What can happen – Or what should I do if ….

Modern high-temperature applications have to meet the highest demands and withstand the harshest conditions. Regardless of the manufacturer, different phenomena can occur during the operation of electric heating elements made of molybdenum disilicide (MoSi₂). With our heating element FAQ we explain some phenomena.

Heating elements lengthen.

This is a normal reaction. As a rule, the lengthening involves rather larger heating elements with a heating zone (Le) of approx. >500mm. In general the phenomenon does not lead to any problem during operation. As soon as the elements threaten to touch the furnace bottom, they should be replaced, otherwise the insulation will be damaged.

Heating elements become round bodied

It is also a normal reaction, which is due to electric fields in terms of attraction and repulsive forces. A slightly round bodied deformation does not generally effect the performance or function. It is advisable to check the circuit (series or parallel circuit) and the connections.

Spalling on the heating elements

Spalling often occurs in reducing atmospheres. This is caused by the fact that the heating elements cannot form a new protective SiO₂ layer. You have two solutions, if you notice that phenomena:

• A regeneration firing in oxidizing atmosphere at temperature of 1450°C for 5 hours without a product inside the furnace;
• You will use an element with a thicker protective SiO₂ We would be glad to advise you.

Heating elements crack (mechanical crack)

This phenomenon happens in various ways. One possibility results in an user error and occurs, when too much force is applied to fix the heating elements with the contact straps or the double holders. Finesse is needed in the truest sense of word. Furthermore it is necessary that the heating elements are really straight installed through the furnace roof into the furnace chamber. A slightly tilting can already lead in a mechanical crack.

Powdery disintegration of heating elements – „MoSi₂-pest“

"MoSi₂-pest" is an effect occurring in the temperature range between approx. 300°C and 700°C. In this temperature range, heating elements of molybdenum disilicide (MoSi₂) sometimes show strong oxidation with powdery disintegration of the material. One possible cause of this is in the intercrystalline disintegration that is facilitated by porosity and the structure. This disintegration may be prevented by a high density and a very slow porosity. In processes, where longer holding times at low temperatures are needed, a heating element must meet special requirements. We will be happy to advice you.

Corrosion in cone and/or in the heating zone (Le)

Heating elements can be operated in different maximum application temperatures depending on their quality. A corrosion in the cone and/or in the heating zone (Le) happen, when the maximum application temperature prescribed for the heating element is regularly exceeded and thus it is operated at excessively elevated temperatures. In this case please contact the manufacturer or supplier ask for advice.