High-speed steel

High-speed steel is tool steel with enough tungsten, molybdenum, chromium, vanadium and cobalt to keep its hardness at temperatures that would soften anything else — up to around 600 °C. That property is called red hardness, and it is the whole point.

An ordinary hardened carbon steel tool loses its temper as the cutting edge heats up, and the cutting speed is therefore limited by how fast the tool can shed heat. High-speed steel is indifferent to the heat, so the machine can be driven as hard as its power and rigidity allow. When it was introduced, cutting speeds rose by a factor of three or four and machine tools had to be rebuilt to take the loads.

It has since been displaced from high-volume production by cemented carbide, which is harder and faster still and is brittle. High-speed steel survives — in drills, taps, end mills, saw blades and gear cutters — because it is tough enough to survive interruption, vibration and a less than rigid setup, which is most of the real world outside a production machine shop.

Hardened from a very high austenitising temperature — around 1200 °C, close to melting — then tempered two or three times. The multiple tempers are not caution: each one transforms retained austenite to martensite, which then needs tempering itself, so a single temper leaves untempered martensite in a finished tool.

Ground rather than machined once hardened. Powder-metallurgy grades, made by consolidating atomised powder rather than casting an ingot, give a much finer and more uniform carbide distribution and are tougher for it — which is what most premium modern HSS tooling is.

Coatings — titanium nitride and its relatives — are now near-universal, and add surface hardness and lubricity to a substrate that supplies the toughness.

Twist drills, taps and dies, end mills, reamers, broaches, hobs and gear cutters, power hacksaw and bandsaw blades, and planer knives.

It is the material of the general workshop, where the work is varied, the setups are imperfect and a tool that chips is worse than one that wears.

Frederick Winslow Taylor and Maunsel White demonstrated it at the 1900 Paris Exposition, cutting with a tool glowing dull red — a demonstration that was understood immediately, because everyone watching knew what it meant for machine shop output.

Its effect was structural rather than incremental. Existing machine tools were not stiff or powerful enough to use it, so the machine tool industry rebuilt its product line around it, and the productivity gain fed directly into the manufacturing methods of the following decades.