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Views: 5 Author: Site Editor Publish Time: 2023-03-28 Origin: Site
The chromium grains are a silvery-white metal (body-centered cubic system) with a grayish tinge. Soluble in dilute sulfuric acid, dilute hydrochloric acid, insoluble in water, nitric acid, aqua regia. Melting point (1857 ± 20) ° C. The boiling point is 2672°C. Conical baskets or boats made of tungsten and tantalum can be used for heating and evaporation, and a small amount of evaporation can also be heated by molybdenum boats. Chromium is a sublimation material. When evaporated in a vacuum, the surface is very easy to oxidize. If an oxide film is formed, it is not easy to evaporate from the surface. Therefore, sometimes chromium is electroplated on a tungsten wire as thick as possible, and then in a vacuum The tungsten wire is heated to evaporate the chromium.
Chromium has good mechanical properties, and the chromium film evaporated on the heating substrate is very firmly combined with the substrate. Taking advantage of this characteristic of chromium film, very thin chromium film is often used as a transition layer film, such as the underlying film of Al and Au, to improve the adhesion of these films to the substrate. Mainly used in beam splitters, reflective lenses, iridescent crystals, motorcycle helmet visors, etc.
The production of metal chromium mostly follows the roasting process of chromite ore calcium method to convert dichromium trioxide into sodium chromate, which is leached with dilute solution and water to form sodium chromate solution, and then produced by electrolysis.
Electrolysis method: When sodium chromate is used as the compensation solution for electrolysis, the catholyte is 40g/L (calculated as Cr), 90g/L (calculated as NH4); the anolyte is 22g/L (calculated as Cr), 18g/L (calculated as NH4), 200g/LH2 S04. Stainless steel is used for the cathode, and Pb-Ag alloy is used for the anode. Electrolysis is carried out at a temperature of about 60°C, a cell voltage of 4.8V, and a current density of 8.6 A/crr12 to obtain a finished metal chromium product. Due to the high content of impurities such as silicon and aluminum in chromite ore and its additives, the amount of slag discharged in the process is large, and the slag contains toxic water-soluble Cr6+, which seriously pollutes the environment.
The process developed in recent years: the chromite ore is reduced to carbon ferrochrome first, and then mixed with soda ash, dolomite and other materials for oxidative roasting and leaching to obtain sodium chromate solution, which is reduced with sulfur and calcined. Chromium oxide is obtained, and then smelted with aluminum, sand blasted, and finished to obtain metal chromium products. The advantage of using this method is that most of the impurities in the chromite ore enter the smelting slag, so that the total chromium in the mixture can be prepared at a higher level, and the pollution to the environment is also reduced. The soda ash consumed in the roasting process is less, and it also improves The quality of the product.
It is used for refining high-temperature alloys, resistance alloys, and precision alloys as chromium additives. Pure chromium is used for electroplating.
1) Chromium and iron can form a continuous solid solution and reduce the austenite phase area. Chromium forms a variety of carbides with carbon, and its affinity with carbon is greater than that of iron and manganese but lower than that of tungsten and molybdenum.
2) Chromium can reduce the concentration of carbon in pearlite and the limit solubility of carbon in austenite.
3) Slow down the decomposition rate of austenite, significantly improve the hardenability of steel, but also increase the tendency of temper brittleness of steel.
1) Improve the strength and hardness of steel, and when other alloying elements are added at the same time, the effect is more significant.
2) Significantly increase the ductile-brittle transition temperature of steel.
3) The impact toughness drops sharply when the chromium content is high.
1) Improve the wear resistance of steel and easily obtain lower surface roughness values. The electroplating industry widely uses chrome plating technology to improve product wear resistance and aesthetics
2) Reduce the electrical conductivity and temperature coefficient of resistance of steel.
3) Improve the coercive force and residual magnetic induction of steel, and are widely used in the manufacture of permanent magnetic steel tools.
4) Chromium forms a passive film on the surface of steel, which significantly improves the corrosion resistance of steel; but when carbides containing chromium are precipitated, the corrosion resistance of steel decreases.
5) Improve the oxidation resistance of steel.
6) Dendritic segregation is easy to form in chromium steel, which reduces the plasticity of steel.
7) Since chromium reduces the thermal conductivity of steel, the temperature should be raised slowly during hot working, and it should be cooled slowly after forging and rolling.
1) Chromium is mainly used in alloy structural steel to improve hardenability, and chromium-containing carbides can be formed on the carburized surface to improve its wear resistance.
2) Chromium and other elements are used in spring steel to improve the comprehensive performance of steel.
3) The use of chromium in bearing steel has the advantages of improving wear resistance and small surface roughness after grinding.
4) In tool steel and high-speed steel, chromium is mainly used to improve wear resistance, and has certain advantages such as temper resistance and toughness.
5) Chromium in stainless steel and heat-resistant steel is often used in combination with manganese, nitrogen, and nickel. When austenitic steel needs to be formed, there must be a certain ratio between the chromium that stabilizes ferrite and the manganese and nickel that stabilize austenite. Such as Cr18Ni9 and so on.
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