Thermocouples have been the overwhelming choice for temperature measurement and control in the area of batch-type vacuum heat-treating furnaces. When properly selected and used, they provide accurate, reliable process temperature data, which is a must as the industry moves toward heat treating more sophisticated alloys. This article provides some guidelines for the selection and use of thermocouples in a vacuum furnace environment.
Thermocouple selection
In vacuum furnaces, thermocouples are used for both furnace temperature control and to measure workload temperature. Table1 shows the thermocouple types most frequently used in vacuum applications and their composition, range, limits of error, and relative cost information, factors that need to be taken into consideration when selecting a thermocouple for vacuum use.
Thermocouples used to control furnace temperature should be selected to operate at the maximum furnace operating temperature. For example, type K or N thermocouples might be the best choice for a maximum furnace operating temperature of up to 2000F (1095C) because they can operate above this temperature and they are relatively inexpensive. The choices at higher temperatures would be types R or S (above 2000F) and type B or C for furnaces rated at 2700F (1480C) and higher. Limits of error for each thermocouple type also are important.
Type C, K and N thermocouples should be calibrated at the critical operating temperatures of the process. Because of the relatively large limits-of-error tolerances (particularly for C), there could be as much as a 20F (11C) error in a temperature reading at 2000F.
The same guidelines for furnace-thermocouple selection also apply to the selection of work thermocouples except that only the maximum temperature of the process need be considered.
Vacuum compatibility
Thermocouples used in vacuum service can be either unprotected or protected by a sheath or protection tube. The advantage of using unprotected thermocouples is rapid response to temperature change. However, the thermocouple material must be compatible with the vacuum and inert gas (either nitrogen or argon) environment that exists in most vacuum heat-treating furnaces. The advantage of a protected thermocouple is longer life and protection from contamination.
Type R, S and B platinum thermocouples are not recommended for use in vacuum. While bare-junction thermocouples have been used to control process temperature, their service life is shortened because they are very sensitive to contamination from the furnace and/or materials being processed. They are also sensitive to reducing atmospheres, such as hydrogen, which is sometimes used as a process gas. Platinum thermocouples work best in air and should be purchased in vacuum-tight assemblies filled with air as long as the protection tube material is compatible with an oxidizing gas.
Type K, N and C thermocouples work satisfactorily in both a vacuum and inert atmospheres. Type K and N thermocouples are sensitive to reducing atmospheres, and therefore, should be protected when using a process gas such as hydrogen. It also must be noted that the calibration of type K and N thermocouples operating in vacuum at high temperature for extended periods is altered due to vaporization of chrome from the positive leg. Type C material works well in vacuum, hydrogen, or inert atmospheres.
Thermocouple assemblies
Thermocouples are manufactured in a variety of packages. They can be made flexible using a beaded construction, using a high temperature ceramic fiber insulation and by placing the wires into a metallic sheath filled with magnesium oxide (MgO) particles, which insulates the wires from the sheath. Thermocouples also can be made of rigid construction, with the thermocouple wires placed in a rigid alumina insulator, which is housed in a high-temperature resistant protection tube.
Vacuum furnace control and over-temperature limit thermocouples are usually made of rigid construction. Figure 1 shows the typical types of construction for control and over-temperature limit thermocouples. The top and middle thermocouples are type S and B, housed in alumina tubes with air as the background gas in the protection tube. The compression fittings shown on the tubes contain a neoprene insert, which provides a vacuum seal for the assembly. The thermocouple at the bottom of Fig. 1 is a type K with an Inconel (Ni-Cr alloy) sheath, MgO insulation inside, and a compression fitting for vacuum seal. Another material used in vacuum furnaces (not shown) is a molybdenum protection tube. Construction is similar to that of the top two thermocouples, except the protection tube is molybdenum. When using molybdenum protection tubes, the backfill gas must be inert (usually argon) because molybdenum is sensitive to oxidizing atmospheres.
Types R, S, B, K, N and C thermocouples all can be made of the constructions discussed above. In a vacuum environment, a vacuum-sealing gland at the top of the protection to tube should be considered to prevent a leak in the event of a break in the tube.
Inconel material can be used in applications at temperatures below 2300F (1260C), while molybdenum or alumina protection tubes should be used at higher temperatures. It is important to prevent an Inconel sheath material from touching graphite in the hot zone, because nickel in the Inconel and graphite form a eutectic at 2130F (1165C), and can cause melting at temperatures lower than the melting point of Inconel.
