Catégories: Matériaux chauffants , Matériaux de résistance
When heated, resistance heating alloys form an oxide layer on their surface, which prevents further oxidation of the material. To accomplish this function the oxide layer must be dense and resist the diffusion of gasses. It must also be thin and adhere to the metal under temperature fluctuations.

Content:
Use Kanthal® alloys
Avoid temperature fluctuations
Choose thick element material
Adjust the element temperature to the furnace atmosphere
Avoid corrosion from solid substances, fluids and gasses

In these respects the aluminum oxide formed on Kanthal® alloys is even better than the oxide formed on Nikrothal® alloys, which contributes to the much longer operating life of Kanthal® heating elements. The diagram below shows the comparative element life. In this chapter you will find some general advice to obtain as long element life as possible.

Use Kanthal® alloys

Heating elements made of Kanthal® alloys have up to four times longer life than heating elements made of nickel-chromium material. The higher the temperature, the greater the difference.

Avoid temperature fluctuations

The operating life of the heating elements will be reduced by rapid temperature fluctuations. It is therefore advisable to choose an electric control equipment, which gives as even a temperature as possible, e.g. by using thyristors.

Comparative life (Kanthal® A-1 at 1200°C = 100%)

Comparative life.jpg

Choose thick element material

The material thickness has a direct relationship to the element life, in that, as the wire diameter is increased, more alloying element is available per surface unit to form a new oxide. Thus, at a given temperature, thicker wires will give a longer life than thinner wires. Accordingly, for strip elements, increased thickness gives a longer life. As a general rule, we recommend minimum 3mm wire diameter and 2 mm strip thickness.

Adjust The Element Temperature To The Furnace Atmosphere

The table shows some common furnace atmospheres and their influence on the maximum operating temperature of the heating elements. Nikrothal® should not be used in furnaces having a CO-containing protective gas atmosphere due to the risk of “green rot” at 800–950°C.

In such cases Kanthal® alloys are recommended, provided the heating elements are pre-oxidized in air at 1050°C for 7–10 hours. Reoxidation of the heating elements should be carried out at regular intervals.

Avoid Corrosion From Solid Substances, Fluids And Gasses

Impurities in the furnace atmosphere, for instance oil, dust, volatiles or carbon deposits can damage the heating elements. Sulphur is harmful to all nickel alloys. Chlorine in different forms will attack both Kanthal® and Nikrothal® alloys. Splashes of molten metal or salt may also damage the heating elements.

Maximum permissible temperatures in various atmospheres

Kanthal® A-1 and Kanthal® APM Kanthal® AF Kanthal® D Nikrothal®80 and 70 Nikrothal® 60 Nikrothal® 40
°C °C °C °C °C °C
Oxidizing:
Air, dry 1400a) 1300 1300 1200d) 1150 1100
Air, moist 1200 1200 1200 1150 1100 1050
Neutral:
N2, Nitrogenb) 1200 1250 1150 1250 1200 1150
Ar, Argon 1400a) 1300 1300 1250 1200 1150
Exothermic:
10 CO, 15 H2, 5 CO2, 70 N2 1150 1150 1100 1100c) 1100 1100
Reducing:
Endothermic:
20 CO, 40 H2, 40 N2 1050 1050 1000 1100c) 1100 1100
H2, Hydrogen 1400a) 1300 1300 1250 1200 1150
Cracked ammoniae):
75 H2, 25 N2 1200 1200 1100 1250 1200 1150
Vacuum:
10-3 dry 1150 1200 1100 1000 900 900

a) Max 1425°C for Kanthal APM
b) The higher values apply for preoxidized material
c) Please note risk of “green rot” formation in carburizing atmospheres. Use Kanthal® AF or Nikrothal® 70
d) 1250°C for Nikrothal 70
e) An atmosphere created by cracked ammonia, that contains uncracked ammonia, will lower the max. permissible temperature