The Application Research of the Graphite Heating Elements to the Vacuum Furnace

On the basis of a lot of excellent characteristic of graphite material, different kinds of construction type, some points for attention inthe design anduse were analysed, for the purpose popularization application of graphite heatingelements to the vacnnm furnace to provide referance.
Graphite has the characteristics of high temperature resistance, small thermal expansion and strong thermal shock resistance. At room temperature, the strength of graphite is worse than that of metals, but the mechanical strength of graphite increases with the increase of temperature below 2500 ℃, The optimum temperature is between 1700 ℃ ~ 1800 ℃, exceeding all oxides and metals. Graphite material has high melting point and low vapor pressure, and the atmosphere in vacuum furnace will contain low concentration of carbon, which will react with O2 and H2O vapor molecules in the residual gas to produce purification effect. Even at low vacuum, the treated workpiece can obtain a bright surface state, greatly simplifying the vacuum system and reducing the cost. This is unmatched by any metal electric heater.
Graphite, as a heating element of vacuum resistance furnace, is more and more popular in heat treatment industry because of its excellent properties. Graphite is widely used in high temperature resistance furnace.
Structural forms of various graphite heating elements
Single rodlike
The simplest single-phase electric heater, the current directly from the two ends of the graphite rod. In order to increase the calorific value, the graphite rod can be made into hollow, which can effectively utilize electric energy because it does not need to heat the central part.
Because of the limitation of the graphite blank, it is impossible to make the joint part and the working part as a whole, but it is divided into several parts to make, and then joined together with nuts. It should be noted that due to the defects of porosity, impurities and grains in graphite material, coarse thread should be selected for the connection. The inner diameter of the screw should be larger than the diameter of the working part of the heater, and the length of the carved thread should be 1.5 times the diameter of the screw, so as to ensure sufficient conductive area at the joint, especially in the case of high current. To improve contact between threaded connection and electrode joint. first coated with graphite paste on the tightening parts to increase the conductive area.
Tubular heater
The tubular heater is divided into single and three phases. The single-phase tubular heater is cut from one end of the tube into two equal parts, and the other is not cut. The heating length is doubled, thus increasing the resistance. In order to increase the strength of the heater, the thickness of the heater at the cutting end is thickened. The heat resistance is equal to the sum of the two parts of the resistance.
A three-phase tubular heater, similar in structure to a single phase, is cut into three equal parts from one end of the tube and not at the other end. It is divided into three parts by three clearances with the same resistance connected by a star at the end. If necessary, one or more long slots can be opened on each phase to increase the resistance value according to the need of resistance value.
Combined rod heating body
Tube heater is mostly used in resistance furnace with cylindrical workroom. Due to the limitation of material size, it is only suitable for small high temperature vacuum furnace. Due to the limitation of raw material size, tube heater can not meet the needs of large-scale heat treatment furnace, and the overall processing cost is not only high, the use of local damage in the process of scrap, the use of large costs. The newly developed combined rod heating body is suitable for vacuum furnaces with temperatures below 1500℃. The structure uses graphite rod as heating element and is assembled by several components, so it is not restricted by the size of raw materials, easy to process, and greatly saves manufacturing cost. If parts are damaged due to collision, oxidation, aging and other reasons in the course of use, local parts can be replaced, easy maintenance, greatly reducing the use cost. The whole structure is mainly composed of graphite heating rod, graphite taper jacket, graphite conductive ring, 99 porcelain insulating connecting ring, graphite bolt and so on. The graphite heating rod is a heating element, which is fixed on the graphite conductive ring at the upper and lower ends through a graphite taper jacket. Graphite conductive ring and 99 porcelain insulating connecting ring are connected by graphite bolt to form two complete rings. By arranging the number and position of graphite heating rod, graphite conductive ring and 99 porcelain connecting ring reasonably, the graphite heating rod can form a series-parallel circuit which is needed. Finally, the electrode leads the rod to connect with the external power supply. To achieve the purpose of heating.
