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.