What is tool steel?
Tool steels are exceptionally hard, tough, or wear-resistant alloys. Their properties come from both their chemistry and their production. As their name intimates, these steels are ready to work cutting, grinding, drilling, punching, striking, or doing other tough jobs. Tool steels must have the right material properties for their application. For example, a drill bit and a punch each need hardness and wear resistance. However, the punch experiences more impact while the drill bit experience more shear. Engineers and metallurgists pick the type of tool steel based on the tool’s use.
Tool steels all have alloying elements that produce carbides, a densely-packed metal lattice which contains a metal and carbon. Carbides are a refractory material, which means they resist breaking down under pressure, chemicals, or heat.
It’s not just chemistry that makes a tool steel, however. A tool steel also gets its hardness from precisely controlled heat treatment and quenching.
Hardening tool steel by quenching
Quenching is a process to harden steel by changing its microstructure.
First the steel is heated. Steel alloys are heated to different temperatures depending on the amount of carbon in mix. Iron and carbon move through different “phases,” in which the molecules are taking different shapes, and these phases are dependent on overall chemistry.
Once the right heat has been reached and held, the alloy is then quenched, or cooled, through exposure to a much cooler gas or liquid. This quenching freezes the metal quickly. When the metal is shock frozen like this, lots of tiny crystal grains of metal all begin freezing at once, with a lot of displacement in each grain and between them. Compare this to a very slow cooling, where grains slowly cool over time in larger, circular blooms. Slow-cooled metal grains can move past each other when the metal is hit, denting the metal but not breaking it. Shock freezing gives the molecular structure less room to move when struck, making it harder, less likely to dent. The metal lattice in these structures is called martensite, and it is martensite’s shocked, jagged molecular structure that gives quenched metals their characteristic hardness.
Water-quenching is the fastest way to quench, and air-quenching the slowest way. What the alloy can handle depends on its chemistry. Some alloys might crack or distort if they’re cooled too quickly but will harden well in a gentler air-quench. Others won’t make a hard martensite layer unless they’re shocked by a big temperature change like that in water.
Categories of tool steel
There are six broad categories of tool steel. The four most common are water-hardened, cold-work, hot-work, and high-speed tool steels. Specialized applications also use shock-resisting and special-purpose tool steels.
This group of tool steels is essentially carbon steel that’s been heat treated. They have 0.5–1.5% carbon. Other alloying elements may be present for their different qualities, but these are generally under 0.5%. These metals, like nickel, tungsten, or molybdenum, are expensive. Using less of the costly metal offers value for lighter duty applications.
The extreme shock of water-quench is necessary with this group of steels to get a hard external layer. Light duty tools, small parts like springs, and small fasteners are likely final products. They must not be used in extreme environments because W-series steels are a bit more brittle than other tool steels. W-series steels are more vulnerable to cracking and only can handle continuous temperatures to 302°F (150°C).
Cold work tool steel
Cold-work tool steels are meant to be used at “cold” temperatures—they are hard, tough, and wear-resistant, but not in hot environments. Because cutting cold materials can take more force than cutting hot materials, compressive strength is a must in cold work tool steel.
Cold, in this context, does not mean it is time for the metalworker to put on a sweater. 392°F (200°C) and below is considered a cold working environment for these tools. Unlike many types of steel, graphite often forms in cold-work steels. This graphite makes many of these alloys machinable: the graphite provides lubrication.
Cold work tool steel’s subcategories are:
- Oil-hardening (O Series): During production, these alloys are quenched in oil. They have 0.85–2.00 % carbon, and usually less than 1% of each other alloying element. These elements may include manganese, silicon, tungsten, chromium, vanadium, and molybdenum.
- Air-hardening (A Series): During production, these alloys are quenched in air. They have 0.05–2.85% carbon. These steels can contain up to 5% chromium. High chromium means that during heat treatment the A series won’t undergo dimensional distortion. Chromium produces closer tolerances.
- High carbon chromium (D Series): These cold work steels function up to 797°F (425°C). They contain 1.4–2.5% carbon, and 11–13% chromium. They can be air or oil quenched, depending on the alloy, and experience very little distortion in either cooling method. These alloys generally have a very high abrasion resistance.
