Monday, November 25, 2013

Chemical properties of High Speed Steel


High speed steel (abbreviation: HSS or HS) is a steel grade used in the manufacturing of a variety of machine tools. Since HSS has more suited mechanical properties for tool making (than the various other grades of carbon steel that have been traditionally used), it is considered to be of better quality for this purpose. Further, while HSS displays very high hardness and abrasion resistant characteristics at room temperatures, it does not lose its hardness even at much higher temperatures. Tools made with HSS can therefore cut much faster than those made of other types of steel and this key property also gives high speed steel its name. Infact, high speed steel has been very critical in the way modern metal processing industry has matured in recent years.

Chemical properties of High Speed Steel

Chemical composition

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

Broadly, high speed steels are classified into two categories – T type (tungsten based) and M type (molybdenum based).

How does the chemical composition influence the mechanical properties?

Influence on a steel grade’s chemical composition in turn affects its mechanical properties as well (such as the strength and hardness, toughness and 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.

Carbon increases the strength of the alloy, which helps it resist from 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 and it also cannot be cold worked.

All the grades of High speed steel require strength and hardness characteristics that have to be maintained even at high temperatures (exceeding 600C). This is typically achieved by heat treatment of the alloy.

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