Fig. 1. Placement of TC on surface of part

First of all, remember that good furnace control means you need to know the temperature of the parts inside a brazing furnace, and this can only be done in batch-type furnaces via well-placed thermocouples (TC’s).

I recommend that brazing shops use a minimum of at least three (3) Type-K (or perhaps Type-N) load-TC’s in each furnace brazing run, and even more TC’s than that, if possible, if their furnace will allow. The more TC’s used, the better will be the overall control of the cycle variables.

The tip of each load-TC should be firmly touching the part it is monitoring, as shown here in Fig. 1.

Please note that a TC measures the temperature at the point where the two TC-wires first touch each other. Thus, as seen in Fig. 1, the twisted TC lead does not actually measure the temperature at the surface of the part it is touching, but actually measures the temperature almost ½” above that surface at the point where the TC-wires first touch each other. Therefore, do not twist wires to make a TC. Instead, weld or braze the ends of the TC-wires instead.

It is also very important that only calibrated thermocouple wire be used when making TC’s. Each TC-wire has a correction factor that needs to be applied to the results measured in your furnace. If you do not use calibrated wire, your temperature readings could have significant error.

When there is a significant difference in cross sectional mass within a part (thin sections and very heavy sections within the same part), then TC’s should be attached to both the heavy and thin section (of at least one of the parts in each load). Then not only is the temperature of the full load being monitored, but also the temperature differential (delta T) between the thin and thick section of a single part can be closely measured and controlled in order to prevent distortion of that component.

Cutaway view of TC in a heavy block

Fig. 2. Cutaway view of TC in a heavy block

Additionally, if the parts in the furnace load are fairly massive, it’s wise to bury at least one TC down in the center of the most massive part of the load. If the parts in the load do not allow for such a TC insertion, then a “dummy” part of similar cross-sectional mass should be used for this purpose, as shown in Fig. 2.

When properly thermocoupled, a brazing furnace-run would generate a brazing chart such as that shown in Fig. 3, which shows a simple schematic of a furnace cycle in which four load-TC’s were used in addition to the furnace-TC.

Typical furnace chart for a load using four (4) load-TC's

Fig. 3. Typical furnace chart for a load using four (4) load-TC’s

TC1 on that chart will have been attached to a thin section of the part being brazed because it’s reading rises the quickest. TC4 should be attached to the heaviest (thickest) part of the assembly because it reaches temperature the slowest. Note that only when all five TC’s have reached the same temperature (at point A) can you then begin timing the cycle for its required amount of soak-time. For example, suppose that the drawing or specification required the load to soak at brazing temperature for 45 minutes with a maximum spread of 20F (15C) from the coldest to the hottest TC. As Fig. 3 shows, only when all the TC’s have come within that 20F total spread can timing of the soak-time begin.

Thermocouples are very important to the successful control of any load being brazed in a retort furnace, such as in a vacuum furnace. The more TC’s employed, the greater can be the level of control for that particular cycle. Each brazing shop should have procedures specifying and controlling the use and placement of TC’s in each load of parts being brazed. In addition to the information already presented in this article, it is wise to always be sure that you have the entire furnace brazing load monitored by TC’s, top to bottom, front to back, etc.. This will give you a good idea of the overall uniformity of the heating within the furnace, and throughout the load being brazed.

Next Month: Next month let’s look at a question that often comes up in furnace brazing, namely, the role of braze-fillets at the edge of brazed joints, and what purpose they actually serve.

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