What is Tool Steel?

As you might guess, tool steel earns its name by being primarily used for tools. If you then picture a steel screwdriver, pliers, or a wrench, you’d be right – but the items created of tool steel group consist of much more. Within the steel industry, tool steel also refers to machine tools, including those used to manufacture other metal products. To meet the standards of this demanding usage, tool steel is known for being extremely hard, resistant to abrasion and deformation, and its ability to hold a cutting edge. While the steel’s chemical composition is important, the method of production must be carefully controlled to achieve the proper qualities expected in tool steel.

Tool Steel Groups
Tool steel is classed into six groups, all made of carbon steel alloyed with one or more of these main elements: molybdenum, tungsten, chromium, and vanadium. Most tool steel originates from recycled steel scrap, but not all steel scrap is suitable for tool steel. Alloy scrap containing elements which resist oxidation (such as nickel, cobalt or copper) will impede the development of carbides in the metal’s structure, and carbides are crucial to creating tool steel. To ensure the metal will be an optimal mix, tool steel will typically consist of 75% mill scrap supplemented by purchased steel scrap.
The first step is primary melting, where the scrap is heated until molten. Most often this is done in an electric arc furnace (EAF) due to it being a lower-cost production method. However, it is key to avoid contamination during the melting process to create the highest-quality steel. While the EAF is widely used for tool steel, there is some risk of contamination from dust or traces of oxidation-resistant metals from previous batches. For top notch tool steel, an alternate method known as electroslag refining (ESR) is used to melt the steel. Instead of a furnace, ESR uses electric currents to superheat and slowly melt the metal. While a more expensive process, ESR will produce a more refined type of steel without apparent surface imperfections.
After melting and alloying, the steel is poured into ingot molds and then forged into the desired shapes. This is where the six groups of tool steel are determined by their processing:

• Water hardening: Also known as W-group tool steel, this group’s defining property is being water quenched. Quenching in water does risk warping or cracking steel with its rapid cooling, and results in a relatively brittle metal. For this reason, water-hardened tool steel is not considered appropriate for high-temperature industrial uses; once the temperature nears 300 degrees Fahrenheit, there is noticeable softening of the steel parts. Typical items made of W-group steel include scissors, smaller hand tools, and springs.

• Shock-resisting: S-group tool steel is notable for its toughness and ability to withstand repeated impact. This is achieved by using low carbon steel during its production, since higher carbon content results in a harder but more brittle product. Alloying with silicon, tungsten, and chromium gives additional wear resistance and tensile strength. It’s then quenched in oil, which allows for a gentler cooling than water and helps produce a metal with a high degree of durability. Items made of S-group tool steels include tableware dies, impact hammers for nails guns, and shear blades used in cutting heavy steel plate.

• High speed: This group is mainly used for cutting tools, which requires a very hard and abrasion-resistant type of steel. Tungsten, chromium, and vanadium allow HS steel to withstand very high temperatures, such as the heat friction created while cutting other metals. It also manages to hold a cutting edge with repeated use, allowing for higher speeds while cutting without having to pause to resharpen. HS steel can be found in various cutting tools such as drill bits, saw blades, and milling cutters.

• Hot work: Hot-working (H-group) steels are used to cut and shape materials at very high temperatures. To achieve a steel that can withstand prolonged exposure to intense heat, H-group steels are made of a low carbon steel with a higher percentage of alloying content. This produces a tool steel with a high amount of carbides, giving it good overall toughness and wear resistance. The most common use for hot work steel is to produce other metal items because it can withstand the heat needed to forge and cast other metals. It’s used in machinery such as pressure dies, extrusion, and forging equipment.

• Cold work: This tool steel type is narrowed down further into three groups known as the A series (air-hardened), O series (oil-hardened), and D series (high carbon/chromium). All three are intended to be used to cut or form materials at low temperatures.

1. A-series: As the name suggests, steel in this series is hardened via air. Because of the quenching method and its high chromium content, A-series cold work steel is known for low distortion during heat treatment. This makes it a good fit for machining purposes, and so can be found in dies, forming tools, and gauges.

2. O-series: Oil-quenching avoids the cracking and warping risks brought on by water-quenching, and is used for larger parts which require minimal distortion. It offers good wear resistance and toughness, lending itself to a wide range of applications as a general-use cold work steel. Typical uses of O-series tool steel include stamping dies, bushings (metal parts used to absorb vibration and friction), shear blades, and other cutting tools.

3. D-series: Made of a high-carbon steel with a high alloy percentage of chromium, this can also be categorized as a type of stainless steel. However, unlike typical stainless steel, its corrosion resistance is limited and thus is not used to make consumer products. Instead, D-series tool steel applications include cutters, dies, plastic injection molds, and machining rollers.

• Special purpose: this group includes tool steels with alloying elements such as nickel, which typically resist the formation of carbides. The extra time and effort needed to produce these tool steels makes them too expensive for general purposes, so they are manufactured for specific use. Special purpose tool steel can be found in plastic molding dies, zinc die casting, and drills.