As we have seen in the last two articles, the fixturing used in furnace-brazing is a very important item that MUST be carefully considered if you want to minimize the time used in each brazing cycle (excess fixturing weight adds a lot of time to your brazing cycle) and maximize the productivity and profitability of each furnace-brazing run.
Another very important “fixturing” item that has to be understood and properly used are thermocouples since they are the only way you can monitor the temperature inside each furnace load. The ability to properly monitor the temperature at different locations within the furnace chamber will not only help to 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. Some typical TC’s are shown in Figure 1.
This will be the first of two articles on the use of TC’s in brazing, and you will hopefully quickly see that they are an important part of your furnace’s fixturing, and when properly understood and used, will help to give you better, more reliable brazes. Some of the information in this article has been previously printed on this website a number of years ago and is being brought forth again, since many readers may not have seen my previous articles on this topic.
To begin with, 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). In next month’s article, I’ll describe the actual use and placement of these TC’s in different brazing furnace cycles.
A thermocouple (not “thermal-couple”) is a temperature measuring device that consists of two metal wires, called thermoelements, 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 2 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”. As shown in Table 1, this type of 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. 2600F (1425C). 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. A cross-sectional drawing of a typical “sheathed TC” is shown in Figure 3.
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”. A typical example of this is shown in Fig. 4.
For most brazing-furnaces operating from 1000-2300F (540-1260C), the Type-K (Chromel-Alumel) thermocouple is still a very 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 longer life and better long-term stability is needed. More and more shops seem to be migrating to the Type N thermocouples.
When several TC’s are properly placed in a furnace load, that brazing furnace-run would generate a brazing chart such as that shown in Fig. 5, which shows a simple schematic of a furnace cycle in which four load-TC’s were used in addition to the furnace-TC.
TC1 (thermocouple#1) on that chart is attached to a thin section of the heavy part being brazed because it’s TC-curve on the chart rises the quickest. TC4 is attached to the heaviest (thickest) part of the assembly because it reaches temperature the slowest. Note that only when all five TC-readings come within an allowable temperature spread (at point A) would 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 50F (25C) from the coldest to the hottest TC. As Fig. 5 shows, only when all the TC’s have come within that 50F (25C) total spread can timing of the soak-time begin.
Thermocouples will deteriorate
Please note that TC’s do “go bad” over time, depending on the atmospheric conditions and thermal cycling done in the furnace. And each TC may deteriorate over time at different rates. Because of this, I always recommend that brazing shops use a minimum of at least three (3) Type-K (or Type-N) load-TC’s in each batch-furnace (vacuum or atmosphere) brazing run. By using three or more per load, it will not only give you more information about what is happening in the furnace load, but it will make it much easier to identify which, if any, of the TC’s may be failing, since it’s thermal-curve on a chart-printout will be clearly different from all the others. Identifying the “errant” TC would be much more difficult if only one or two TC’s were being used.
Therefore, because of this deterioration, TC’s should be changed or re-calibrated on a regular basis to insure 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.
TC’s are a very important part of any correctly performed furnace brazing operation. The more TC’s that can used within each furnace run the more the control you are able to have on what you are brazing in that furnace. Be sure to use the right TC for the job and you’ll get good results. And be sure you have enough TC’s in the furnace load to give you good information about the temps of the load (front to back, top to bottom) but also — very important — the temps within one or more of the components in the load, i.e., the hottest and coldest temps within each of the parts in the load, since distortion comes from the temperature differential within each component part, NOT from merely the temp-differential of the total furnace load (more about this next month).
We’ll also look further next month at correct (and incorrect) ways to attach TC’s in a furnace load for optimal load-control, and we’ll also touch on what the latest revision of AMS 2750 (Pyrometry) is doing to temperature control requirements in the world of brazing.
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Dan Kay – Tel: (860) 651-5595 – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 45-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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