Water Hardening Tool Steels: Low hardenability, water quench necessary to form martensite.
Oil Hardening Tool Steels: High wear resistant steels.

1) Air Hardening Cold Work Tool Steels: High wear resistance. Requires the surface to be protected against surface oxidation and decarburization. Because of the high carbon content, fine carbide distribution is accomplished. Will form martensite with an air cool; however, scale and decarburization will occur. Therefore it is necessary to process under vacuum conditions or protective atmosphere. Because of the slow cool requirement to form martensite, distortion is kept at a minimum, except for induced residual stress resulting from prior machining.

2) High Carbon/High Chrome D series: These steels are known as secondary hardening steels. Some of the alloy carbides will form on the tempering of the steel. In other words, the steel will increase in hardness value to form large volumes of the alloy carbides, giving high wear characteristics. It should be noted that these steels (although known as dimensionally stable steels) will distort if the induced machining stress is not relieved before the austenitizing process. They are typically used in cold work applications.

3) Mold Steels (P series): Because of the low carbon content, the hardenability of the steel is low. These steels will be suitable for hobbing of cavities for plastic mold dies or low-temperature metal casting. The only method of achieving any degree of high surface hardness for wear resistance is to surface treat by FNC, carbo-nitride or carburize.

4) Chromium Hot Work Die Steels: This group of steel is generally used in hot work applications such as forging, aluminum extrusion, shot sleeves, magnesium die-casting and aluminum die-casting. These steels have good hardenability and will harden (as quenched to approximately 55HRC depending on the maximum cross sectional thickness).
Tempering temperatures are generally in the 1000°F range so that the carbide precipitation will begin to occur (vanadium and chromium carbides). This steel group is also susceptible to retained austenite, which can and must be decomposed by multiple tempering.

5) Molybdenum Hot Work Tool Steels: These steels are employed when higher forging temperatures are being used, as opposed to temperatures used on alloy carbon steels. This group of steels is used for high temperature forging, hot extrusion or hot metal die casting—such as aluminum die casting and magnesium die casting.

6) Molybdenum High-Speed Steels: Molybdenum is among the carbide forming elements and strongly assists in the formation of hard carbides that are finely dispersed throughout the matrix of the steel. Care must be taken with these steels with the austenitizing temperature selection as well as the holding time at temperature. Eutectic grain boundary melting can begin to occur if the steel is held too long at too high an austenitizing temperature.

7) Tungsten High-Speed Steels: Tungsten is a very strong carbide former and will produce very high as quenched hardness values. All of the high-speed steels (molybdenum and tungsten) are known as secondary hardening steels. In addition, this group of steels has the ability to cut at red heat without losing its hardness value. The addition of cobalt enables an even higher operating temperature, before losing the as quenched hardness.

8) Ultra Hard High-Speed Steels: These steels are used to produce hardness values in excess of 70 HRC. The ultra hard high-speed steels will demonstrate high impact values. In other words, they are very tough under repeated impact such as milling cutters.

9) Carbide Formers: Carbide formers are those elements that will form strong carbides of finely dispersed carbides. The strong carbide forming elements are chromium, molybdenum, tungsten, vanadium, manganese, titanium, zirconium and niobium.

The above elements will react favorably with the carbon to form hard carbides, provided that the appropriate austenitizing temperature was selected. It is not necessary to have all of the above elements in solution. Provided there is carbon (in solution) then these elements can be present individually or corporately. The non carbide forming elements are nickel, aluminum, copper, nitrogen, cobalt and silicon.