Defects of precision castings, such as oxidation inclusions, surface pits, shrinkage cavities, and cracks, are generated immediately after the molten metal is poured into the shell. The most ideal way to avoid and reduce casting defects is to: It is to ensure the purity of the molten metal and the most suitable temperature; the second is to try to make the characteristics of the shell adapt to and comply with the different changes in the condensation process of the molten metal when the molten metal is injected into different shells. For the latter, colleagues in the industry have successfully taken many external measures, such as the use of carbon cover box method to cool the metal in a reducing atmosphere to prevent pitting on the surface of easily oxidized steel castings; Partially soaked with water before pouring or sprayed with water after pouring to change the solidification sequence of the castings to prevent local shrinkage of the castings. There are many similar methods like the above, but most of them are measures taken from outside the shell. Inspired by foreign experience, the author of this article changes the material of the shell itself, and references the graphite material to the shell-making material. Graphite materials and repeated tests on castings with different structures and different metals have obtained some practical experience, which will play a certain role in promoting the popularization and application of graphite sand and powder in shell making in the future.
1. Basic understanding of graphite
1.1. Graphite is an allotrope of carbon. The solid substances composed of carbon in nature can be divided into two categories, one is amorphous substances with amorphous structure, such as charcoal, coke, coal, activated carbon, etc., and the other is substances with crystalline structure, such as diamond. and graphite. That is to say, there are three allotropes of carbon, which are diamond, graphite and amorphous carbon. There are two sources of graphite. One is natural graphite. There are abundant graphite ore in nature. China is the country with the largest graphite ore reserves in the world, mainly distributed in Heilongjiang, Shandong, Henan, Hunan and many other provinces. Our country is rich in natural graphite sources: the other is artificial graphite, which is graphitization of high-purity amorphous carbon by artificial methods. The purity of artificial graphite is higher and has a wider range of applications.
1.2. Natural graphite. According to the crystal structure and size of carbon atoms, it can be divided into three categories: ① dense crystalline graphite, also known as bulk graphite. The diameter of graphite crystals is greater than 0.1 mm, and the crystals are visible to the naked eye, but the arrangement is disorderly and has a dense block-like structure. And the slippery is not as good as phosphorus flake graphite. ②Scaly graphite, the graphite crystal is fish scale-like and has a lamellar crystal structure. The carbon atoms in the layer are arranged in a plane hexagon. Each carbon atom is combined with other carbon atoms by three covalent bonds, and the layers are The carbon atoms in between are combined by intermolecular force, which is characterized by the fact that the grade of carbon in the ore is generally not high, and flake graphite concentrate can be obtained through multiple grinding and multiple selection. It is not easy to mix with water-based solutions, and its floatability , Lubricity and plasticity are superior to other graphites, and have great industrial value. ③ Cryptocrystalline graphite, also known as earthy graphite. The diameter of graphite crystal is less than 1 micron, and the crystal shape can only be seen under the electron microscope. It is an aggregate of microcrystalline graphite, which is characterized by the high grade of carbon in the ore. Generally, the carbon content is 60%-80%, and a few are as high as 90%. Above, the ore selectivity is poor, the surface is earthy, lacks luster, and has poor lubricity, but it is easy to mix with water-based solutions.
Natural graphite is a metamorphic ore deposit formed at high temperature, containing SiO2, Al2O3, FeO2, CaO, P2O5, CuO and other impurities, often in the form of minerals such as quartz, pyrite, carbon salt, and water, asphalt, CO2, Although H2, CH4, N2 and other gases have undergone beneficiation treatment, when using natural graphite, at least the main indicators such as fixed carbon, sulfur, ash, volatile matter, etc. should be detected. See Table 1 for details.
1.3. Artificial graphite. It is made of high-quality powdered calcined petroleum coke as raw material, adding asphalt as a binder, pressing and forming by extrusion, molding or isostatic pressing, and then in a non-oxidizing atmosphere at 2500℃-3000℃. Graphite products obtained by graphitization, such as electrodes and graphite products of various shapes, have high carbon content and few impurities in the calcined petroleum coke, and are formed by high pressure, so this artificial graphite has high purity, high The characteristics of density and high strength are shown in Table 2. According to the purity and density, it can be divided into ordinary artificial graphite and high-purity dense graphite. It has a wide range of uses in manufacturing and scientific research. Even if it is used up, it can still be broken into sand. And powder, also known as graphite crushed, can be used as high-quality graphite material.
2. The characteristics of graphite and the feasibility of its application in precision casting shell materials
Graphite is gray-black and soft in texture. Artificial graphite formed by artificial pressure is hard in texture, with a specific gravity of 1.9~2.3 and a melting point of up to 3850±50℃. Its main characteristics are: high temperature resistance, even under ultra-high temperature arc burning, the weight loss is very small; the coefficient of linear expansion is 2 × 10-6, which is 1/4 of fused corundum and zircon. 2/5, so the thermal shock resistance is high, and the volume change is small when the temperature changes suddenly, so the mold shell will not crack due to this addition; the electrical conductivity is excellent, and the thermal conductivity exceeds that of metal materials such as steel, especially the thermal conductivity. It decreases with the increase of temperature, and graphite is almost a thermal insulator at high temperature; it has good chemical stability and does not react with acids, bases and organic solvents; in addition, it has good plasticity, lubricity and high temperature strength.
In foundry production, graphite is not unfamiliar. For example, graphite is used as a recarburizer in metal smelting. Electrodes used in electric arc furnace steelmaking and crucibles for melting non-ferrous metals are all graphite products. It is also very common to use graphite as a coating on the surface of the mold in mold casting. According to the many excellent characteristics of graphite, it should be feasible to selectively use graphite materials in precision casting shell materials to achieve the following purposes.feihongalloys.com