The steels of this group are characterized by heat treatment that hardens the steel, followed by tempering to give them a sorbitic structure with satisfactory hardness, toughness and fatigue strength, so as to make these steels capable of enduring to the strongest static and dynamic stresses undergone by mechanical parts.

The large variety of sizes of the pieces to heat-treat means that great caution is necessary in choosing the type of steel.

Italian standard UNI EN 10083 specifies various kinds of steel and ladle analysis.