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Chemical Properties of High Speed Steel

High speed steel (also called HSS and HS steel) is a steel grade used in the manufacturing of a variety of machinery tools. Since high speed steel has more suited mechanical properties for tool making compared to the various other grades of carbon steels which have been traditionally used, it is considered to be of better quality for this purpose.

  1. Chemical Composition

    • Tool steels such as high speed steel are alloys containing many elements other than iron, each of which influences its chemical properties. These elements are carbon (0.65 to 0.80 percent), chromium (3.75 to 4.00 percent), tungsten (17.25 to 18.75 percent), vanadium (0.90 to 1.30 percent), and in very small proportions cobalt, molybdenum and aluminum. Other elements that may be present in small quantities are manganese (0.10 to 0.40 percent), silicon (0.20 to 0.40 percent), nickel (about 0.30 percent), copper (0.25percent), phosphorus (about 0.30percent) and sulfur (about 0.30percent).

      Broadly, high speed steels are classified into two categories according to its chemical composition: T type (tungsten based) and M type (molybdenum based).

    Effect of Chemical Composition

    • Influence on a steel grade’s chemical composition in turn affects its mechanical properties as well (strength, hardness, toughness, brittleness, ductility and malleability), and hence is a very important determinant of the variety of applications for which a particular grade can be used. Chemical properties of a steel grade also dictate its anti-corrosive properties. Higher carbon content increases the strength of the alloy, which helps it resist getting deformed when load is applied to it. It also increases the hardness of the alloy, although making it less ductile. In turn, presence of chromium in the iron alloy can significantly delay its oxidation (rusting) by forming an anti-corrosive thin layer of chromium oxide, thereby preventing any mechanical or chemical damage to the tool. Molybdenum helps in preventing the alloy from being scarred, although this makes it less malleable.

    Hardness

    • All the grades of high speed steel require strength and hardness characteristics that have to be maintained even at high temperatures (exceeding 600 degrees Celsius). This is typically achieved by heat treatment of the alloy. It is important to note that high speed steel displays very high hardness and abrasion-resistant characteristics both at room and high temperatures. Tools made up of high speed steel can therefore cut much faster than those made of other types of steel grades and this key property also gives high speed steel its name. In fact, high speed steel plays a very important role in modern metal processing industry.

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