Tool steels are among the hardest of metals. The hardness ranges from 55Rc, for example wood cutting steels, to the 60-65Rc range for steels used in heavy duty shearing and punching operations.

Some of the same principles are incorporated in tool steels that are found in engineering steels - that is increasing hardness on relatively thin sections can be achieved by increasing the carbon content. However for through hardness in thicker sections, for toughness, for wear resistance or for use at elevated temperatures it is important to select a steel with the correct combination of alloying elements.

Fortunately it is not necessary to know the advantages conferred by each alloy or alloy combination in order to make a correct selection. The AISI system of numbering makes selection relatively simple.

Each steel is designated by a letter and a number.
e.g. Ol steel, A2 steel, M42 high speed steel The letter identifies which group it belongs to and gives a clue as to its method of heat treatment or its application, or its chemical composition - as follows:

WATER HARDENING STEELS: "W" series
These depend on their carbon content to give hardness, but because of their very low alloy content, require a rapid quench (cold agitated water) in order to harden. These are relatively cheap steels but are very limited in use because the fast quench increases the likelihood of distortion or cracking. For use with thin sections and simple shapes.
Common grade WI

OIL HARDENING STEELS: "O" series
The increased alloy content makes these steels more responsive to quenching so that full hardness may be achieved on thicker sections using oil as a quenchent. The resulting slower change of temperature reduces the risk of distortion and cracking.
Common grade - Ol

AIR HARDENING STEELS "A" series
The "A" series - further increases in the alloy content gives a series of steels which will harden when "quenched " in air. This allows a much slower temperature drop which further reduces the risk of distortion or cracking.
Common grade A2.

DEEP HARDENING STEELS "D" series
The "D" series are the high carbon (1.5% - 2.5%), high chromium (nominally 12%) which will fully harden in heavy sections, and which , because of embedded chromium carbides in the structure, are extremely abrasion resistant.
Common grade - D2.

SHOCK RESISTING STEELS "S" series
In this group the carbon content is kept relatively low which improves toughness (resistance to shock loading), at the expense of wear resistance. Within the group they vary from low alloy, which require water quenching, through to medium alloy which may be air quench hardened. These differences carry the same implications as in the "W', "0" and "A" series with regard to risk of distortion and cracking.
Common grade - S7.

HOT WORK DIE STEELS "H', series
These are designed for hot working applications such as forging and also for dies in the die casting of zinc and some copper alloys. The low carbon content (about .35%) and high alloy content principally chromium, tungsten or molybdenum gives a combination of deep hardening , resistance to scaling and resistance to heat cracking. They are also among the toughest of the tool steels when used at normal temperatures.
Common grade H13.

MOLYBDENUM HIGH SPEED STEEL "M" series
This is the general purpose cutting steel used in metal machining operations. The temperatures generated at the tip of a cuffing tool would be sufficient to soften a low alloy tool steel but the addition of molybdenum in combination with other alloys produces a "temper resistant" microstructure. This grade is useful on the softer alloys or for slower speed on harder alloys.
Common grades – M6, M42, M2

TUNGSTEN HIGH SPEED STEEL "T" series
This is a higher grade than the "M" steels. It retains its hardness and therefore its cutting edge even when machining at high speed or when the metal being cut is in the quenched and tempered condition. It is fairly expensive and has to some extent been replaced by sintered carbide.
Common grade Tl.

These notes are a very brief summary of a quite complex subject and we hope that they offer some clarification.