Physical Properties of High Speed Steel

Written by james rutter
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Physical Properties of High Speed Steel
Engineers use high speed steel for drill bits. (drill image by Aleksandr Ugorenkov from Fotolia.com)

High Speed Steel (HSS) refers to any of a variety of steel alloys that engineers primarily use to construct machine tool bits and blades and drill bits for industrial power tools. Materials scientists can infuse this steel with differing percentages of elements to form a variety of HSS manufacturing alloy types. While all of these different types exhibit HSS's characteristic resistance to high-temperature, the properties of each alloy will vary depending on the percentage of other elements the alloy contains.

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Physical Composition

High Speed Steel is a multi-component alloy carbon-based steel and, as such, steel mills manufacture HSS according to the Fe-C-X system (Iron-Carbon-X). In this system, "X" can represent one or more other elements, most commonly Tungsten plus chromium, molybdenum, vanadium, or cobalt. Generally, the final proportions of total weight of non-ferrous elements used to manufacture HSS range from .65 to .80% carbon, .10 to .40% manganese, .20 to .40% silicon, 3.75 to 4.0% chromium, no more than .30% nickel, 17.25 to 18.75% tungsten, .90 to 1.30% vanadium, no more than .25% copper, .03% phosphorous, or .03% sulphur.

Strength

High Speed Steel exhibits a density of 8.67x1000 kg/m3 (kilograms per meters-cubed). This high density affords it incredible durability and hardness (even at high temperatures) and shock and vibration resistance while still allowing for its machinability into tools and drill bits. Once formed, the low carbon percentage gives it a very high melting point (ranges vary depending on the alloy). Its strength also leads to greater durability in tools even when used in conditions of mechanical and thermal stress. A greater percentage of chromium in HSS alloys will increase its strength, while vanadium increases will improve the cutting quality and sharpness of tools and drill bits made with HSS. Alloys containing higher percentages of molybdenum will also display increased hardness.

Thermal Properties

When used in tools, High Speed Steel operates at a very low thermal expansion rate of 9.7 microns per meter per degree Celsius. It also conducts heat at a very low rate. Both of these properties enable HSS tools to cut faster (hence the name, "high speed") and to be used for longer periods. Because of this low thermal conductivity, technicians using HSS manufactured tools can also use a wide variety of lubricants for the tools. As a general rule, the higher the percentage of carbon in HSS, the lower the melting point of the steel, which will lessen its thermal properties as well. Alloys containing cobalt demonstrate an improved hardness at high temperatures and greater durability.

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