What Is Tempering of Tool Steel?
Making steel involves heating iron to a high temperature to allow carbon atoms to diffuse into the iron structure. This produces the stronger alloy we know as steel. During the smelting process, steel is cooled rapidly to solidify its crystalline structure. This not only hardens the metal but also makes it brittle because of stress points created during the rapid cooling. "Tempering" is the process of reheating steel and cooling it slowly to eliminate these stress points, and make the steel less brittle.
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Making Steel
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Iron ore is smelted in a furnace at 1,800 degrees Celsius. During this process, the iron melts and absorbs carbon in the furnace. Other minerals are added to the molten iron to precipitate impurities to the surface, where they are removed. The molten iron is poured into molds. It will then be resmelted to lower the carbon content, and to remove other impurities to produce a steel product. Steel contains carbon ranging from 0.25 to 1.5 percent. This is the optimum content range for a balance of strength and malleability for steel.
Steel Structure
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Steel is composed of groups of iron and carbon molecules arranged in a crystalline structure. During steel-making, heat forces the iron and carbon atoms into irregular arrangements, preventing the groups of iron molecules from slipping over each other and reducing the steel's malleability. Hardness is further reinforced in steel by boundary gaps between groups of molecules, preventing displacement when the metal is subjected to stress.
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Tempering Steel
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Tempering is a process by which steel is reheated and slowly cooled. Tempering is necessary because the process of making steel requires it to be rapidly cooled to set the crystalline lattice of iron and carbon atoms. This results in a hard metal, but also makes it brittle because of an uneven arrangement of carbon and iron. Reheating the metal evenly redistributes the carbon atoms, and eliminates stress points (and brittleness) by reducing boundary gaps between molecules.
Tool Tempering
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Tempering must be performed for all steel tools after they are heat-treated. This is done at a variety of temperatures, depending on the tool's final use. Tempering begins at roughly 150 degrees Celsius. Gauging the correct tempering heat for a steel tool is determined by the color the metal glows at when heated. Heating a tool ceases once the hot steel reaches a specific color, which can range from strawberry to yellow to blue. Sections of a tool can be tempered so that different parts of it can have different properties. This is important for tools that have edges or teeth.
Uses of Tempered Steel
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Tempered steel is used in a variety of products, among them tools, pins and bolts. In addition, steel alloys must also be tempered. Typically these are steels that are mixed with metals such as chromium to prevent corrosion (such as stainless steel) or with other elements such as titanium for additional strength. Alloyed steels have a variety of applications, such as in construction or for use as rigid frames in automobiles or airplanes.
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References
- Photo Credit steel tools studio isolated over white image by dinostock from Fotolia.com
