Alloy Steel

Steels that contain specified amounts of alloying elements -- other than carbon and the commonly accepted amounts of manganese, copper, silicon, sulfur, and phosphorus -- are known as alloy steels. Alloying elements are added to change mechanical or physical properties. A steel is considered to be an alloy when the maximum of the range given for the content of alloying elements exceeds one or more of these limits: 1.65% Mn, 0.60% Si, or 0.60% Cu; or when a definite range or minimum amount of any of the following elements is specified or required within the limits recognized for constructional alloy steels: aluminum, chromium (to 3.99%), cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, zirconium or other element added to obtain an alloying effect. Technically, then, tool and stainless steels are alloy steels.

See Reference Table - AISI-SAE Carbon Steel & Alloy Designations