Monitoring the actual quality of the brazing atmosphere inside a vacuum-furnace during brazing cycles is very important, and is not hard to do. When vacuum brazing, you have to wait until the brazed parts are removed from the furnace at the end of the brazing cycle in order to see if everything was actually okay during that brazing cycle. If, when opening the furnace after a brazing cycle, you see that the parts you were trying to braze are discolored or poorly brazed, then how can you determine exactly what went wrong during that cycle, and how can you know when the brazing problem actually occurred (did it happen during heating, or during cooling, etc.)? Also, how do you determine whether the poor braze results are caused by physical problems with the furnace itself, or if they might be related to the brazed-component’s base-metal (parent-metal) composition, or perhaps with the brazing filler metal (BFM)?
To answer such questions, there really should be an “atmosphere QC inspector” placed inside the vacuum-furnace during each of your brazing cycles. Such an atmosphere QC inspector is shown in Fig. 1.
Simply put, a piece of 304-stainless steel, placed inside the furnace along with the parts you are going to braze, is the perfect atmosphere QC inspector, providing accurate, reliable answers that will save you from making wild guesses or false accusations about what might have happened inside the furnace during the particular brazing cycle in question. The use of these stainless QC coupons can save you many hours of searching, head-scratching and lost production, because they will provide you with immediate answers to the causes of the brazing problem.
What kind of stainless “coupon” should be used?
As you can see in Fig. 1, there are a couple of options for you to consider. The first is the use of a square piece of 304-stainless sheet-stock approximately 1-to-2 inchs (25-50 mm) square, and the other option is to join two pieces of shim-stock to form an inverted “T”, the latter constructed of two pieces of 304-stainless sheet-metal shims, each measuring approximately 3/8” (10mm) wide by about 4-to-5” (100-125 mm) long, and about 1/32” (0.75mm) to 1/16” (1.5mm) thick, as shown in Fig. 2 below.
Please note that you can use whatever size specimens you wish. The dimensions shown here are just the ones I have personally used. These specimens are designed for use in any non-oxidizing (low-dewpoint) furnace atmosphere, such as vacuum, pure dry hydrogen, dissociated ammonia, nitrogen, argon, etc., which are operated at temperatures above about 1850°F (about 1000°C).
Why use 304-stainless?
The main reason for using 304-stainless is for its chromium (Cr) content, which is very sensitive to atmosphere quality. Chromium oxidizes readily. Remember, it’s the presence of chromium-oxide on the steel’s surface that makes stainless-steel stain less than regular carbon steels! The layer of Cr-oxide increases rapidly if there is excess oxygen in the furnace atmosphere (perhaps from an air leak, or from the presence of moisture in the furnace atmosphere), and this causes the surface to begin to turn blue, dark blue, purple, dark greenish-blue/purple, etc. Just a tiny leak, or a bit too much moisture in your vacuum can quickly cause the stainless steel specimen to discolor.
Some metallurgy: As stainless steels are heated, the chromium will oxidize more and more, turning darker and darker in color. However, above about 1850-1900°F (about 1000-1050°C) these chromium-oxides can be “reduced”, i.e., eliminated, and the surface of the stainless restored to a bright, shiny condition. This will happen if the atmosphere is clean and “dry” enough (i.e., free of oxygen) at those temps so that the thermodynamics of the situation will cause any chromium-oxides present on the surface of the stainless to actually break apart into chromium and free-oxygen (the oxygen may then be pumped out of the furnace), literally “cleaning up” the surface of the stainless above those temps! The extent of this oxidation/reduction reaction on the surface of the stainless can be seen by the degree of shiny or matte finish on the stainless when it comes out of the furnace (but that’s the topic of another article in the future).
Therefore, if the stainless coupon exits from the high-temp brazing cycle looking bright and shiny, then you KNOW that the cycle was of excellent quality!
Over the years, we have found that 304-stainless steel is not only an extremely reliable and accurate atmosphere-quality indicator, but is also readily available and not very expensive.
IMPORTANT NOTE — What stays constant, and what changes?
REMEMBER — the atmosphere control tests being conducted involve a “constant” (the same stainless coupon material in every furnace run) and a “variable” (the BFM used in each particular furnace run).
When should the specimens be used?
Include at least one stainless QC-specimen in every vacuum-furnace brazing run. Do NOT decide to use them periodically, such as “every third cycle…”, etc. That defeats their purpose! These on-site “atmosphere QC inspectors” will do their job every time, and hopefully will emerge from the furnace looking bright and shiny after every brazing-cycle (meaning that your furnace, atmosphere, and BFM are all in good control). But don’t let that ever cause you, or your supervisor, to say: “We’re wasting our time with these coupons. Stop using them.” Place at least one of these coupons into every brazing cycle, because WHEN —not IF — but WHEN one of your brazing cycles goes amiss, you will be very glad you included your stainless QC-specimen in that furnace run, since the coupon WILL tell you WHAT went wrong in THAT cycle, WHEN it happened, and WHY it happened! Wow!
Making the stainless QC-specimens
Constructing stainless QC-specimens is quite easy. Shims of 304-stainless sheet metal can be easily made or purchased from your metals-supplier to the approximate dimensions mentioned earlier (again, the choice of dimensions of the coupons is entirely up to you. There are no “standard” sizes that must be used). These coupons are usually made by shearing/cutting them from larger sheets. If you decide to make the T-shaped coupon, it can be made as follows: the long edge of one of the two shim pieces is positioned along the flat side of the other piece to form a “T” as shown. Then, those two shims are held together by either a couple of light-gauge stainless steel wires, or via a very small tack-weld at each end of the coupon.
Here’s two very important considerations for you when making your stainless QC-specimens:
Interpreting the results:
Table 1 shows a chart that I developed many years ago which shows how to interpret the results of five different scenarios that can occur inside the brazing furnace when conducting a high-temp furnace brazing run. The first four vertical columns (left to right) in the table show possible surface conditions that might be observed for the following three items:
The remaining columns of the table describe the conclusions that can be reached based on what is observed in each of the different scenarios shown in the first four columns in any particular row.
Each of the horizontal rows in the chart describe a particular “observed scenario” when the brazed parts are removed from the furnace and inspected. These scenarios are as follows:
Okay, put your understanding to the test: In the photos below in Fig. 3, what was the problem in each of the two furnace runs represented by the two different stainless coupons shown? (The answers are at the end of the article)
REMINDER: The stainless QC-coupon is not suitable for lower-temp brazing using any of the copper-based, or silver, gold, or aluminum-based BFMs, etc., since the temperatures involved are not high enough to allow for reduction of the chromium-oxides on the surface of the stainless QC-coupons. Instead, I recommend the use of the vertical variable clearance specimen for testing these lower melting-temp BFMs in varying atmosphere quality. This vertical test specimen has already been discussed at length in a previous blog article.
IMPORTANT FINAL NOTE: If you are brazing with any boron-containing nickel-based BFMs, be careful to braze them only in a vacuum furnace, or in hydrogen, helium, or argon. Do not braze them in a nitrogen atmosphere, because the boron and nitrogen can react together to form boron-nitrides, a good stop-off!
Answers to questions in Fig. 3:
2. The second sample in the photo shows a dark blue coloration, indicating a problem with the furnace on the way down from brazing temperature. How do we know it occurred during the cooling cycle? Because the BFM has flowed completely through the braze joint on the QC-specimen, which meant that the furnace atmosphere was fine right up to end of the actual brazing cycle itself, prior to the beginning of cooling.
Dan Kay – Tel: (860) 651-5595 – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 40-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@example.com, and his website can be visited at: http://www.kaybrazing.com/
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