Alloy steel is alloyed with a variety of elements in total amounts from 1.0% to 50% to improve the mechanical properties. Alloy stainless steels are broken down into two groups: low-alloy steels and high-alloy steels. The difference between this two is somewhat arbitrary: Smith and Hashemi define the difference at 4.0%, while Degarmo etal., define it at 8.0%. Most commonly, the phrase "alloy steel" refers to low-alloy steels.
Strictly speaking, every steel is an alloy, but not all steels are called "alloy steels". The simplest steels are iron (Fe) alloyed with carbon (C) (about 0.1% to 1%, depending on type). However, the term "alloy steel" is the standard term referring to steels with other alloying elements added deliberately in addition to the carbon. Common alloyants include manganese (the most common one), chromium, nickel, vanadium, molybdenum, silicon, and boron. Less common alloyants include cobalt,aluminum, niobium, copper, cerium, titanium,tin, tungsten, zinc, lead, and zirconium.
The following is a range of improved properties in alloy steels (as compared to carbon steels): strength, toughness, hardness, corrosion resistance, wear resistance, hardenability, and hot hardness. To achieve some of these improved properties the metal may require heat treating.
Some of these find uses in exotic and highly-demanding applications, such as in the turbine blades of jet engines, in spacecraft, and in nuclear reactors. Because of the ferromagnetic properties of iron, some steel alloys find important applications where their responses to magnetism are very important, including in electric motors and in transformers.