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Electric heating element for vacuum heat treatment furnace
With the improvement of the manufacturing level of vacuum heat treatment furnace, vacuum heat treatment gradually shows incomparable advantages. Vacuum heat treatment is favored by people with a series of advantages such as degassing, degreasing, non oxidation and automation. The electric heating element of vacuum heat treatment furnace, as a kind of wearing part, will have faults such as high temperature deformation, fracture, volatilization, etc. in long-term use. These failures require the user to purchase appropriate heating elements for replacement, and the following will introduce the electric heating elements of the vacuum furnace.
Performance index of electric heating element
Vacuum heat treatment is different from ordinary electric heat treatment. It conducts heat transfer through radiation. The phenomenon of electric heating elements in vacuum environment is also different from that of ordinary electric heaters, which puts forward higher requirements for vacuum electric heating elements. The performance requirements of vacuum heating element materials can be summarized as follows:
(1) With high resistivity, once the vacuum heat treatment furnace is installed, the terminal voltage of the electric heating element is generally unchanged, and stable power and heating rate can be obtained with high resistivity.
(2) With small resistance temperature coefficient, vacuum heat treatment furnace is an automatic and controllable equipment, which can reduce the design difficulty and save costs.
(3) In order to prolong the service life of the electric heating element, reduce the number of maintenance and replacement, and reduce the cost, the electric heating element is required to have a smaller thermal expansion coefficient.
(4) Good machinability and high temperature mechanical strength.
(5) Ensure that the electric heating element will not react with the protective atmosphere, furnace lining and supporting parts in the furnace under high temperature.
Common types of electric heating elements
The materials of electric heating elements are divided into metal materials and non-metallic materials. Metallic materials include molybdenum, tungsten, tantalum, nickel chromium alloy, etc. Non metallic materials include graphite, silicon carbide, molybdenum dioxide, etc. Molybdenum, tungsten and tantalum can be used as heating elements in high temperature vacuum furnaces or shielding gas high temperature furnaces. Molybdenum is a rare metal with high melting point and is very brittle. Tungsten is a refractory rare metal with high hardness, difficult processing and high price. The maximum service temperature of molybdenum disilicide rod is 1700 ℃, and that of zirconia is 1800 ℃. Both of them are used in air. Among non-metallic materials, graphite stands out with its unique advantages. Electric heating elements made of graphite are used in almost all types of vacuum heat treatment furnaces.
Graphite electric heating element
Graphite electric heating element has the advantages of stable resistivity, low resistance temperature coefficient, small thermal expansion coefficient and large blackness, making it the first choice of electric heating element materials. However, due to the low hardness of graphite at room temperature, the graphite electric heating element is easy to be broken and damaged at room temperature, and should not be used in the heating furnace where it is difficult to replace the electric heating element. In addition, graphite is easy to volatilize in a vacuum environment to form a carbon environment, so some materials cannot be processed in such a vacuum furnace. At the same time, special attention should be paid to prevent the support and connection from reacting with graphite when selecting them.
Despite the above limitations, graphite, as a material with rich reserves and low price, is much cheaper than molybdenum, tungsten and tantalum. Low price makes it widely used in vacuum furnace equipment.
Selection of vacuum furnace equipment: heating elements, as key components of vacuum furnace, are more likely to cause various problems if they are not well designed and installed. The vacuum heat treatment furnace produced by SIMUWU has passed excellent design and a lot of practical experience. It is proved that it is a high-quality product for vacuum heat treatment of tools and dies, and good temperature control accuracy and uniformity ensure that the vacuum heat treatment process can be effectively carried out. SIMUWU focuses on the manufacturing of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum gas quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.
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