High-carbon and alloy steels known as "tool steels" are frequently used to create cutters, reamers, bits, and other items needed to machine metals, plastics, and wood. Tool steels typically include cobalt or nickel to improve strength and performance at high temperatures, and carbide former for increased wear resistance, including: chromium, vanadium, molybdenum, and tungsten in various combinations. Most tool steel is produced in electric arc furnaces, where it is melted, purified, brought to the correct composition, then poured into ingots, cooled, and annealed to prepare it for further processing. Some tool steels are produced by powder metallurgy or other specialized processes.
This article will discuss the general characteristics of tool steels, different tool steel compositions, the various tool steel grades, and applications of tool steels.
What is Tool Steel?
The name ”tool steel” describes a range of carbon and alloy steels that are especially well-suited for use in the production of tools. These steels are distinguished by their hardness, resistance to wear, toughness, and resistance to softening at high temperatures. These properties make them suitable candidates for tool manufacturing, including reamers, drills, machine dies, and hand tools.
They are divided into numerous categories based on their features and makeup. There are seven main types of tool steel: water-hardening, hot-work, cold-work, shock-resisting, mold steels, high-speed steels, and special purpose tools steel. The presence of carbide-forming elements like chromium, vanadium, molybdenum, and tungsten are key identifying features of high-speed tool steels, for example. Their performance at high temperatures is enhanced by the addition of cobalt or nickel. Tool steels are typically heat treated to increase their hardness and used for metal stamping, forming, shearing, cutting, and plastic forming.
The carbon percentage in tool steels is typically in the range of 0.7 to 1.5 wt% carbon. However, some tool steels can contain up to 2.1% carbon, while others contain less than 0.25 % carbon. Hardness, strength, and hardenability are enhanced with an increase in carbon concentration. However, because of its propensity to create martensite, carbon also makes materials more brittle and less weldable.
How Tool Steel is Made
Tool steels are composed of various combinations of carbide-forming metals such as chromium, molybdenum, tungsten, and vanadium. These iron-based alloys have relatively high carbon content for strength and carbide formation. Substitutional solutes like nickel and cobalt are added for high-temperature strength. Carbide-forming metals like: chromium, molybdenum, tungsten, and vanadium are added for hardness and wear resistance. They are divided into different categories based on their composition and characteristics including: cold work tool steels, hot work tool steels, and high-speed tool steels.
Tool steel is made in a variety of ways. One of the main methods is through an electric arc furnace (EAF). EAF is done by melting recycled steel scrap in an electric arc furnace together with alloying components. To stop oxidation, the molten mixture is combined with chemicals and thrown into a huge ladle. The steel can then flow down into enormous molds to form ingots once the impurities have been eliminated during this refining stage.
An alternative to EAF is electroslag refining (ESR). With this method, ingots with smooth surfaces and no tubes (holes) or porosity (imperfections) are created using a progressive melting technique. This creates very high-grade steel with few imperfections. Other common steps in the manufacture of tool steel includes:
- Annealing: Steel is heated up to a certain temperature and kept there for a certain amount of time before being cooled down again. This process causes a change to the molecular structure of the steel, making it less brittle, and more workable.
- Hot or Cold Drawing: Drawing processes are used on tool steel to achieve higher tolerances, smaller sizes, or unique shapes. Because of the high strength and limited ductility of tool steels, numerous passes or warm drawings at temperatures as high as 540 °C are used. Cold drawing is usually restricted to a single, light pass to prevent breakage of the feedstock.
Tool steels are used in a variety of different industries and applications, including: cutting, forming, shearing, and stamping of metals and plastics; extrusion of plastic components, such as pipes and vinyl window frames; and the manufacture of dies for pressing metal powder into shapes like gears.
Properties of Tool Steel
The three key properties associated with tool steel are: wear resistance, heat resistance, and toughness. Alloying elements are added to increase strength, wear resistance, hardness, and toughness. The degree of resistance to deformation of steel is measured by its hardness. The Rockwell C test is most frequently used to gauge the hardness of tool steels. Depending on the grade, hardened cold work tool steels have a hardness of around 58/64 HRC (Rockwell C). The majority are normally between 60/62 HRC, while some are occasionally utilized up to 66 HRC.
The temperature used to harden tool steel depends on the chemical composition of the steel. The different types of tool steel are usually heat treated at critical temperatures, which are determined by the type of steel and are typically in the range of 760-1300 °C. This step is then followed by a controlled cooling step.
Main Types of Tool Steel
Tool steels can be divided into seven main categories. Table 1 below lists the categories with their corresponding symbols and attributes:
1. Water-Hardening Tool Steels (Symbol W)
W-grade tool steels are high-carbon steel that requires water quenching because its low alloy content gives it lower hardenability than other tool steels. Small amounts of other elements, like manganese, molybdenum, and silicone can be added to the steel for added functionality. This group of tool steels is less expensive than the others, making it a popular choice for many basic applications. Although it often costs less than other kinds of tool steels, it cannot be used in situations where high temperatures are present — at 150 °C they start to noticeably soften. This steel can reach high hardness levels but it is more brittle when compared to other tool steels. Water quenching, which might result in more warping and cracking, is required for all W-grade tool steels. Some common uses are for the manufacture of cold heading dies, embossing tools, industrial cutting tools, and reamers.
