high speed steel
A kind of tool steel, invented in the early 20th century, which retains its hardness and temper at high temperatures (at which previous kinds of tool steel would soften), thus allowing cutters made from it to be employed at higher cutting speeds than were previously practical.
high speed steel: tool steel that stays hard when hot
High speed steel (HSS) is a iron-based alloy engineered to maintain its cutting hardness at temperatures where conventional tool steels fail. A cutting tool made from HSS can operate at three to ten times the speed of one made from carbon steel before it softens and loses its edge. This property alone transformed metalworking in the early 1900s, making mass production of machined parts economically viable where it had been prohibitively slow before.
The alloy works by incorporating tungsten, molybdenum, chromium, and vanadium into the iron matrix, typically in proportions like 18% tungsten, 4% chromium, 1% vanadium, and small amounts of molybdenum. Tungsten is the key player here, forming hard carbide particles that resist softening even as the tool reaches 600 degrees Celsius or higher during cutting. These carbides remain stable where the iron base would begin to lose its temper. The exact recipe varies: molybdenum grades substitute some tungsten to reduce cost and weight, while cobalt additions can push heat resistance a few degrees higher.
HSS comes in two main metallurgical forms. Wrought high speed steel, produced by forging and rolling, offers toughness but slightly lower hardness. Powder metallurgy HSS, made by atomizing molten metal into fine powder, then sintering it under pressure, delivers more uniform carbide distribution and superior hardness, though at higher cost. Coatings like titanium nitride or titanium carbonitride are often applied to HSS tools to add another layer of heat and wear resistance without changing the substrate.
Where HSS Sits in the Cutting Tool Hierarchy
High speed steel occupies the middle ground between carbon steel and cemented carbide. Carbon steel is cheap, easy to sharpen, and forgiving to dull machines, but its speed ceiling is low. Carbide cutters can run much faster than HSS and hold an edge longer, but they are brittle, expensive, and demand rigid setups. HSS remains the practical choice for job shops, manual machines, and situations where tool cost must be balanced against machine capability. A drill bit or end mill made from HSS can be sharpened by hand with a bench grinder, whereas carbide demands precision grinding equipment.
The name "high speed" reflects its historical context: when it was developed around 1900, the speed at which a lathe could turn was itself a limiting factor. HSS allowed machinists to run their machines faster than ever before. On modern CNC equipment with rigid spindles and flood coolant systems, HSS may seem quaint. On a loose old manual mill or in a field situation where coolant is not available, it remains reliable and forgiving. Machinists still choose HSS drills and taps for general work because they tolerate interrupted cuts and wobble better than carbide does.