Work thermocouples are usually made to be flexible so that they can be placed within the work to measure load temperature. Figure 2 shows the most popular construction for work thermocouples. The thermocouple on the left is made of thermocouple wire insulated with a ceramic fiber, known as Refrasil (Hitco Carbon Composites Inc., Gardena, Calif.). The center assembly is made with multiple alumina or mullite insulators for flexibility, while the thermocouple on the right is a 0.125-in. (3 mm) thick Inconel sheath assembly.
Most work thermocouples are type K or N because they are relatively inexpensive and can be replaced frequently. They are capable of measuring temperatures as high as 2400F (1315C), but degrade rapidly when cycled above 2000F and should be used only once above that temperature.
Type B, R, S and C thermocouples should be used to measure workload temperatures higher than 2400F. The thermocouples are usually made of beads and splines over bare wire type because it is difficult to find sheath materials that remain flexible at these higher temperatures.
Relative life expectancy for both the platinum- and tungsten-grade thermocouples fabricated as mentioned above is low due to factors such as exposure to the vacuum furnace environment, high temperatures and the necessity of handling between cycles.
Another approach to work temperature measurement at the higher temperature ranges is to use long, rigid-construction thermocouples fed down through the hot zone into the workspace as shown in Fig. 3. The thermocouples in this example are type S material with alumina protection tubes, but this approach could be used with the other platinum thermocouple types and tungsten-rhenium material. These thermocouples must be removed from the workspace, either automatically or manually for load insertion and removal.
Placement and connections
Control and over-temperature limit thermocouples are usually located near the heating elements in the hot zone, but outside the boundaries of the workspace. The important thing to remember is that the location of the control thermocouple junction in relation to the heating elements affects the average temperature of the work (as measured by the work thermocouples). The junction of the thermocouple extends past the heating elements, the distance being determined by hot zone design. However, when running furnace surveys, the average temperature of the survey thermocouples can be increased by pulling the control thermocouple out, closer to the plane of the heating elements. The average temperature decreases if the control thermocouple is pushed farther into the hot zone.
Flexible work thermocouples are generally placed in the load but masked from seeing the direct radiation of the heating elements. Direct radiation tends to make the work thermocouples read incorrectly. The thermocouples are usually placed inside slugs or scrap parts with holes drilled in them to provide shielding. Keep in mind that when using thermocouples with twisted junctions, the measuring junction is not at the extreme end of the thermocouple, but is at the last twist before the wires go into the insulating material. This area must be shielded to get good readings.
Work thermocouples are frequently provided with quick-connects that plug into a jack panel located just outside of the hot zone (Fig. 4). This makes it very convenient to place the thermocouples in the load before it is put into the furnace and then plugging the thermocouples into the jack panel after the furnace is loaded. The one negative aspect to this approach is that the jack panel is usually located in an area where temperatures can range from 600 to 900F (315 to 480C) during operation. The jack panel selected must be rated for high-temperature operation. In addition, one must be careful to have no dissimilar metal junctions in this area, because the introduction of a secondary junction will create an error in the thermocouple reading.
In applications where contaminants are out-gassing from the load, the jack panel connection is subject to contamination, which can introduce errors into the measurement. In this case, the logical choice is to terminate the thermocouple outside the furnace through vacuum-tight fittings.
For platinum-type thermocouples, use of a jack panel within the furnace becomes more complicated. For most thermocouple types, jack panel sockets are made of the same material as the thermocouple being used; for example, chromel and alumel for a type K thermocouple. However, in the case of platinum type thermocouples (B, S and R), because of the softness and price of the material, sockets are made of copper instead of platinum. Copper works well and introduces only a small error into the reading as long as the temperature difference is less than 200F (110C). However, as mentioned above, in most furnaces the jack panel can see temperatures in excess of 600F. Therefore, the jack panels must be located in an area where the temperature does not exceed 200F if they are to be used to interface with platinum type thermocouples. Two alternatives are to bring the platinum wire through vacuum tight glands or to use rigid type platinum thermocouples as shown in Fig. 4.
Recommendations
- Use thermocouples housed in protection tubes for control and over-temperature limit because it eliminates exposure and potential contamination from the furnace environment.
- For work thermocouples, try to use the largest diameter possible to help extend service life.
- Calibrate thermocouples periodically to help minimize errors due to drift and degradation. Calibration is only accurate for the next cycle. Each time a thermocouple is cycled, its calibration can change.
With proper thermocouple selection and application, the user can obtain more accurate temperature measurements and more repeatable results in their processes. IH
