By Dan Herring

Fig. 1. Curved molybdenum heating elements

This is the ninth in a series of articles in our Vacuum Heat-Treatment Series. This part talks about heating elements used in vacuum furnaces, the materials and temperatures of operation, forms and maintenance practices. The design and location of the heating elements is critical to achieve proper heating and uniformity of temperature.

Almost all high-temperature vacuum furnaces are electrically heated. Resistance heating elements are constructed from metal or graphite in a variety of styles. In general, one of the following materials is used:

• Stainless steel alloys – 300 series alloys (e.g., 304L, 316L) can be used for heating elements up to approximately 760°C (1400°F).
• Nickel/chromium and iron-aluminum based alloys – These typically operate up to temperatures of 1150°C (2100°F) and exhibit good-to-excellent oxidation resistance, making them useful for a number of applications including hot wall-type furnaces.
• Inconel® and other nickel alloys – Depending on material and vacuum level, they can be used up to 1150°C (2100°F). Above 800°C (1475°F), there is a risk of evaporation of chromium from these materials.
• Silicon carbide (SiC) – These elements have a maximum operating temperature of 1090°C (2000°F). There is a risk of evaporation of silicon at high temperatures and low vacuum levels of less than 0.133 mbar (100 microns).
• Molybdenum – With a maximum operating temperature of 1700°C (3100°F), molybdenum becomes brittle at high temperature and is sensitive to changes in emissivity brought about by exposure to oxygen or water vapor.
• Graphite – These elements can be used up to 2000°C (3630°F). Graphite is sensitive to exposure to oxygen or water vapor, resulting in reduction in material thickness. The strength of graphite increases with temperature.
• Tantalum – Elements made of tantalum have the highest duty temperature, typically 2400°C (4350°F). Tantalum, like molybdenum, becomes brittle at high temperatures and is sensitive to changes in emissivity brought about by exposure to oxygen or water vapor.

Note: The above element ratings are downgraded from their upper operating limits (Table 1).

Fig. 2. Curved graphite heating elements

The choice of a heating-element material depends largely on operating temperature. For low-temperature operations such as aluminum brazing or vacuum tempering, inexpensive stainless steel or nickel-chromium alloys can be used for the heating elements. For higher-temperature general heat-treating applications such as hardening or brazing, molybdenum or graphite are popular choices for element materials. For specialized heat-treating applications above about 1482°C (2700°F), a refractory metal such as tantalum is a popular choice, though graphite is also used. Other processes such as low-pressure vacuum carburizing use graphite or silicon carbide elements.

For many years, molybdenum elements (Fig. 1) were used almost exclusively in vacuum furnaces for aersopace heat-treating and brazing applications. There was a widespread concern that contamination from graphite elements could react with certain gases (such as hydrogen) during partial-pressure or quenching operations and contaminate certain materials. In addition, early graphite-element designs were cumbersome and limited choices to certain simple shapes. Furthermore, connections between element segments and electrical feed-throughs were prone to failure. With advances in both materials and manufacturing techniques for graphite-based electrical products, however, the popularity of graphite heating elements now exceeds that of molybdenum elements in general heat-treating and brazing furnaces. The most widely used graphite-element design incorporates either lightweight, curved bands (Fig. 2) or segmented bars (Fig. 3).

Fig. 3. Segmented graphite heating elements

Refractory metals (Mo, W, Ta) and molybdenum in particular are popular choices for heating elements. In sheet form, the watt density of the radiating surface is low compared with cylindrical rod, allowing lower operating temperatures. The trade-off is in mechanical strength. As such, good supports and restraining systems are necessary. Molybdenum also undergoes changes in electrical resistance. Therefore, the design of the power system must control current during the onset of heating to avoid damage to the elements, and the heating rate must be limited and controlled. Metallic elements are typically available in strip, wide band, coil (wire), ribbon or rod form.

Graphite is an excellent choice for heating elements. It's lightweight, strong at temperature and stronger at higher temperature, and has a high melting point and a low vapor pressure. In addition, graphite exhibits low contact resistance at internal connections and power feed-throughs, has excellent thermal shock properties, is not degraded by constant heating and cooling, and has a low heat expansion coefficient.

Graphite has the ability to take very high current density, and therefore very fast ramp-up times can be achieved. Graphite elements can operate in very corrosive or aggressive atmospheres without significant degradation. The low resistivity of graphite means it requires high-current power supplies and correspondingly large feed-throughs and cables. Graphite also acts as a getter to oxygen although it is attacked and consumed in the process (forming CO and CO2 gas). Graphite heating elements can be supplied in rod, tube, bar, plate, circular shapes or cloth form.

Carbon-carbon-composite (CFC) materials can also be used as heating elements and can be made into very thin sections, typically as thin as 1-mm (0.04”) thick, due to their fibrous grain structure. CFC elements have higher resistance than graphite elements, allowing lower-current, higher-voltage power supplies to be used. They also have extremely low thermal conductivity, which reduces heat loss.

Silicon carbide heating elements are typically supplied in bar form (electrically heated units) or tubes (gas-fired units).

Fig. 4. Flat nickel strip heating elements

Heating elements used in vacuum furnaces are typically resistance type and do not require the oxidation-resistant properties of their atmosphere furnace cousins. To improve operating efficiency, however, elements used in vacuum run at higher temperatures and often operate at a higher watt density (watts/inch2 or watts/mm2) since the transfer of radiant heat energy is a T4 relationship, which follows the Stefan-Boltzmann Law (Equation 1). This states that the total energy radiated per unit surface area of a black body in unit time, j*, is directly proportional to the fourth power of the black body’s thermodynamic (or absolute) temperature, T.

Equation 1.  j* = s T4

where s is a constant of proportionality = 5.6704 x 10-8 J s-1 m-2 K-4

It is also important that the heating elements used have a low vapor pressure (Table 1) to ensure long life at elevated temperature. Depending on the design of the hot zone (cylindrical or rectangular), heating elements can be placed circumferentially in a 360° pattern, on just the two sidewalls, on the top and bottom as well as the sidewalls, or with the use of end elements on all six sides.

Other Considerations

Fig. 5. Replacing a molybdenum element section at or near a standoff

The use of low voltage in the vacuum chamber prevents short-circuiting of the heating elements due to ionization from residual gases within the chamber.

Uniformity of temperature is also of great importance to heat-treatment results. The construction of the heating system should be such that temperature uniformity in the load during heating is optimal; it should be better than ± 5.5°C (±10°F) after temperature equalization. This is realized with single or multiple temperature control zones and a continuously adjustable supply of heating power for each zone.

In the lower-temperature range (below 850°C), the radiant heat transfer is low and can be increased by convection-assisted heating. For this purpose, after evacuation the furnace is backfilled with an inert gas up to an operating pressure of 1-2 bar, and a built-in convection fan circulates the gas around the heating elements and the load. In this way, the time to heat different loads, especially those with large cross-section parts to moderate temperatures, can be reduced by as much as 30-40%. At the same time, the temperature uniformity during convection-assisted heating is much better, resulting in less distortion of the heat-treated part.

Fig. 6. Replacing a molybdenum element damaged between standoffs

Maintenance

Graphite heating elements should not be exposed to air until the element temperature is less than 260ºC (500ºF). Elements that have been attacked by air exhibit a classic “sugar cube” surface appearance and often lose cross-sectional area, causing resistance change and (over) heating issues. In general, graphite elements cannot be repaired if broken, damaged or severely attacked by oxygen or other contaminants. Damaged sections must be replaced by a new element section. Elements can be wiped off and cleaned of debris using a shop-vac.

Molybdenum and other metallic-type heating elements can be repaired as long as no more than three repairs should be attempted in any one heating element (Fig. 5, 6)

Daniel H. Herring / Tel: (630) 834-3017) /E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.

References

1. Pritchard, Jeff, "Hot-Zone Design for Vacuum Furnaces," Industrial Heating, 2007
2. Byrnes, Edward R., and Roger C. Anderson, Heat Treating in Vacuum Furnaces, 1983
3. Craig, Richard A., "Vacuum Furnace Maintenance," Vacuum Heat Treating SME Technology and Applications Conference, February 1999.