Content:
Operating life and maximum permissible temperature
Furnace atmospheres
Ceramic support materials
Embedding compounds
OPERATING LIFE AND MAXIMUM PERMISSIBLE TEMPERATURE
When heated, resistance heating alloys form an oxide layer on their surface, which helps to prevent further oxidation of the material. For this protective function to be effective, the oxide layer must be dense to resist the diffusion of gases, thin to avoid adding bulk, and strongly adhere to the metal even under temperature fluctuations.
The aluminum oxide layer formed on Kanthal® alloys excels in these qualities compared to the oxide formed on Nikrothal® alloys, resulting in a significantly longer operating life for Kanthal® heating elements.
The diagram below illustrates the comparative element lifespans.
This chapter offers general guidance on maximizing the operating life of heating elements.
Use Kanthal® alloys
Heating elements made from Kanthal® alloys can last up to four times longer than those made from nickel-chromium materials. This advantage becomes more pronounced at higher operating temperatures.
Avoid temperature fluctuations
Rapid temperature fluctuations can reduce the operating life of heating elements. To minimize this effect, it is recommended to use electric control equipment that maintains a stable temperature, such as thyristors, which provide smooth and continuous control.
Choose thick element material
The thickness of the element material directly affects its lifespan. As the wire diameter increases, more alloying material is available per surface unit to form a protective oxide layer, resulting in longer element life at a given temperature. Consequently, thicker wires offer a longer lifespan than thinner ones. Similarly, for strip elements, increasing thickness enhances their durability. As a general guideline, a minimum wire diameter of 3 mm and a strip thickness of 2 mm is recommended to maximize element life.
Adjust the element temperature to the furnace atmosphere
The table below shows common furnace atmospheres and their impact on the maximum operating temperature of heating elements. Nikrothal® should not be used in furnaces with a CO-containing protective gas atmosphere, as this can lead to “green rot” at temperatures between 800–950°C (1,472–1,652°F). In these situations, Kanthal alloys are recommended, provided the heating elements are pre-oxidized in the air at 1,050°C (1,922°F) for 7–10 hours. Reoxidation of the heating elements should also be performed at regular intervals.
Avoid corrosion from solid substances, fluids, and gases
Impurities in the furnace atmosphere, such as oil, dust, volatile compounds, or carbon deposits, can cause damage to heating elements. Sulfur is harmful to all nickel-based alloys, while chlorine, in any form, will attack both Kanthal® and Nikrothal® alloys. Additionally, splashes of molten metal or salt can also lead to damage to the heating elements.
Numerous practical applications also show a much longer life of Kanthal® elements.
FURNACE ATMOSPHERES
The lifespan of a resistance heating element relies on the continuous presence of a dense oxide layer that fully covers the element’s surface. Corrosion occurs when specific compounds in the furnace atmosphere interfere with the formation or replenishment of this oxide layer. The greater the interference, the shorter the element’s life, and the impact of corrosive compounds is often dependent on temperature.
Air
The ability of resistance alloys to function in air at high temperatures depends entirely on the protective oxide layer formed on their surface. However, impurities in the air, such as fumes, gases, dust, and other contaminants from the furnace batch or insulation, can disrupt oxide formation. Poor ventilation may cause gases to escape along the terminals, leading to excessive corrosion and premature failure.
Under normal operating conditions, Nikrothal® alloys have a higher tendency for oxide spalling than Kanthal® alloys, which can be an issue when heat- ing materials with sensitive surfaces, such as white porcelain. Additionally, ceramic supports can become contaminated, potentially causing creep currents that lead to premature element failure.
Controlled atmospheres
In carbonaceous atmospheres, whether endothermic or exothermic, the alumina layer on Kanthal® alloys provides effective protection against the active components of these gas mixtures. Pre-oxidizing the elements in the air at 1,050°C (1,920°F) for seven to ten hours can significantly extend their life in these “protective” atmospheres. For maximum lifespan, the elements should be re-oxidized periodically based on operating conditions.
In contrast, the protective layer on Nikrothal® 80 Plus is not effective in exothermic and endothermic atmospheres; instead, selective chromium oxidation along the grain boundaries (“green rot”) occurs, especially at low oxygen potential and element temperatures of 500-950°C (932-1,742°F). In such cases, Kanthal® alloys are recommended.
Hydrogen and nitrogen atmospheres
Pure hydrogen does not harm Kanthal® or Nikrothal® alloys, but service life can be shortened if the gas mixture contains uncracked ammonia.
Very dry nitrogen, lacking in oxygen, can lead to the formation of aluminum nitride, limiting the maximum permissible temperature 1,050°C (1,920°F) for Kanthal® A-1 and 1,100°C (2,012°F) for Kanthal® AF. Conversely, the strong affinity of these alloys for oxygen can inhibit nitride formation in atmospheres of technically pure nitrogen, which typically contains some oxygen.
Kanthal® AF remains relatively stable in a pure nitrogen atmosphere at temperatures up to 1,250°C (2,280°F), provided controlled pre-oxidation is performed at the service temperature.
Vacuum
In a high vacuum, the oxide layer on Nikrothal® alloys decomposes at temperatures above 1,000°C (1,830°F), and the alloy components may vaporize, depending on the pressure and temperature.
