| Iron alloy phases|
|Austenite (γ-iron; hard)|
Cementite (iron carbide; Fe3C)
Ferrite (α-iron; soft)
Pearlite (88% ferrite, 12% cementite)
| Types of Steel|
|Plain-carbon steel (up to 2.1% carbon)|
Stainless steel (alloy with chromium)
HSLA steel (high strength low alloy)
Tool steel (very hard; heat-treated)
| Other Iron-based materials|
|Cast iron (>2.1% carbon)|
Wrought iron (almost no carbon)
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 in that it can withstand higher temperatures without losing its temper (hardness). Because of this, high speed steel can cut metal at a higher speed than its predecessor high carbon steel, hence the name high speed steel. It is about as hard as high carbon steel at room temperature. Only at elevated temperatures does it become advantageous.
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.
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.
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).
This term is descriptive of their ability to cut metals at the "high speeds" in use through the 1940s when the predominant tool steel was high carbon steel, which was unable to run at high cutting speeds, e.g. on a lathe or milling machine.
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 is of this type and only 2% in the USA.
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.
Table 1: Percentage Additions of Alloying Elements to High Speed Steel Grades.