What Are the Functions of Alloying Elements in Tool Steel?

Carbon

• The presence of carbon, typically at a concentration of between 0.2 percent and 1.3 percent by weight, is essential for tool steel. Carbon increases hardness, tensile strength and resistance to wear, but it decreases ductility, machinability and toughness. The level of carbon in tool steel can be adjusted to give the steel certain properties. For example, water-hardening tool steel relies on relatively high carbon content ̵1; typically between 0.7 percent and 1.3 percent by weight ̵1; for its useful properties (primarily wear-resistance) at the expense of toughness.

Chromium

• Chromium is added to tool steel to increase its hardenability, heat resistance and cutting performance. Chromium combines with carbon atoms in the tool steel during annealing ̵1; the process in which the steel is heated and then cooled to make it less brittle ̵1; to form different types of chromium carbides, depending on the amount of chromium added. Tool steel typically contains between 4 and 12 percent chromium, by weight.

Vanadium

• Originally, vanadium was added to tool steel to remove impurities and reduce the amount of nitrogen dissolved in the matrix of iron atoms. However, it is now known that vanadium combines with carbon atoms to form hard, thermally stable carbides, increasing not only the hardness, durability and cutting performance of tool steel but also the ease with which it can be forged and hammered into shape. Adding too much vanadium to tool steel does make it very difficult to grind.

Tungsten and Molybdenum

• At an atomic level, tungsten and molybdenum are similar and produce similar results when added to tool steel. Tungsten increases the resistance of tool steel to softening at high temperatures (also known as ̶0;hot hardness̶1;) and forms hard, wear-resistant carbides; molybdenum improves properties of hardenability and toughness. Molybdenum is less expensive than tungsten and is often used as a substitute for it, although heat treatment of molybdenum tool steel is more difficult than that steel incorporating tungsten. Tool steel contains just small amounts of tungsten or molybdenum, typically between 1 and 2 percent by weight.