The Types of Tool Steels

“Tool Steel” refers to a class of steels that are metallurgically very “clean” and fall within strict limits for alloy proportions. Vanadium, tungsten, and molybdenum are often added to tool steels to make the steel resist annealing (softening) when used in “high-speed” (high heat) applications. Chromium is added in very large quantities for corrosion resistance (“stainless”). High-speed steels are essential in metal-working tools (drills, milling cutters, etc.) and “stainless” steels can be cost effective by resisting rust during the manufacture, shipping, and storage of the tool itself.

Correctly heat-treated, tools made from high-speed, stainless, and “chrome-vanadium” steels may hold an edge well in woodworking applications, but, due to the large, hard carbide particles that form during hardening, they are difficult to sharpen and cannot be honed as sharply as a blade of plain high-carbon steel. Our choice of High-Carbon Tool-Steel (.95% Carbon: either O1, our “high carbon” or A2) offers the finest, sharpest edge possible.

Tool steel refers to a variety of carbon and alloy steels that are particularly well-suited to be made into tools. Their suitability comes from their distinctive hardness, resistance to abrasion, their ability to hold a cutting edge, and/or their resistance to deformation at elevated temperatures (red-hardness). Tool steel is generally used in a heat-treated state. Many high carbon tool steels are also more resistant to corrosion due to their higher ratios of elements such as vanadium and niobium.

With a carbon content between 0.7% and 1.5%, tool steels are manufactured under carefully controlled conditions to produce the required quality. The manganese content is often kept low to minimize the possibility of cracking during water quenching. However, proper heat treating of these steels is important for adequate performance, and there are many suppliers who provide tooling blanks intended for oil quenching.

Tool steels are made to a number of grades for different applications. Choice of grade depends on, among other things, whether a keen cutting edge is necessary, as in stamping dies, or whether the tool has to withstand impact loading and service conditions encountered with such hand tools as axes, pickaxes, and quarrying implements. In general, the edge temperature under expected use is an important determinant of both composition and required heat treatment. The higher carbon grades are typically used for such applications as stamping dies, metal cutting tools, etc.

Tool steels are also used for special applications like injection molding because the resistance to abrasion is an important criterion for a mold that will be used to produce hundreds of thousands of parts.

Water-hardening types

W-type tool steel gets its name from its defining property of having to be water quenched. W-grade steel is essentially high carbon plain-carbon steel. This type of tool steel is the most commonly used tool steel because of its low cost compared to other tool steels. They work well for small parts and applications where high temperatures are not encountered; above 150 °C (302 °F) it begins to soften to a noticeable degree. Hardenability is low so W-grade tool steels must be quenched in water. These steels can attain high hardness (above HRC 66) and are rather brittle compared to other tool steels. W steels are still sold, especially for springs,but are much less widely used than they were in the 19th and early 20th centuries. This is partly because W steels warp and crack much more in quench than oil-quenched or air hardeneing steels.

The toughness of W-type tool steels are increased by alloying with manganese, silicon and molybdenum. Up to 0.20% of vanadium is used to retain fine grain sizes during heat treating

Typical applications for various carbon compositions are:

• 0.60–0.75% carbon: machine parts, chisels, setscrews; properties include medium hardness with good toughness and shock resistance.
• 0.76–0.90% carbon: forging dies, hammers, and sledges.
• 0.91–1.10% carbon: general purpose tooling applications that require a good balance of wear resistance and toughness, such as rasps, drills, cutters, and shear blades.
• 1.11–1.30% carbon: files, small drills, lathe tools, razor blades, and other light-duty applications where more wear resistance is required without great toughness. Steel of about 0.8 % C gets as hard as steel with more carbon, but the free iron carbide particles in 1% or 1.25% carbon steel make it hold an edge better. However, the fine edge probably rusts off faster than it wears off, if it is used to cut acidic or salty materials.

Cold-working types

These tool steels are used on larger parts or parts that require minimal distortion during hardening. The use of oil quenching and air hardening helps reduce distortion as opposed to higher stress caused by quicker water quenching. More alloying elements are used in these steels, as compared to water-hardening grades. These alloys increase the steels’ hardenability, and thus require a less severe quenching process. These steels are also less likely to crack and are often used to make knife blades.

Oil-hardening types (O1)

Composition : 0.90% C, 1.0–1.4% Mn, 0.50% Cr, 0.50% Ni, 0.50% W. It is a very good cold work steel and also makes very good knives. It can be hardened to about 57-61 HRC.

Air-hardening types (A1,A2,A3, etc)

The first air hardening grade tool steel was mushet steel, which was known as air-hardening steel at the time. Modern air-hardening steels are characterized by low distortion during heat treatment because of their high-chromium content. They also harden in air because they have less alloyants than oil-hardening grades. Their machinability is good for tool steels and they have a balance of wear resistance and toughness (i.e. between the D- and shock-resistant grades).

D-types

D-type tool steels contain between 10% and 18% chromium. These steels retain their hardness up to a temperature of 425 °C (797 °F). Common applications for these grade of tool steel is forging dies, die-casting die blocks, and drawing dies. Due to high chromium content, certain D-grade tool steel grades are often considered stainless or semi-stainless tool steels. However their corrosion resistance was very limited due to the precipitation of major amount of chromium and carbon as carbides.

Shock resisting types

S-type tool steel are designed to resist shock at both low and high temperatures. A low carbon content is required for the necessary toughness (approximately 0.5% carbon). Carbide-forming alloys provide the necessary abrasion resistance, hardenability, and hot-working characteristics. This family of steels displays very high impact toughness and relatively low abrasion resistance, it can attain relatively high hardness (HRC 58/60). This type of steel is used in applications such as jackhammer bits. In the US, toughness usually comes from 1 to 2% silicon and 0.5-1% molybdenum. In Europe,shock steels often contained .5-.6 % carbon and around 3% nickel. 1.75% to 2.75% nickle is still used in some shock resisting and high strength low alloy steels (HSLA),such as L6, 4340, and Swedish saw steel, but it is expensive.

High speed types

T-type and M-type tool steels are used for cutting tools where strength and hardness must be retained at temperatures up to or exceeding 760 °C (1,400 °F). M-type tool steels were developed to reduce the amount of tungsten and chromium required. T1 (also known as 18-4-1) is a common T-type alloy. Its composition is 0.7% carbon, 18% tungsten, 4% chromium, and 1% vanadium. M2 is a common M-type alloy.

Hot-working types

H-type tool steels were developed for strength and hardness during prolonged exposure to elevated temperatures. All of these tool steels use a substantial amount of carbide forming alloys. H1 to H19 are based on a chromium content of 5%; H20 to H39 are based on a tungsten content of 9-18% and a chromium content of 3–4%; H40 to H59 are molybdenum based.

Special purpose types

• P-type tool steel is short for plastic mold steels. They are designed to meet the requirements of zinc die casting and plastic injection molding dies.
• L-type tool steel is short for low alloy special purpose tool steel. L6 is extremely tough.
• F-type tool steel is water hardened and substantially more wear resistant than W-type tool steel.