2. Shock-Resisting Tool Steels (Symbol S)
Shock-resisting tool steel was created to withstand stress at low temperatures with fair hot hardness. The S-grade metals are characterized by their high impact toughness with limited abrasion resistance. S-type steels are not among the hot-hard tool steels, with a temperature limit of up to 537 °C.
Typical applications include: chisels, boiler shop tools, tool chuck jaws, collets, clutch parts, hot and cold swaging dies, hot and cold shearing blades, and chipper knives.
3. Mold Steels (Symbol P)
P-type tool steels are used to make mold steels for manufacturing plastic parts. These steels are suitable molds and dies for processes such as: cold punching, hot forging, die casting, and plastic injection molding. Common mold tool steel grades include P20 and 420 (highly refined, mold-quality stainless steel).
4. Cold-Work Tool Steels
Cold-work tool steels are divided into three categories: air-hardening (A-grade), oil-hardening (O-grade), and high-carbon chromium (D-grade) tool steels. This group of tool steels offers average hardness, high wear resistance, and high hardenability. They are typically used in the fabrication of larger parts or parts that require minimal distortion when hardened.
A-grade tool steel are steels that are air-cooled. These steels combine the benefits of deep hardening qualities equal to air hardening grades with a low hardening temperature range similar to oil hardening grades. Compared to other air-hardening grades or oil hardening grades, A6 tool steel exhibits the least distortion. This tool steel is easily machined, and has a good balance of toughness and wear resistance. Its common uses include: coining, cams, die bending, arbors, and blanking.
O-grade tool steels stand for oil-hardening steels. It is robust, has good abrasion resistance, and is used in a variety of applications. Applications include: chasers (thread-cutting), arbors, bushings, and die blanking.
D-type tool steels have chromium contents of 10-13%. They can maintain their hardness up to 425 °C. These steels are also air-hardening steels with high hardenability, low distortion, good high wear resistance, and they are good for long production runs. Common applications are: die-casting die blocks, drawing dies, and forging dies.
5. Hot-Work Tool Steels (Symbol H)
H-grade tool steels are used for working material at high temperatures, with the exception of cutting. H-grade tools steels are harder and stronger and are ideal for applications where the steel will be exposed to high temperatures for extended time intervals. These metals have a fair amount of alloying content with little carbon content.
H-grade tool steel is frequently used for applications such as: cold heading die casings and magnesium or aluminum hot extrusion processes.
6. High-Speed Steels
High-speed steels (HSS) are divided into M-type (molybdenum-based) and T-type (tungsten based). The molybdenum steels have a shorter hardening range and a lower hardening temperature than the tungsten grades due to their somewhat lower melting point. Although the M-type high-speed tool steels are slightly less hard than the T-type high-speed tool steels, they are more durable.
HSS are frequently used in cutting tools, tool bits, drill bits, and power-saw blades. These steels have the ability to endure high temperatures without losing their hardness. The high-speed steels are so named because they can cut at higher tool speeds and feed rates than plain high-carbon steels. In comparison to conventional carbon and tool steels, HSS grades typically exhibit high levels of hardness and abrasion resistance (usually linked to the tungsten and vanadium components frequently used in HSS).
7. Special Purpose Tools Steels
Special purpose tool steels are too expensive for general W-type tool steels. Their special compositions and qualities make them suitable for special applications that can't be accomplished by W steels. These steels do not require the extra expense of the other tool steel alloy content and attendant challenges with heat treatment.
Special purpose tool steels are divided into two groups: low alloy (L-type) and carbon-tungsten-based (F-type). L-type tool steels are used where wear resistance and toughness are prioritized, including: bearings, clutch plates, rollers, wrenches, cams, and collets. Steels with increased carbon content are used for dies, drills, gauges, knurls, and taps.
F-type steels are water-hardening tool steels. These steels are ideal for applications that require high wear resistance, but not high temperature or shock resistance. Common applications of F-type steels include: paper-cutting knives, broaches, burnishing tools, reamers, and plug gauges.
How Can Xometry Help
Xometry provides a wide range of manufacturing capabilities including CNC machining, 3D printing, injection molding, laser cutting, and sheet metal fabrication. Get your instant quote today.
Summary
Disclaimer
The content appearing on this webpage is for informational purposes only. Xometry makes no representation or warranty of any kind, be it expressed or implied, as to the accuracy, completeness, or validity of the information. Any performance parameters, geometric tolerances, specific design features, quality and types of materials, or processes should not be inferred to represent what will be delivered by third-party suppliers or manufacturers through Xometry’s network. Buyers seeking quotes for parts are responsible for defining the specific requirements for those parts. Please refer to our terms and conditions for more information.
Dean McClements
Dean McClements is a B.Eng Honors graduate in Mechanical Engineering with over two decades of experience in the manufacturing industry. His professional journey includes significant roles at leading companies such as Caterpillar, Autodesk, Collins Aerospace, and Hyster-Yale, where he developed a deep understanding of engineering processes and innovations.
Read more articles by Dean McClements
