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Stainless QC-coupons to Determine Atmosphere-Quality In Your Furnace

By Dan Kay

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)?

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Fig. 1 – 304 stainless steel coupons (either flat sheet or T-shaped specimens) are the perfect Atmosphere QC inspector for use in all high-temp vacuum brazing furnaces.

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?

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Fig. 2 – How to construct a “T-shaped” Stainless QC-coupons for use in high-temp brazing furnace atmospheres. Based on drawing by Dan Kay for his article in Wall Colmonoy’s Nicrobraz News.

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. 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?

1.  Coupon material should be only 304-stainless. In order to ensure consistent quality control in each high-temperature vacuum-furnace run, it is very important to use the same “control” material as your “atmosphere QC inspector”. Therefore, do not change the base-metal used for your atmosphere-control-specimen according to each different base metal being brazed per furnace run. This will defeat the purpose of these tests. Stay with 304-stainless (or perhaps, 302-stainless)! Do NOT use 321-stainless, since 321-stainless contains some titanium, which can give very misleading results in your furnace atmosphere!
2.  The brazing filler metal (BFM) used on each specimen DOES CHANGE! Be sure to use the SAME BFM on each stainless QC-coupon as you are using for the brazing in that particular furnace run.

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:

1.  The stainless steel shims should be thoroughly degreased before assembly. Remember, brazing filler metals (BFMs) do not bond to (or flow over) oils, grease, or oxides! The shims should be stored in clean, closed containers, and handled with clean hands by the edges. Otherwise fingerprints can affect your coupon results!
2.  If you are making T-shaped specimens, then be sure of the following:
   a.  edges of the shims should be deburred, particularly the edge used for the braze-joint at the junction of the “T”. The edge should be “squared” so the joint-gap clearance remains as small as possible (a 0.001-0.002”/0.025-0.05mm gap is ideal) between the squared edge of one shim and the flat surface of its mating shim on which it is resting.
   b.  Caution: Do not deburr the sheared components by tumbling them in such media as marble chips, corn-cobs, stones, alumina chips, etc., as all of these materials will leave a non-brazeable residue on the surface.
           Recommendation: Tumble the parts dry (against themselves), or tumble them in stainless-steel B-B’s.
   c.  The joint gap along the entire 4-5” (100-125mm) length should be as close as possible (0.001-0.002”/0.025-0.05mm). As long as the shims are flat and   straight, the roughness of the as-cut surface finish of the pieces should provide a minimum gap of approximately 0.001” (0.025mm) when the two shims are placed directly against each other. Do not insert shims or wires between the parts in order to try to create a specific 0.001-0.002” (0.025-0.05mm) gap.
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Table 1 – Evaluating Results from Stainless QC-coupons placed in high-temp brazing furnace atmospheres. From Dan Kay’s article he wrote for Wall Colmonoy’s Nicrobraz News.

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:

a.  base metals being brazed (the “part”)
b.  304 stainless-steel “on-site atmosphere QC-specimen” (the “control”)
c.  brazing filler metal (BFM)

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:

1.  The base metal (parent metal of the part/assembly being brazed) is bright and clean, the BFM flow on the metal part looks good, and the 304-stainless QC-coupon also looks bright and clean with good BFM on it. The conclusions that can be readily drawn from that scenario are quite favorable:  Good furnace atmosphere, good-quality BFM, and proper furnace brazing cycle.
2.  If base metal comes out of the furnace dark or discolored with no BFM flow (or very poor BFM flow), but the 304-stainless QC-coupon is bright and clean with good BFM flow, then you know that the furnace atmosphere is good and that there is a problem with the base metal (perhaps it contains aluminum or titanium, etc.) which is easily oxidized, even in normally “good” vacuum atmospheres.
3.  IF both the base metal and the stainless QC-coupon come out of the furnace discolored, but the BFM has flowed well on both, then you know that the base metal, BFM, and furnace atmosphere were all good up through the end of the time the parts were being held at brazing temp (because the BFM flowed out well), but that an atmosphere problem occurred somewhere during the cooling cycle. There may be a leak in the gas-cooling/quench line that is being used to rapid cool the parts, or perhaps the parts were pulled “too-hot” from the furnace.
4.  If both the base metal and the 304 stainless QC-coupon come out of the furnace dark or discolored, and the BFM has not flowed at all, then you know that you had an atmosphere problem right from the start of the heating cycle (since the BFM will not melt and flow if it (or the base metal) becomes oxidized heavily during heating, and those oxides can’t be “reduced” prior to reaching brazing-temp. Thus, you may have a bad furnace leak, or the furnace is badly contaminated with material that readily outgasses during heating (a “virtual leak”) and contaminates the atmosphere and the parts being brazed.
5.  If the base metal and the stainless QC-coupon come out of the furnace bright and clean, but the BFM has not melted or flowed, then either the furnace was never turned on, or the BFM may be bad. Including another 304 stainless atmosphere QC-coupon with some known good-flowing BFM as a “control specimen” can quickly verify this.

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)

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Fig. 3 – Examples of stainless specimens from two different furnace runs, exhibiting two different distinct problems. What does each specimen indicate about the quality of the furnace brazing cycle in which that specimen was run?

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:

1.  The brown, discolored QC-specimen shows no BFM flow.  Thus we can conclude that there was an atmosphere problem in this furnace right from the start.  The brown coloration could indicate a “virtual leak” in the furnace, i.e., oils, dirts, greases, etc., outgassing from the furnace walls where they had condensed from previous furnace runs, or even an outgassing issue from contaminated parts being brazed that were not cleaned prior to brazing.  No-one, I’m sure, has ever heard anyone say: “Oh don’t worry about cleaning those parts.  The furnace will clean them during brazing…..”
      Thus, the specimen (and the parts being brazed) became contaminated before reaching actual brazing temp, and the BFM could not therefore flow over contaminated surfaces. The furnace (and all parts being brazed) need to be properly cleaned prior to the next brazing run.
      Another scenario: If the surface of the stainless QC-specimen had a dark-gray appearance with no BFM flow, then the presence of either aluminum or titanium might be suspected as a contaminant somewhere in the furnace chamber during the brazing run.

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.


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