Hot work tool steel
Hot-work tool steels all have a greater percentage of alloying elements to create more carbides and handle hotter working temperatures. Hot work tool steels can work in temperatures up to 1004°F (540°C). As a group, most have low carbon percentages, below 0.6%.
Hot work tool steel is often used in high temperature manufacturing with malleable hot material like metal and glass. Dies, stamps, extruders, and compressors all may be made with hot-work steel. The tool must continue to function even in extended heat exposure.
There are three main types of hot work tool steel, based on the major alloying element: chromium, tungsten, or molybdenum. A few have high levels of chromium and tungsten, and so get sorted as tungsten or as chromium depending on the manufacturer. The major alloying element produces the type of carbide in the steel, and each carbide offers different advantages.
Hot work tool steel’s subcategories are:
- Chromium: Chromium hot work tool steels have 3–5% chromium. Other alloying elements like tungsten, vanadium, or molybdenum are under 5%. Vanadium is generally used in the steel used for cutting tools. This group of alloys (and specifically AISI H13) are the most used hot work tools.
- Tungsten: These alloys often do not contain any molybdenum or vanadium. Instead they contain 9–18% tungsten. Most tungsten hot work tool steels have between 2–4% chromium. This is not universally true: for example, the alloy AISI H23 has 12% chromium and 12% tungsten. H23 is sometimes called a chromium steel, sometimes a tungsten one, and sometimes both. Tungsten steels provide excellent heat resistance but can be brittle; this is managed by preheating to operating temperature before use.
- Molybdenum: Many chromium hot work tool steels have a little bit of molybdenum in them. Premium hot work steels for very tough environments have a lot. Molybdenum creates higher-heat stability and wear resistance in severe hot work applications. Molybdenum alloys AISI H42 and H43 are often used as dies or cutters in metal mills to handle the heat and force of cutting and forging.
High speed tool steel
Modern production processes are possible in part to the invention of this class of tool steel. Cutting tools and machine bits that run at high speeds get hot due to friction. The high-speed materials innovation allowed for machinists to increase their rate of production.
In 1900, high-speed tool steel was debuted at the Paris Exhibition. A crowd of manufacturers gasped to see a cutting edge start glowing red-hot due to friction yet continuing to work. Where cutting speeds in the 1890s were 5–30 feet per minute, by 1905 there were machines moving at 150 feet per minute.
High speed tool steels have 14–18% tungsten, 3–5% chromium, 0.6% carbon, and other elements depending on the application.
Shock resisting tool steel
Group S shock-resisting tool steels are incredibly strong, built to have high impact resistance. Their strength does not simply lie at the surface or “shocked” layer of the tool. They may be used for springs, chisels, stamping dies, and punches. This group of alloys often has many alloying elements—most of which are seen in other tool steels. However, silicon, in the range of 0.15–3%, is present in this class of steels.
Special purpose tool steels
This group of alloys is where the odd ducks go to roost; these metals are used in specific applications and may even be created for a particular manufacturer by a metallurgist. This group includes low-alloy, high-iron steels where all other alloying elements are sparely used. These are most like the inexpensive W-steels and are also water quenched. The minute addition of other elements increases mechanical properties but keep costs down. Low carbon “mold” steels are also in this group of special purpose alloys. These are used for thermoplastic molding that must be heat-tolerant and wear resistant but do not need high impact resistance.
Choosing a tool steel
If designing an industrial machine, or buying a machine for industrial or manufacturing purposes, the type of tool steel used is important and should be specified by an engineer.
The need-to-know is different for individuals buying hand tools. Often, a hand tool manufacturer will offer “hardened-steel”, and then choose the steel based on the tool’s intended use. However, what grade of steel is being used is something of an industry secret for each tool manufacturer. The manufacturers balance value and job stresses on a part-by-part basis. The best way to choose tools is to get recommendations from professionals in the field, friends, and online reviews, rather than worrying about the alloy itself. If the manufacturer has any metallurgical competence and isn’t cutting corners, the design of a hand tool becomes more important than the alloy.
Blacksmiths, founders, and other metallurgists will be more curious, even at the home-hobbyist level. Searching for resources and forums in these groups provide a wealth of information about what can be expected and what types of metal might be best used.