Plate heating body
Plate heating body is a new heating element used in recent years. It has the advantages of convenient manufacture, large radiation area and high heat stress.
Flat-plate heater is mainly used in the square chamber. According to the size of the effective heating zone, it can be made up of one or several pieces in series and parallel. When making, the graphite billet is processed into a plate shape, and then grooves on the top to form a loop. According to the requirement of furnace temperature uniformity, the flat heating body is arranged on the two sides, the top and the bottom of the furnace to form a square studio, which can realize the purpose of multi-faceted heating.
Graphite cloth and graphite belt heater
In recent years, heating elements made of graphite fibers braided into graphite cloth or graphite ribbon can be powered by single-phase and three-phase power supply in parallel with multiple ribbons. Compared with graphite, graphite cloth has the characteristics of small thermal inertia, no deformation, good flexibility and so on, and has been widely used. The size of graphite fiber braiding can be specially braided according to the requirements of resistivity and furnace space layout.
Conditions for the use of graphite heater
Graphite heater is usually made of high-quality graphite or three-high graphite. The resistivity of graphite billet produced by different manufacturers varies greatly, usually from 8~13 Ωmm2/ m. It can be calculated temporarily by 10Ωmm2/m. After the manufacturer is determined, the electrical parameters of transformer can be corrected appropriately to make the heating system work. The power meets the design requirements. The heating element was knitted with graphite fiber and the resistivity was 4.7Ωmm2/m.
(1) The service life of graphite heater in resistance furnace depends on its oxidation and volatilization rate, surface power, maximum operating temperature, etc. When the vacuum degree is 10-1~10-3 Pa, the working temperature should be below 2200℃. If the heater needs to work above 2200 C, the furnace must maintain a low vacuum or put protective gases into the furnace, such as H2, N2, CO2, Ar and other gases, resulting in a certain pressure to reduce volatilization, so that the use of temperature can reach about 3000℃.
(2) The allowable surface load of graphite heating element is very large, about 40-60W/cm3. Generally, the radiation area of graphite rod, tube and plate is very large, and the surface load value is very low. Therefore, the load limit can not be considered in the design of graphite heating element.
(3) Graphite has good thermal conductivity at low temperature, and it decreases to a fraction of low temperature at high temperature. Therefore, the temperature difference between the center and the outer surface of the heater results in mechanical stress inside the heater. Therefore, the wall thickness of the heater is usually 8-16 mm, and the thickness of the thickened part is 16-30 mm. For solid rod, when the diameter of graphite heating rod exceeds 12 mm, it is better to use graphite tube instead, which can not only increase the resistance value, increase the radiation area, improve the thermal efficiency, but also overcome the defects of the solid rod which is easy to damage the electric heating element when the temperature difference between the center and the outside is too large.
(4) Graphite heating element, as an electrothermal element in vacuum furnace, should not be selected too high voltage, otherwise it will lead to vacuum discharge or glow discharge in the furnace, resulting in damage to the electrothermal element; if the voltage is too low, it will increase the current of the electrothermal element, resulting in difficult and complex connection structure of the electrothermal element, and The electrical loss is increased. Therefore, the voltage of the element should be below 200V. According to the operating temperature and furnace atmosphere, the voltage should be between 170 and 30V.
(5) The resistivity of graphite electrothermal element varies little with temperature, so the temperature coefficient of resistance is not considered generally. The transformer with fixed value can be used according to the voltage and power.
In terms of some properties of graphite, it is like both metal and ceramics. It is like metal because of its high thermal conductivity and electrical conductivity, and its processing performance is very good. Graphite has a higher resistance, so it can be used in the case of large cross-sectional area of heating body, low voltage and high current power supply, easy to get high temperature, compared with the vacuum furnace commonly used pure metal tungsten, molybdenum, tantalum, much cheaper. Based on the above advantages, as a vacuum resistance furnace, protective atmosphere furnace heating elements, more and more by the heat treatment industry.
Author: ZHANG Shu-rong