Fig. 1. Illustration of the elements of a single TC circuit (Adapted from Fabian, ed., Vacuum Technology: Practical Heat Treating and Brazing (OH: ASM International, 1993), p. 141, fig. 8

Proper temperature control within each furnace brazing cycle is essential. It will not only ensure proper brazing filler metal (BFM) flow but can also prevent part distortion. To accomplish this, multiple thermocouples (or “TC’s” as they are often called) need to be placed in strategic positions within each furnace load.

In this article on TC’s and their use in brazing, I’d like to briefly look at what TC’s are, and the types commonly available for use in brazing furnaces today (much more exhaustive discussions about TC’s can be found on the websites of a number of thermocouple manufacturers and suppliers). I’ll also look at the correct placement of TC’s in furnace brazing loads, and how, together with correct furnace heating/cooling rates, they can help to maximize uniformity of temperature throughout each brazing load and minimize any distortion of components that are being brazed together.

A thermocouple (not “thermal-couple”) is a temperature measuring device that consists of two metal wires, called thermo-elements, each of a different composition, and joined together at one end to form a measuring junction (MJ), “couple”, or “hot junction”. These two leads form a closed-loop electrical circuit when the free ends are attached to a voltage-measuring instrument. The ends attached to the instrument are called the “cold junction”, or reference junction (RJ). Figure. 1 illustrates these items.

A thermocouple (TC) can technically, therefore, be called a “thermoelectric pyrometer”, because when the temperature of the wires at MJ is different than at RJ, a voltage is produced in the closed loop circuit, proportional to that temperature difference. This “Seebeck Effect”, named in honor of its discoverer in 1821, J. T. Seebeck, is fundamental to understanding TC operation. The voltage, or “emf” (electromotive force), in this circuit, is measured by the recording instrument; however, it is not reported as “milli-volts”, but rather as a “temperature”, the numerical value of which has been computed to correspond to that particular voltage reading.

Table-1 Lists a Number of the Commonly Used TC’s Available Today

The TC that is inserted into the furnace chamber to monitor the temperature of the atmosphere and furnace hot zone is often called the “furnace-thermocouple”. This TC is often a Type-R or S thermocouple (Platinum vs. platinum-rhodium), sheathed in protective alumina or molybdenum to provide long life and maximum protection from high-temperature oxidation up to approx. 2700F (1480C). The tip (measuring junction) of this furnace TC should extend a minimum of at least 2″ (50mm) beyond the heating elements into the hot zone. This “furnace-TC” is usually then connected to the furnace microprocessor (reference junction) to control the rate of heat input into the furnace during each cycle.

Thermocouple Types Commonly Available Today

Table 1: Thermocouple Types Commonly Available Today

The furnace-TC is often accompanied by a second, sheathed “over-temperature” thermocouple (also called “high-limit TC”). The function of this TC is to cause an automatic furnace alarm or shutdown should the furnace exceed the maximum temperature set on the over-temp controller. As with all sheathed TC’s, be sure the sheath material is compatible with the atmosphere and temperature requirements for the furnace load.

Finally, the TC’s attached directly to parts that are being brazed are called “load-thermocouples” or just “load-TC’s”. For most brazing-furnaces operating from 1000-2300F (540-1260C), the Type-K (Chromel-Alumel) thermocouple is still the most popular load-TC. It is relatively inexpensive, is quite reliable, and can have a long life when properly used. The newer Type-N (Nicrosil-Nisil) thermocouple is a more stable TC for use in similar applications as Type-K, and should be considered when greater life and stability is needed. As will be discussed in greater detail next month, I recommend that brazing shops use at least a minimum of three (3) Type-K (or Type-N) load-TC’s in each batch-furnace (vacuum or atmosphere) brazing run. Continuous-belt furnaces have a number of different options for placing/using TC’s in their brazing atmospheres, which will also be discussed in next month’s article.

Important Note: TC wires do deteriorate with time, depending on the furnace conditions, which can result in significant accuracy loss. Therefore they should be changed or recalibrated on a regular basis to ensure maximum reliability. Some companies and specs require that new TC’s be used for each furnace brazing run. Many other plants change them daily or weekly, or only when they notice deterioration.

Finally, for commercial and aerospace brazing today, the latest “D” revision of the Aerospace Materials Specification AMS 2750 (“Pyrometry”) is being required more and more. This is a comprehensive document, including the details of TC usage, which brazing companies need to use in order to be in compliance with today’s more rigid commercial and aerospace requirements for brazing furnaces.

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).

Placement of TC on surface of part

Fig. 1. Placement of TC on surface of part

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.

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.

Cutaway view of TC in a heavy block

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

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.

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 the timing of the soak-time begin.

Typical furnace chart for a load using four (4) load-TC's Thermocouples are very important to the successful control of any load being brazed in a retort furnace, such as in a vacuum furnace.

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

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