Fig. 3), and then having a more experienced and knowledgeable inspector come over to that person and ask them why they are counting and measuring those voids. The person might say: “Well, according to this drawing callout, I’m supposed to measure this…..etc” and then having the experienced, knowledgeable inspector say: “Do you know how this brazed assembly is actually used in service? Because if you did, you would realize that this kind of inspection of the fillet is not really meaningful. Instead of that, we should be testing it in such and such a manner……” etc. Such a discussion between inspectors in this example is just a “what if…” type of scenario, and not actually meant to critique the specific application for which the drawing in Fig. 3 was made. I think you can see what I am trying to point out — know what kind of conditions the part will actually encounter in end-use service, and then develop meaningful tests (see Fig. 4) that will accurately test that brazed part in accordance with those end-use conditions.
Counting voids along a joint surface, or measuring the size of an external braze fillet, etc., are all inspection requirements which carry no real value or meaning, in my opinion, and based on my years of experience. Any and all inspection requirements should always, in my opinion, be backed up by extensive field test experience that can justify such criteria.
Over the years, I’ve written many articles for this website dealing with inspection requirements, methods of inspection, what makes good/bad brazed joints, etc., and encourage the reader to search this website for my other articles dealing with these topics. There’s a “search box” at the top right-hand side of this article into which you can type words such as: radiographic inspection, penetrant inspection, cracks, voids, visual inspection, ultrasonic inspection, concave fillets, joint length, etc.. When you read those articles you find by that method, you will then have information that can help you begin to more knowledgeably inspect brazed joints and improve your brazing process, with the goal of eventually having “zero defects”! Good luck to you! By the way, do you know what “good luck” is? Good luck is what happens when good preparation meets a good opportunity!
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Dan Kay – Tel: (860) 651-5595 – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 45-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Dan can be reached via e-mail at [email protected], and his website can be visited at http://www.kaybrazing.com/
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