In contrast, the protective oxide on Kanthal® alloys is more stable, and pre-oxidized elements can be operated at lower pressures and higher temperatures. At 5 × 10-4 torr and 1,100°C (2,010°F), Kanthal® elements have an excellent lifespan. However, if the element temperature reaches 1,150°C (2,100°F), it should be re-oxidized after 250 service hours; at 1,250°C (2,200°F), re-oxidation is needed after 100 hours (or at 1,050°C (1,920°F) after 5 hours).
| Atmosphere |
KANTHAL® A-1 AND |
KANTHAL® AF °C (°F) |
KANTHAL® D °C (°F) |
NIKROTHAL® 80 °C (°F) |
NIKROTHAL® 70 °C (°F) |
NIKROTHAL® 60 °C (°F) |
NIKROTHAL® 40 °C (°F) |
| Oxidizing | |||||||
| Air, dry | 1,400* (2,550) | 1,300 (2,370) | 1,300 (2,370) | 1,200 (2,190) | 1,250 (2,280) | 1,150 (2,100) | 1,100 (2,010) |
| Air, moist** (3% H2O) | 1,200 (2,190) | 1,200 (2,190) | 1,200 (2,190) | 1,150 (2,100) | 1,150 (2,100) | 1,100 (2,010) | 1,050 (1,920) |
| N₂, Nitrogen*** | 1,200/1,050 (2,190/1,920) | 1,250/1,100 (2,280/2,010) | 1,150/1,000 (2,100/1,830) | 1,250 (2,280) | 1,250 (2,280) | 1,200 (2,190) | 1,150 (2,100) |
| Ar, Argon | 1,400 (2,550) | 1,400 (2,550) | 1,300 (2,370) | 1,250 (2,280) | 1,250 (2,280) | 1,200 (2,190) | 1,150 (2,100) |
| Exothermic: 10% CO, 15% H₂, 5% CO2, 70% N2**** |
1,150 (2,100) | 1,150 (2,100) | 1,100 (2,010) | 1,100 (2,010) | 1,100 (2,010) | 1,100 (2,010) | 1,100 (2,010) |
| Reducing: | |||||||
| Endothermic: 20% CO, 40% H₂, 40% N2**** |
1,050 (1,920) | 1,050 (1,920) | 1,000 (1,830) | 1,100 (2,010) | 1,100 (2,010) | 1,100 (2,010) | 1,100 (2,010) |
| H₂, Hydrogen | 1,400 (2,550) | 1,400 (2,550) | 1,300 (2,370) | 1,250 (2,280) | 1,250 (2,280) | 1,200 (2,190) | 1,150 (2,100) |
| 75%H2, 25%N2***** | 1,200 (2,190) | 1,200 (2,190) | 1,100 (2,010) | 1,100 (2,010) | 1,100 (2,010) | 1,100 (2,010) | 1,100 (2,010) |
| Vacuum | |||||||
| 10-3 torr | 1,150 (2,100) | 1,200 (2,190) | 1,100 (2,010) | 1,100 (2,010) | 1,100 (2,010) | 1,000 (1,830) | 900 (1,650) |
** Maximum temperature for the Nikrothal® alloys will decrease with increasing water content and flowrate of gas.
*** The higher values apply for preoxidized material.
**** Please note the risk of "green root" corrosion on Nikrothal® alloys in carburizing atmospheres. Use of Kanthal alloys is preferred.
***** Ammonia or ammonia containing atmospheres will have a lower maximum permissible temperature.
CERAMIC SUPPORT MATERIALS
For electric furnaces, special consideration must be given to the ceramic supports that contact the heating elements directly. Firebricks used for element support should have an alumina content of at least 45%. In high-temperature furnaces, sillimanite or high- alumina firebricks are often recommended. The free silica (uncombined quartz) content should be minimized, as silica may react with the surface oxide at high temperatures. Iron oxide (Fe2O3) content should be kept as low as possible, preferably below 1%, and alkali oxides (Na2O, K2O, etc.) should remain below 0.1%.
Water glass, often used as a binder in cement, can negatively affect resistance heating materials and should be avoided.
Leakage and creep currents at high temperatures can attack contact points between the ceramic support and the heating element, potentially leading to premature failure. Therefore, support materials must have high insulating resistance at the service temperature.
EMBEDDING COMPOUNDS
Most embedding compounds, including ceramic fibers, are suitable for Kanthal® and Nikrothal® if composed of alumina, alumina-silicate, magnesia, or zirconia, and if the guidelines under “Ceramic support materials” are followed. Generally, commercially available products meet these criteria. When moistened cement is used with Kanthal® alloys, such as in heating panels, immediate drying is crucial to prevent corrosion from sulfuric impurities. Distilled water is preferred as a moistening agent because fluorinated or chlorinated tap water can cause corrosion. Likewise, degreasing solvents containing chlorine must be completely removed after cleaning element coils.
Certain cements can attack resistance heating materials. In enclosed environments, even traces of sulfur-containing contaminants can severely damage Nikrothal® wires at elevated temperatures. Boron com- pounds can attack both Kanthal® and Nikrothal® alloys at temperatures above 900°C (1,650°F).
Corrosion tests for embedding compounds should always be conducted before their use is specified.