Abrasives can be divided into two categories: natural abrasives and artificial abrasives.
I. Natural abrasives
All materials in nature that can be used for grinding or abrasive are collectively referred to as natural abrasives. Commonly used natural abrasives are as follows.
Diamond is currently the hardest known material, its microhardness of 98.59Gpa. diamond carbon isotype, the main component of carbon, in addition to containing 0.02--4.8% impurities, specific gravity of 3.15--3.53g/cm3. The production area is very limited, not only expensive, but also extremely lacking.
Diamond because of different impurities and black, black-brown, gray-black, etc., brittle, easy to crack along the crystalline surface, the larger the crystalline resistance to external forces, diamond measurement unit carat, 1 carat = 0.2g.
Natural diamond as abrasive main use has two aspects.
(1) for dressing grinding wheels.
(2) To grind and polish difficult materials (such as cemented carbide, gemstones, glass, stone, etc.).
The main mineral composition of natural corundum is α--Al2O3, its microhardness is 20.58Gpa, specific gravity is 3.93--4.00g/cm3, there are three kinds of natural corundum in nature.
(1) High quality corundum (commonly known as gemstones) with sapphire (containing titanium), ruby (containing chromium), etc.
(2) Ordinary corundum, which is black or brownish-red in color.
(3) adamantine, which can be divided into emerald adamantine and limonite adamantine, a kind of aggregate crystal with low hardness. Among the above three kinds of natural corundum, the first one is mainly used for jewelry, while the latter two can be used as abrasives to make grinding wheels, oil stones, sandpaper, emery cloth or micro powder, abrasive paste, etc.
Garnet crystal shape is good, microhardness is 13.33Gpa. belongs to the garnet mineral variety, but suitable for abrasive only one kind of iron aluminum garnet, its mineral composition: 3FeO.Al2O3.3SiO2, content of not less than 85--90%.
Quartz chemical composition is SiO2, often interspersed with Al2O3, Fe2O3, CaO MgO Fe2O3, etc.. Microhardness of 8.04 Gpa, can be used as abrasive quartz ore are vein quartz, quartzite and quartz sand, etc..
With the development of science and technology, artificial abrasives varieties have reached dozens of species, natural abrasives due to their own defects, has been replaced by more and more artificial abrasives, in addition to natural diamond, garnet, other types of natural abrasives dosage is very little.
II. Synthetic abrasives
Artificial abrasives are divided into corundum series, carbide series, super hard series and other major categories. The brief manufacturing methods, characteristics and grinding objects of each type of abrasives are described as follows.
1. Corundum Series Artificial Abrasives
There are brown corundum, white corundum, zirconium corundum, microcrystalline corundum, single crystal corundum, chromium corundum, praseodymium corundum, black corundum and alumina sintered corundum, etc.
(1) Brown corundum (A)
Brown corundum is made of bauxite, anthracite and iron filings, which are smelted at high temperature in electric arc furnace. In the smelting process, the carbon in the anthracite coal will be bauxite in silicon oxide, iron oxide and titanium oxide impurities reduced to metal, for some metals combined together to become an iron alloy, due to its specific gravity than the corundum melt and sink to the bottom of the furnace and corundum melt separation. Only a small amount of impurities are interspersed in the corundum melt fast.
The main mineral composition of brown corundum is physical corundum, tripartite crystal system, a small number of mineral impurities are: calcium silicate, calcium plagioclase, alumina-rich rhodochrosite (also known as mullite), titanium compounds, vitreous and a small amount of iron alloy.
Brown corundum has strong anti-crushing ability, anti-oxidation, anti-corrosion, good chemical stability, a widely used abrasive.
(2) White corundum (WA)
White corundum is made of alumina powder, melted by high temperature. White corundum is white, slightly higher hardness than brown corundum, slightly lower toughness. The abrasives made of white corundum are suitable for grinding of high-carbon steel, high-speed steel and hardened steel. It can be used for grinding and polishing brushes. White corundum can also be used as precision casting sand, spraying materials, chemical catalyst carrier, special ceramics, advanced refractory materials, etc.
(3) Chromium corundum (PA)
Chrome corundum smelting process is the same as white corundum, only in the smelting process to add a certain amount of chromium oxide, light purple or rose color. Chrome corundum due to the introduction of Cr3 + improve the abrasive toughness, its toughness is higher than white corundum, and hardness and white corundum is similar, used for processing tough materials, its processing efficiency than white corundum, and the surface finish of the workpiece is also better, chrome corundum is suitable for processing toughness high quenched steel, alloy steel, precision gauges and instrument parts and other high requirements of finish workpiece.
(4) Microcrystalline corundum (MA)
Microcrystalline corundum used in raw materials and smelting methods and brown corundum is basically the same, immediately after the suspension of the molten liquid through the flow or pouring method into the branch mold rapid cooling (generally within 30 minutes Kushi), and thus get microfine crystalline aggregates. Microcrystalline corundum in the smelting process, the impurity reduction degree is poor, Al2O3 content is 94 - 96%, crystal size is generally in 80 - 300 microns, crystal accounted for 57 - -85%, the maximum crystal size does not exceed 400 - 600 microns. It has high strength and large toughness characteristics. It is suitable for heavy-duty grinding and can grind stainless steel, carbon steel, bearing steel and special ductile iron, etc. It is also used for precision grinding and even mirror grinding because the abrasive grains are micro-edged and broken during the grinding process.
(5) Monocrystalline corundum (SA)
Monocrystalline corundum is made of bauxite, anthracite, iron filings and pyrite as raw materials, and is eutectically melted in an electric arc furnace, where iron oxide, silicon dioxide and titanium oxide in bauxite are successively reduced and formed into iron alloy from the molten liquid to the bottom of the furnace. A small part of alumina reacts with carbon and ferrous sulfide to produce a small amount of aluminum sulfide filled between the single crystal particles, when the melt is cooled and put into water, the aluminum sulfide is dissolved, and the aluminum sulfide separates the single crystal corundum can be dispersed to become a natural particle size abrasive.
Monocrystalline corundum is off-white, its particle shape is mostly isometric, the crystal does not contain impurities, with multi-angle cutting edge, in the same grinding force, the moment formed is less than other abrasives, so it is not easy to break, high mechanical strength, single particle compressive strength of 22--38kg, while brown corundum is only 10--20kg. --Because of its high hardness and toughness, single crystal corundum has strong cutting ability and can be used to process tool steel, alloy steel, stainless steel, high barium steel and other tough and hard materials.
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