High-speed steel
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High speed steel (HSS) is a material usually used in the manufacture of machine tool bits and cutters. It is often used in power saw blades and drill bits. It is superior to the older high carbon steel tools, which were used exstensively through the 1940's, in that it can withstand higher temperatures without losing its temper (hardness). This property allows HSS to cut faster than high carbon steel, hence the name high speed steel. At room temperature HSS and high carbon steel have an equivalent hardness; only at elevated temperatures does HSS become advantageous.
The main use of high speed steels continues to be in the manufacture of various cutting tools: drills, taps, milling cutters, tool bits, gear cutters, saw blades, etc., although usage for punches and dies is increasing.
High carbon steel remains a good choice for low speed applications where a very keen (sharp) edge is required, such as files, chisels and hand plane blades.
Types of high speed steel
When tool steels contain a combination of more than 7% tungsten, molybdenum, vanadium and cobalt along with more than 0.60% carbon, they are referred to as high speed steels (HSS).
The grade type T-1 with 18% tungsten has not changed its composition since 1910 and was the main type used up to 1940, when substitution by molybdenum took place. Nowadays, only 5-10% of the HSS in Europe and only 2% in the United States is of this type.[citation needed]
The addition of about 10% of tungsten and molybdenum in total maximises efficiently the hardness and toughness of high speed steels and maintains these properties at the high temperatures generated when cutting metals.
| Grade | C | Cr | Mo | W | V | Co |
|---|---|---|---|---|---|---|
| T-1 | 0.75 | - | - | 18.0 | 1.1 | - |
| M-2 | 0.95 | 4.2 | 5.0 | 6.0 | 2.0 | - |
| M-7 | 1.00 | 3.8 | 8.7 | 1.6 | 2.0 | - |
| M-42 | 1.10 | 3.8 | 9.5 | 1.5 | 1.2 | 8.0 |
To increase the life of high speed steel, tools are sometimes coated. One such coating is TiN (titanium nitride). Most coatings generally increase a tool's hardness and or lubricity. A coating allows the cutting edge of a tool to cleanly pass through the material without having the material gall (stick) to it. The coating also helps to decrease the temperature associated with the cutting process and increase the life of the tool.