Vacuum Heat Treatment Furnace for Graphite Electrothermal Elements
With the improvement of manufacturing level of vacuum heat treatment furnace, vacuum heat treatment has gradually shown incomparable advantages. Vacuum heat treatment has gained people's favor by virtue of a series of advantages such as degassing, degreasing, non-oxidation and automation. However, there are still many problems in the electrothermal components of vacuum heat treatment furnace, such as high temperature deformation, fracture, volatilization and so on, which become obstacles to the development of vacuum furnace.
In order to solve these problems, graphite stands out with its unique advantages. At present, electrothermal elements made of graphite are used in almost all types of vacuum heat treatment furnaces. In this paper, the advantages and disadvantages of graphite as an electrothermal element are scientifically evaluated by combining theoretical analysis with simulation analysis, which provides a basis for the design of vacuum electrothermal elements.
1. Analysis of Performance Factors of Electrothermal Components
Vacuum heat treatment is different from ordinary electric heat treatment. It carries out heat transfer by radiation. The phenomena of electric heating elements in vacuum environment are different from those of ordinary electric heating furnace, which puts forward higher requirements for vacuum electric heating elements. The performance requirements of vacuum electrothermal element materials can be summarized as follows:
(1) High resistivity, once the vacuum heat treatment furnace is installed, the terminal voltage of the electrothermal element is generally unchanged, and high resistivity can obtain stable power and heating speed.
(2) The vacuum heat treatment furnace is an automatic and controllable equipment with smaller resistance temperature coefficient, which can reduce the difficulty of design and save cost.
(3) In order to prolong the service life of the electrothermal components, reduce the number of maintenance and replacement, and reduce the cost, it is required that the electrothermal components have smaller thermal expansion coefficient.
(4) Good machinability and high temperature mechanical strength;
(5) Ensure that the electrothermal components do not react chemically with the protective atmosphere, lining and supporting parts in the furnace at high temperature.
2. Performance comparison and analysis of common materials
The material of electrothermal element is divided into metal material and non-metal material. Metallic materials include molybdenum, tungsten, tantalum, nickel-chromium alloy, non-metallic materials include graphite, silicon carbide, molybdenum dioxide and so on. In this paper, molybdenum, tungsten, tantalum and graphite of non-metallic materials are compared and analyzed.
Different materials and resistivity will change with the change of temperature.
When designing the electric heating elements of vacuum furnaces, in order to ensure the total power of the electric heating elements of vacuum furnaces, enable them to rise rapidly, facilitate the design, installation and work stably, we require that the resistance of the electric heating elements change little with the temperature, that is to say, the resistivity is stable.
With the increase of temperature, the resistivity of molybdenum, tungsten and tantalum increases sharply and varies greatly, while that of graphite varies slightly with the increase of temperature, ranging from 10% to 20%. It can be seen that graphite has obvious advantages and stable resistivity compared with other three materials.
(2) Resistance temperature coefficient
Resistance temperature coefficient is another important parameter affecting material resistance changing with temperature. Its value directly affects the resistance value of electrothermal components at different temperatures. The larger the resistance temperature coefficient is, the greater the change of resistance will be with the increase of temperature, thus seriously affecting the stability of power, thus making the work of electrothermal components extremely unstable. The resistance temperature coefficients of graphite, molybdenum, tungsten and tantalum are 126 * 10 ^ - 5 (?), 471 * 10 ^ - 5 (?), 482 * 10 ^ - 5 (?) and 399 * 10 ^ - 5 (?), respectively. It can be seen that the resistance temperature coefficients of graphite are the smallest, while the resistance temperature coefficients of molybdenum, tungsten and tantalum are 3 - 4 times as high as that of graphite. Thus, the electrothermal elements made of graphite can obtain stable working performance.
(3) Coefficient of thermal expansion
In the design of electric heating element of vacuum heat treatment furnace, thermal expansion and contraction is an important factor, because it directly affects the service life and performance of electric heating element. For example, in the design of electric heating element, there is no space reserved for thermal expansion and contraction design or too small space reserved, which will cause the electric heating element to bear greater thermal stress after being heated, or the electric heating element will be directly forced to fracture, loss. Bad electrothermal components; on the contrary, too much reserved space will lead to the difficulty of fixed installation and assembly and replacement of electrothermal components. Therefore, the thermal expansion coefficient of electrothermal components is required to be as small as possible and stable.
(4) machinability and high temperature mechanical strength
Molybdenum, tungsten and tantalum are all metal electrothermal materials. They all have hard texture. The hardness of molybdenum is 5.5HB, tungsten is 5-5.5HB, and the hardness of tantalum is 6-6.5HB. It is obvious that the processing difficulty is very great. With the increase of temperature, the hardness decreases, and the high temperature mechanical strength gradually becomes worse. Graphite is a kind of non-metallic electrothermal material with soft texture and hardness of 1~2HB. It is very easy to be processed and moulded. The mechanical strength of graphite under 2500 (?) C increases continuously with the increase of temperature, and reaches the best value at 1700~1800 (?) It can be seen that the machinability and high temperature mechanical strength of graphite are better than the other three, which is one of the reasons why graphite becomes the preferred material for electrothermal components.
(5) Other factors
Vacuum heat treatment furnace is a kind of equipment heated by radiation, so the radiation performance of the electrothermal element directly determines the efficiency and performance of the electrothermal element, and the radiation ability depends on the blackness of the material. The higher the blackness, the stronger the radiation ability, and vice versa, the weaker the blackness of graphite is 0.95, molybdenum is 0.1-0.3, tungsten is 0.03-0.3, tantalum is 0.2-0.3, graphite is the largest.
In addition, all kinds of materials will accelerate evaporation in vacuum environment, and with the increase of vacuum degree, the degree of evaporation will increase correspondingly, forming the atmosphere environment of their own substances, limiting the scope of application of electrothermal elements, such as graphite electrothermal elements volatilization in vacuum environment will form a carbon environment, requiring that heat treatment that cannot carburize or react with carbon can not be in such a furnace. Conduct.