Fig. 1 A vacuum furnace cross-section showing different types of TC’s used, such as the furnace “Control TC”, an “Over-temp TC” and typical “Load TC’s” that are actually attached to parts in a load of parts being brazed.

When parts are to be furnace-brazed in commercial job-shop vacuum furnaces or in standard atmosphere furnaces, they are typically placed on a rack inside the furnace, and then the furnace door is closed, thus completely hiding those parts from view. Once the actual furnace brazing-cycle begins inside that furnace, you really don’t know what’s happening to those parts, since you can’t see them. The only practical way to determine if they are actually being successfully brazed is to watch the furnace’s instrument panel in order to find out what’s happening inside the furnace (temperature, vacuum level, leak-up rates, etc.). Obviously, when you open the furnace door after the brazing cycle is over you will quickly see the results of the brazing cycle, which you hope will be fine. But when (not IF, but WHEN) something goes wrong with one of your brazing runs, and you see that the parts did not braze well (or did not braze at all), then the importance of properly instrumenting your brazing load will become very clear to you!

By the phrase “properly instrumenting your brazing load” I am primarily referring to the proper use of thermocouples (TC’s) to accurately monitor and record the temperatures being experienced by the parts themselves as they are being brazed! This is very important. As shown in Fig. 1, TC’s that are attached to the parts being brazed are known as “load-TC’s” since each such assembly is part of the “load” going into the furnace to be brazed.

I have personally seen too many brazing shops that do not apparently choose to see the inherent value of using a number of load-TC’s in their brazing operations, looking at load-TC’s only as a kind of “necessary nuisance”, and thus try to use as few TC’s as possible in any given load, or fail to use them at all (since “they know what they are doing, and load-TC’s aren’t really needed, since each load is the same”). That is, frankly, misguided thinking. Every time you operate a brazing furnace it is continuing to wear out more and more with each cycle, and furnace operators need to constantly monitor what is happening inside the furnace during each brazing cycle to verify that the furnace is working properly during each such cycle, and more specifically, if the parts being brazed are actually coming up to the temperatures needed to braze them correctly.

I’ve written in previous articles about the use of special stainless-coupons in high-temp furnace runs to help verify atmosphere quality, and I’ve also written some articles about thermocouples (TC’s) and why/how they are used. In this current article, I want to remind readers about the important role that “dummy-blocks” can play as an integral part of proper TC use in a brazing furnace.

A “dummy-block” is a piece of metal into which a TC has been embedded, a cross-sectional diagram of which is shown in Fig. 2.

Fig. 2 Notice that the tip of the TC is near the center of the dummy-block

Fig. 2 Notice that the tip of the TC is near the center of the dummy-block

This block is then placed next to the actual parts being brazed inside the brazing furnace. A photo of some typical, different-sized, dummy-blocks is shown in Fig. 3.

Different sized blocks

Fig. 3. Note the different size and colors of blocks (Photo courtesy of Solar Atmospheres, Souderton, PA., from their excellent article entitled: “Understanding Emissivity and the Use of Thermocouple Test Blocks in a Vacuum Furnace”, written by Real Fradette and Trevor Jones.

Size of Block

The size of the block that you should use should – as closely as possible — match the cross-sectional mass of the part that is being brazed. Thus, as shown in Fig. 3, you should try to match the block with the cross-sectional mass of one of the parts that is being brazed. The reason why this is necessary can be shown by looking again at the cross-section of a typical dummy-block, as shown in Fig. 2. Please notice that the TC is buried inside the block so that the tip of the TC is centered in the block. The heat from the furnace should be able to reach the tip of that buried TC at the about the same time as the heat gets to the center of the part that you are brazing. Thus, it’s important that the cross-sectional mass/weight of the dummy-block is about the same as that of the part being brazed! In that way, as you monitor the TC buried in the dummy-block, you’ll have a good idea of when the joint that you are brazing also reaches that temp. By controlling your brazing cycle by what is happening to the TC in the dummy block, you’ll then have an excellent idea of what is actually happening with the brazing filler metal (BFM) inside the parts being brazed.

As you can see in Fig. 1, the buried TC in the dummy-block is approximately the same distance below the surface of the dummy-block as the joint-surface is in the actual part that is being brazed. It is often forbidden to bury a TC down into the actual joint area of the part that is being brazed, and thus, any TC placed on that actual part being brazed is usually only attached to the top of the outside surface of that part. Although this is good to do, it doesn’t actually give you the temperature inside the joint itself. Thus, having a dummy-block with a buried TC in it, placed next to the part being brazed is a highly recommended practice to help the furnace operator get a more accurate idea of what is actually happening inside the joints themselves.

Color and Surface Roughness of Dummy Blocks

As shown in a report written by Real Fradette and Trevor Jones, the surface roughness and color of the dummy block does affect its ability to duplicate the temperatures in the actual parts being brazed. Polished surfaces on dummy blocks tend to reflect heat away, whereas roughened, dull surfaces tend to absorb heat more readily. Likewise, darker surfaces absorb heat-energy faster than lighter surfaces. So, another important consideration about dummy blocks is to try to keep the surface finish and color approximately the same as that of the parts being brazed. It doesn’t need to be exact, but don’t use polished blocks when brazed parts that have very dull roughened surfaces.

Conclusions

Don’t just rely on your main “Furnace TC” for information about what is happening within the load of parts that you are trying to braze in your furnace. For proper control of your brazing cycle you must also use “load-TC’s” that are either attached to the parts that are being brazed, or if TC’s cannot be actually attached to, or embedded in, the parts themselves, then use appropriate “dummy blocks” with TC’s buried in the middle of those blocks, the blocks being of approximately the same cross-sectional mass/weight (and surface roughness) as the parts being brazed, and placed right next to the parts being brazed.

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