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Discussion on the use conditions of iron-chromium-aluminum electrothermal alloys Li Yingjie, Shanxi Provincial Institute of Metallurgy, Shanxi Normal University, Wang Jianzheng Shanxi Provincial Institute of Metallurgy, Wang Yu, to discuss the chemical composition, basic properties and main characteristics of iron-chromium-aluminum electrothermal alloys. The main reasons for the service life of iron-chromium-aluminum electrothermal alloy components are proposed. It provides a useful loan for some units and users to use electric heating elements reasonably. Iron-chromium-aluminum electrothermal alloys are widely used in industrial and residential electric heating elements, but they are improperly used and selected. Will reduce the service life. Metal materials are used as high-resistance electrothermal alloys, which are classified into noble metals and alloys thereof, heavy metals and alloys thereof, nickel-based alloys and iron-based alloys. The most widely used are nickel-based alloys (such as Cr20Ni80), iron-based alloys (such as the chemical composition of Cr25A!5 electrothermal alloys (see Table 1). Table 1 The chemical composition of the alloys. The composition of the elements, Si bismuth, the precious fraction, the composition of the mass fraction. The basic screen of the sheet 2 sheet metal The maximum use temperature 20*0 resistivity specific gravity linear expansion coefficient gyro temperature coefficient specific thermal melting point tensile strength elongation (1) section shrinkage rate m hardness (HB): 1mm heat after heat treatment The effect of the rolling bar is small, the tensile strength is increased by 20%, and other data related to ffi should be changed.
Li Yingjie, male, born in 9S7, graduated from Taiyuan 4 College in i982: Automation. Shanxi Province Metallurgical Research Institute test factory temple factory director, work S foot - 0Cr2SAIS electrothermal alloy main characteristics 3.1 crystal grain coarsely obtained by electroslag remelting can be purified steel ingot, but from the post-processing point of view, there are more than three heating processes Can cause coarse grains. The iron-chromium-aluminum alloy is a uniform ternary solid solution. Since the affinity of aluminum and carbon is less than the affinity of iron and carbon, there is generally no carbide of aluminum. When the aluminum content in the steel is very low, A1N can refine the grains. However, when the aluminum content is more than 1%, the temperature of the heat-sensitive temperature of the steel is lowered, so that the grain of the steel is violently increased upon heating. For the iron-chromium-alloy alloy, when the aluminum content contains ft of 5%, the grain size starts to increase, and the temperature is about 850X: the grain size increases rapidly above 900*, and the grain size increases sharply above 1100X:. Very coarse grains obtained by heating hot-rolled iron-chromium-aluminum billets at 1250 TC for 6 h. A typical brittle fracture formed by hot rolling of an iron-chromium-aluminum wire rod with a temperature of over-rolling, a SEM image of a very coarse grain and a secondary crack.
After 250C heating and rolling, austenite fine grained fractures can be obtained, which have good cold workability and forming energy.
Hot-rolled wire rods typical coarse-grained fractured SEM 500X3.2 brittle iron-chromium-aluminum alloys have many reasons for brittleness: such as argon, and so on. Hydrogen and nitrogen are the main causes of brittleness of iron-chromium-aluminum alloys. Hydrogen comes from both the atmosphere and the raw materials, while nitrogen is mainly derived from furnace gas and higher iron alloys such as chromium and aluminum.
Hydrogen in iron-chromium-aluminum alloys may cause the alloy to produce white spots (also known as cracking). The white point can reduce the plasticity and tensile strength of the alloy, causing the gold to break prematurely, that is, the so-called hydrogen embrittlement phenomenon occurs. Therefore, the white point (which may be gray depending on the composition and the hot working conditions) is a defect that the alloy is not allowed to exist. The hydrogen content in the iron-chromium-aluminum alloy can only be reduced to 1mL and less than 4mL and 5mL, so that the white point sensitivity and hydrogen embrittlement can be eliminated. When smelting, the hydrogen should be reduced to the minimum level.
