Tool steel
Steel that cuts, forms or shapes other materials — hard enough to hold an edge and tough enough not to shatter, which are opposing requirements.
Tool steels are carbon steels with enough alloying — chromium, molybdenum, tungsten, vanadium — to be hardened deeply and reliably, and to hold that hardness. They exist to work other materials: to cut, punch, form, mould and shear.
The design problem is that hardness and toughness pull against each other. A cutting edge needs hardness or it deforms; a punch needs toughness or it snaps. There is no steel that maximises both, so the family is a set of compromises with names, and choosing between them is choosing which failure to accept.
All of them are supplied soft, machined, and then hardened — because a steel hard enough to cut with cannot itself be cut. Everything about how a tool is made follows from that ordering.
Processing
Machined or ground in the annealed condition, then hardened by heating to austenitising temperature and quenching, then tempered — always tempered, because an untempered quenched tool steel is brittle enough to crack on its own as residual stress relaxes.
The quench medium is part of the grade. Water-hardening steels are cheap and distort or crack readily; oil-hardening are the general workshop compromise; air-hardening steels move so little in heat treatment that a finished tool can be hardened without needing to be re-ground, which is why they dominate precision tooling.
Cryogenic treatment after quenching converts retained austenite that the quench left behind, and gives a small but real gain in hardness and dimensional stability.
Uses
Cutting tools, punches and dies for sheet metal, injection moulds and die-casting dies, shear blades, thread-rolling dies, chisels and hand tools, and the moulds plastic products are made in.
Mould tooling is the largest use by value and the least visible. An injection mould is a precisely machined, polished, hardened tool steel assembly that costs as much as a house and produces millions of parts, and it is the reason plastic products are cheap.
History
Crucible steel from Benjamin Huntsman in the 1740s is the beginning of tool steel as a controlled material: melting the metal fully gave a uniform composition that cementation steel never had, and Sheffield built an industry on it.
Robert Mushet's air-hardening tungsten steel in 1868 is the first true alloy tool steel, and the ancestor of high-speed steel. Before it, machining speed was limited by the tool losing its temper.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
contains
- Iron — element · 80–97% · the balance
- Carbon — element · 0.3–2.5% · the element that makes hardening possible at all — below about 0.3 per cent no useful martensite forms
- Chromium — element · 0.5–13% · forms hard carbides and deepens hardening, so a thick section hardens through rather than only at the surface
- Molybdenum — element · 0–5% · hardenability and hot strength
- Vanadium — element · 0–4% · forms the hardest carbides of any of them, which is what gives wear resistance
is an input to
- Heat treatment — process · the entire point of the family — supplied soft, machined, then hardened and tempered, because a steel hard enough to cut with cannot itself be cut
is used as
- Abrasive — application · as the tool rather than the abrasive — cutting, punching and forming everything else
succeeded
- Wootz steel — alloy · crucible steel in Sheffield from the 1740s produced a uniform steel by a repeatable process, which is what wootz had achieved by an ore and a tradition and could not be taught
was succeeded by
- High-speed steel — alloy · in machining, and the change was structural: existing machine tools were not stiff enough to use the new steel and the industry rebuilt its product line around it
is produced at
- Sheffield — place · crucible steel from the 1740s, which made the town the centre of the European tool and cutlery trade for a century and a half
is produced by
- Alloying and melting — process · carbon and carbide formers into iron, and the carbide is the working part
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing