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Tool steel refers to a variety of carbon and alloy steels that are particularly well-suited to be made into tools. Their suitability comes from their distinctive hardness, resistance to abrasion and deformation and their ability to hold a cutting edge at elevated temperatures. As a result tool steels are suited for their use in the shaping of other materials. With a carbon content between 0.5% and 1.5%, tool steels are manufactured under carefully controlled conditions to produce the required quality. The presence of carbides in their matrix plays the dominant role in the qualities of tool steel. The four major alloying elements in tool steel that form carbides are: tungsten, chromium, vanadium and molybdenum. The rate of dissolution of the different carbides into the austenite form of the iron determines the high temperature performance of steel (slower is better, making for a heat resistant steel). Proper heat treatment of these steels is important for adequate performance.〔.〕 The manganese content is often kept low to minimize the possibility of cracking during water quenching. There are six groups of tool steels: water-hardening, cold-work, shock-resisting, high-speed, hot-work, and special purpose. The choice of group to select depends on, cost, working temperature, required surface hardness, strength, shock resistance, and toughness requirements. The more severe the service condition (higher temperature, abrasiveness, corrosiveness, loading), the higher the alloy content and consequent amount of carbides required for the tool steel. Tool steels are used for cutting, pressing, extruding, and coining of metals and other materials. Their use, such as the production of injection molds, is essential, due to their the resistance to abrasion, which is an important criterion for a mold that will be used to produce hundreds of thousands of moldings of a product or part. The AISI-SAE grades of tool steel is the most common scale used to identify various grades of tool steel. Individual alloys within a grade are given a number; for example: A2, O1, etc. == Water-hardening group == W-group tool steel gets its name from its defining property of having to be water quenched. W-grade steel is essentially high carbon plain-carbon steel. This group of tool steel is the most commonly used tool steel because of its low cost compared to others. They work well for small parts and applications where high temperatures are not encountered; above it begins to soften to a noticeable degree. Its hardenability is low, so W-group tool steels must be subjected to a rapid quenching, requiring the use of water. These steels can attain high hardness (above HRC 66) and are rather brittle compared to other tool steels. W-steels are still sold, especially for springs, but are much less widely used than they were in the 19th and early 20th centuries. This is partly because W-steels warp and crack much more during quench than oil-quenched or air hardening steels. The toughness of W-group tool steels are increased by alloying with manganese, silicon and molybdenum. Up to 0.20% of vanadium is used to retain fine grain sizes during heat treating. Typical applications for various carbon compositions are for W-steels: * 0.60–0.75% carbon: machine parts, chisels, setscrews; properties include medium hardness with good toughness and shock resistance. * 0.76–0.90% carbon: forging dies, hammers, and sledges. * 0.91–1.10% carbon: general purpose tooling applications that require a good balance of wear resistance and toughness, such as rasps, drills, cutters, and shear blades. * 1.11–1.30% carbon: files, small drills, lathe tools, razor blades, and other light-duty applications where more wear resistance is required without great toughness. Steel of about 0.8% C gets as hard as steel with more carbon, but the free iron carbide particles in 1% or 1.25% carbon steel make it hold an edge better. However, the fine edge probably rusts off faster than it wears off, if it is used to cut acidic or salty materials. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Tool steel」の詳細全文を読む スポンサード リンク
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