弋, 4h5h dehydrogenation effect is best * through the dehydrogenation of the wire rod, the tensile strength is unchanged, and the area shrinkage can be increased from the lowest of 05% to 60% 70%, the elongation can be increased from the lowest 3% to 13 %17%, grain size did not change.
Nitrogen in the alloy can result in "aging brittleness." When the alloy is cooled from high temperature, the nitrogen in the alloy is saturated, so as a result of low temperature retention, the dispersion of fine nitrides from the supersaturated alloy will increase the strength of the alloy, but reduce its plastic toughness. That is, it produces "aging brittleness", which is called "blue brittle". On the other hand, nitrogen can form stable stray brittle nitrides and carbide inclusions with chromium, aluminum, and the like in the alloy, which deteriorates the forgeability, temperature strength, and high temperature plasticity of the alloy. Therefore, it is extremely important to reduce the ft in the iron-chromium-alloy.
3.3 High-temperature strength The high-temperature strength of the iron-chromium-aluminum alloy is much smaller than that of the nickel-chromium alloy. Table 3 (see next page) shows the mechanical properties of 0Cr25Al5 alloy at different temperatures.
After heating, the iron-chromium-aluminum alloy forms a bright, firm and dense oxide film. The composition and properties of the oxide film, the high temperature strength of the alloy, and the non-metallic inclusions are the main factors affecting the service life of the electrothermal alloy, but the most important one is the composition and properties of the oxide film. Nitrogen and other gas impurities, carbide content is too high; grain coarsening Table 3 alloy under different suede mechanical pylon 10027196100 at 70 (KC below oxide film composition: Table 4 different twist 0Cr25AIS J5 consumption temperature CC) time (h The main cause of oxide loss (g/m2)1 is the low melting point metal* (: above %* when the heated low melting point metal (such as Al, Pb, Zn, i, etc.) or its vapor is splashed onto the heated The electric heating element and the electric heating element react at a temperature to form a low melting point alloy, and the beneficial oxide film of the iron chromium aluminum alloy is destroyed, so that the electric heating element is melted.
4.2 High-melting point metals such as cast iron chips, high-carbon steel, especially the iron oxide skin splashed onto the electric heating element to form a large amount of carbides at high temperature and destroy the element oxide layer, causing the electric heating element to break.
When a large amount of iron oxide falls on the electric heating element, the composition of the electrothermal alloy changes greatly. The Fe content increases, the relative content of Cr and Al decreases, and the C content increases to produce a large amount of C. The protective layer of alumina is destroyed. Decrease, the surface load of the electric heating element is increased, and the influence of a series of deterioration conditions makes the life of the electric heating element greatly shortened.
4.3 Surface load is unreasonable or the surface load is too high due to improper selection of alloy, the temperature of the component is too fast, and the use temperature is high, which will make the 0Cr25Al5 electric heating element die increase and become brittle during use, and the temperature rises and falls quickly. The alloy can also cause component disconnection during elongation and contraction.
4.4 Refractory materials Ferrochromium and aluminum are used at high temperatures, especially in the electric heating elements of closed electric heaters. Pay attention to the composition of refractory materials, and use clay bricks for low temperature use. High-aluminum bricks should be used when the furnace temperature is about 1000X: the use temperature should be more pure. 4.S furnace atmosphere with carbon atmosphere, pure hydrogen atmosphere can be iron-chromium-aluminum alloy, but the temperature should be used. Reduced; sulfur-containing oxidizing atmosphere can be iron-chromium-alloy, but not suitable for sulfur-containing reducing atmosphere; reducing atmosphere: hydrogen, nitrogen, ammonia, water vapor and low-temperature use, bromine atmosphere can be nickel-chromium alloy.
5 Conclusion Any type of electrothermal alloy should avoid the contamination of other metals and their oxides, otherwise the service life will be greatly reduced. It is necessary to research and develop a new type of iron-chromium-aluminum alloy to gradually replace the traditional nickel-chromium alloy.
Basic properties of 1 0Cr25A15 electrothermal alloy (see Table 2)
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