The life of the heater

    Resistance furnaces have various types of heating bodies (heaters). It converts electrical energy into thermal energy and transfers it to the heated workpiece by means of radiation, counterwashing or conduction. The heating body is one of the most important parts of the resistance furnace, and it determines the reliability of the resistance furnace to a large extent. The heating body is usually under relatively harsh temperature conditions, that is, it often works at the highest temperature allowed by the heating body material. Therefore, if the service life of other mechanisms and parts of the electric furnace (except the heat-resistant parts in the high-temperature zone) is calculated for many years, the heating body is an exception and often needs to be replaced. The short life of the heating body will cause the furnace to be shut down for repairs, thereby reducing the reliability of the electric furnace. At the same time, because some heating body materials are scarce and expensive, increasing the life of the resistance furnace heating body is an important task for electric furnace designers.

    The main factors affecting the life of the heating body are as follows:

    1. The highest temperature for long-term work. The temperature of the heating body in the electric furnace is higher than the given heating temperature of the workpiece, and the temperature difference is determined by the heat transfer conditions between them. When the heating body is calculated correctly, this temperature difference value should not be large, and when the calculation is incorrect, it may reach hundreds of degrees. Under high temperature working conditions, if the temperature of the heating body is increased by 30~50°C, the service life of the heating body will be significantly reduced.

    2. Oxidation of heating body. When the heating body is heated, a dense oxide film is formed on the surface of the material, which prevents the continuous oxidation of the inner layer of the material. Therefore, there is a certain temperature for each material-the maximum temperature allowed. Below this temperature, the heating body's oxidation or volatilization speed is extremely slow, and above this temperature, its oxidation or volatilization speed increases sharply, so the life of the heating body is drastically shortened. When the heating body is oxidized, the oxide film gradually thickens and the cross-sectional area of the material gradually decreases, causing the resistance of the heating body to gradually increase, and the power it emits gradually decreases, reducing to a certain percentage, that is, to replace the new heating body.

    3. Local heating. The oxidation or volatilization of the heating body increases its resistance, which is not a frequent reason for replacing a new heating body. It is often because of some local defects of the heating body, such as small cracks in the bend, impure oxide film and other similar conditions. Increase the resistance of these local locations, and form local overheating, causing the same adverse effects as mentioned above.

    If the local overheating position of the heating body is not large and its maximum temperature does not exceed the maximum allowable temperature value, this local overheating will not have a significant impact on the life of the heating body. Therefore, the working temperature of the heating body should be lower than the maximum temperature allowed. There must be a safety margin between them, which is greater than the possible local superheat value of the heating body. Otherwise, it is necessary to design a reasonable structure and choose a large cross-section material to reduce the local superheat value of the heating body.

    4. High temperature strength. The reason for the replacement of a new heating body is often due to its insufficient high temperature strength, which results in damage to the heating body. Under high temperature work, the heating body will be elongated or deformed. If the structure is unreasonable or the working temperature is too high, its mechanical strength will be reduced, and the deformation will even cause local short circuit and arcing, which will cause the heating body to be burned. Or as a result, the elongation makes the cross section of the heating body thinner and cannot be used continuously.

    Generally, in order to improve the reliability of the furnace and extend the life of the heating body, it is recommended that the working temperature of the heating body be lower than its allowable maximum temperature. However, in many cases, the heating body has to work at the maximum allowable temperature, although there is a risk of local overheating. This is because the selection of other more heat-resistant materials is not suitable or economical for the given conditions reason. When designing the structure of the heating body, it should avoid mechanical stress as much as possible. For example, the horizontally placed strip zigzag heating body and the wire spiral heating body are placed on the ceramic wire block, and the diameter and pitch of the spiral are restricted. The length of the hanging zigzag, etc.

    5. The working characteristics of the heating body. The more frequently the working temperature of the heating body changes, the faster its oxidation process will be. This is because when the heating body is heated and cooled frequently and periodically, the metal material itself and its outer surface oxide film have a different expansion coefficient, which often causes the latter to crack and form cracks. The oxygen in the air will enter the unoxidized metal layer along this crack to reach a deeper level of oxidation.

    6. The influence of furnace gas composition on the life of heating body. Although the heating body of the nickel-chromium system has strong rust resistance, its properties are not stable in an atmosphere containing sulfur (S and Ni will generate nickel sulfide with a melting point of about 800°C). Iron-chromium-aluminum alloy is relatively stable in a sulfur-containing atmosphere, but it is more sensitive to N2, NH3 and other gases and compounds. For example, when heated to high temperature in the air, materials such as tungsten and molybdenum are easily oxidized. Therefore, a protective atmosphere or vacuum must be used. However, when the temperature exceeds 1650°C, molybdenum volatilizes strongly in vacuum. Therefore, this factor must be considered when selecting the heating body material, especially when there is a protective atmosphere in the furnace.

    7. When the heating body (tungsten, molybdenum, tantalum) is in contact with some high-purity refractory oxides, chemical changes will also occur and affect its service life.

(Huiputech)