Nicrobraz Technical Articles Library

Brazing Fundamentals Using Thermocouples in Furnace Brazing

As we discussed in our first issue of Nicrobraz News (Vol.1, No. 1), proper temperature control within each furnace cycle is essential. It will not only insure proper filler metal 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. We recommend a minimum of three (3) Type K Chromel* Alumel* TC's per furnace load. (See the center section of this issue for an explanation on the types of available thermocouples and their uses.)

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. 2600° F (1425°C). The tip (measuring junction) of the furnace-TC should extend at least2" (50mm) beyond the heating elements into the hot zone. It is usually then connected to the furnace microprocessor(reference junction) to control the rate of heat input into the furnace during each cycle.

The furnace-TC is often accompanied by a second, sheathed over-temperature thermocouple (also called "highlimit 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.

TC's attached directly to parts that are being brazed and/or heat treated are usually bare and called load-thermocouples. The bare wires are separated by ceramic beads - or other appropriate material - as shown in Figure6a. Type K TC's are used because they are generally less expensive than other types. They are suitable for use up to approximately 2300° F (1260° C).

The tip of each load-TC should be firm, touching the part it is monitoring, as shown in Figure 6a. This figure also shows how not to configure a TC for part monitoring. Please note that a TC measures the temperature at the point where the first contact is made between the two wire leads. Thus, in Figure 6b, the twisted TC lead does not measure the temperature of the surface of the part, but actually measures the temperature almost1/2" above its surface. Therefore, do not twist wires to make a TC. Weld or braze the tip instead! It is also extremely important that only calibrated thermocouple wire be used when making TC's.

When there is a significant difference in cross sectional mass within a part (thin sections and very heavy sections), 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 (dT) between the thin and thick section of the part can be closely measured and controlled to prevent distortion. Additionally, if the parts in the load are fairly massive, it is 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 mass should be used for this purpose (see Figure 7).

Figure 8 shows a simple schematic of a furnace cycle in which four load-TC's were used in addition to the furnace-TC. TC1 was attached to a thinner section of the part being brazed because its readings rose the quickest. TC4 is attached to the heaviest (thickest) part of the assembly because it reached temperature the slowest. Note that only when all five TC's have reached temperature (at point A), can you begin timing the cycle for its required amount of soak. For example, suppose that the drawing or specification required the load to soak at brazing temperature for 45 minutes with a maximum spread of 20° F (15° C) from the coldest to the hottest TC. As Figure 8shows, only when all the TC's have come within that 20° F total spread, can timing of the soak begin.

* Trademarks of Hoskins Mfg. Co.

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