What is Tool Steel?

Tool steel is a type of steel with mechanical properties that make it a desirable material for tools. Tool steels are noteworthy for their hardness, ability to hold a sharp edge or resistance to damage from abrasion, and deformation. There are a a number of alloy and carbon steels, usually made through a product of heat treatment and quenching, that are suitable as tool steels. Tool steel is made in many different grades and can be used to make tools such as stamping dies, construction equipment, and axes. In addition to making tools, it is also used in other applications that require materials with tool steel's mechanical properties.

The different grades of tool steel are frequently classified according to a system called the SAE steel grades or AISI/SAE steel grades, created by the Society of Automotive Engineers (now SAE International) and the American Iron and Steel Institute. It categorizes each type of steel with a letter that indicates its properties or production method, such as W-grade for water-hardened steel or S-grade for shock-resistant steels. Each type of steel within these broad categories then receives an individual number that follows the letter.

The most common form of tool steel is W-grade, a form of carbon steel popular due to its low cost. Some types of W-grade steel also include silicon, molybdenum, or manganese to increase the steel's toughness. It is called water-hardened because water is used during quenching, a step in the production process of many steels in which the heated steel is rapidly reduced in temperature. W-grade tool steel is hard, but tends to be brittle and does not stand up well to temperatures above 302° F (150° C). W-grade tools steels are commonly used to make blades such as shear blades and razors; machine parts for machinery that does not encounter or produce high temperatures; and tools such as hammers, drills, and chisels.

Grades A, O, and D are steels produced by cold-working, a process in which the steel is subjected to mechanical stress until it undergoes plastic deformation, a permanent change in the steel's microstructure. This process increases the steel's tensile strength and hardness while lowering its ductility, and the resulting forms of steel are commonly used for purposes such as blades and machine tools. O-grade steel is oil-hardened while A-grade is air-hardened, methods that produce less distortion in the steel than water-hardening. D-grade steel contains a large amount of chromium, which composes 10 percent to 18 percent of the alloy. In addition to chromium, A-, O-, and D-grade steels are often alloyed with other metals, including manganese, tungsten, and vanadium, and with nonmetals such as sulfur and phosphorous.

H-grade steel is hot-working tool steel, created by causing plastic deformation in steel through extended exposure to high temperatures. These steels have high strength and hardness, but slight distortions from the process of cooling and thermal contraction makes them less suited than cold-working steel for applications with very tight engineering tolerances. Chromium, tungsten, and molybdenum are common alloying elements in steels of this grade.

The T and M grades are types of high-speed steel, which is distinguished by its ability to retain high hardness at high temperatures. This property makes it well suited for use in powered cutting tools and stamping dies, where its resistance to the heat produced by friction allows it to work at higher speeds than other steels. T-grade steel is named for its high tungsten content, while M-grade steel contains a high amount of molybdenum. Additional alloying elements common in these steels include vanadium, chromium, and cobalt.

S-grade steels are tool steels distinguished by their high shock resistance. They have less carbon than other steel alloys, which decreases their resistance to abrasion but increases their toughness. S-grade steel is used in equipment such as jackhammers that needs to be able to withstand heavy impact.

Some grades of tool steel have very specialized characteristics or purposes. F-grade steel is a type of water-hardened steel, like W-grade steel, but has superior wear resistance. P-grade steel, or plastic mold steel, is used in injection molding and die casting machines. L-grade, or low alloy special-purpose steel, is a very tough type of steel that has little alloying material and very high iron content.