Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties. Alloy steels are broken down into two groups: low-alloy steels and high-alloy steels. The difference between the two is somewhat arbitrary: Smith and Hashemi define the difference at 4.0%, while Degarmo, et al., define it at 8.0%. Most commonly, the phrase "alloy steel" refers to low-alloy steels.
Every steel is truly an alloy, but not all steels are called "alloy steels". Even the simplest steels are iron (Fe) (about 99%) 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 in addition to the carbon. Common alloyants include manganese (the most common one), nickel, chromium, molybdenum, vanadium, silicon, and boron. Less common alloyants include aluminum, cobalt, copper, cerium, niobium, titanium, tungsten, tin, zinc, lead, and zirconium.
The following is a range of improved properties in alloy steels (as compared to carbon steels): strength, hardness, toughness, wear resistance, corrosion 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.
Low-alloy Steels 
Low-alloy steels are usually used to achieve better hardenability, which in turn improves its other mechanical properties. They are also used to increase corrosion resistance in certain environmental conditions.
With medium to high carbon levels, low-alloy steel is difficult to weld. Lowering the carbon content to the range of 0.10% to 0.30%, along with some reduction in alloying elements, increases the weldability and formability of the steel while maintaining its strength. Such a metal is classed as a high-strength low-alloy steel.
Some common low alloy steels are:
|40xx||Mo 0.20% or 0.25% or 0.25% Mo & 0.042% S|
|41xx||Cr 0.50% or 0.80% or 0.95%, Mo 0.12% or 0.20% or 0.25% or 0.30%|
|43xx||Ni 1.82%, Cr 0.50% to 0.80%, Mo 0.25%|
|44xx||Mo 0.40% or 0.52%|
|46xx||Ni 0.85% or 1.82%, Mo 0.20% or 0.25%|
|47xx||Ni 1.05%, Cr 0.45%, Mo 0.20% or 0.35%|
|48xx||Ni 3.50%, Mo 0.25%|
|50xx||Cr 0.27% or 0.40% or 0.50% or 0.65%|
|50xxx||Cr 0.50%, C 1.00% min|
|50Bxx||Cr 0.28% or 0.50%|
|51xx||Cr 0.80% or 0.87% or 0.92% or 1.00% or 1.05%|
|51xxx||Cr 1.02%, C 1.00% min|
|52xxx||Cr 1.45%, C 1.00% min|
|61xx||Cr 0.60% or 0.80% or 0.95%, V 0.10% or 0.15% min|
|86xx||Ni 0.55%, Cr 0.50%, Mo 0.20%|
|87xx||Ni 0.55%, Cr 0.50%, Mo 0.25%|
|88xx||Ni 0.55%, Cr 0.50%, Mo 0.35%|
|92xx||Si 1.40% or 2.00%, Mn 0.65% or 0.82% or 0.85%, Cr 0.00% or 0.65%|
|94Bxx||Ni 0.45%, Cr 0.40%, Mo 0.12%|
|ES-1||Ni 5%, Cr 2%, Si 1.25%, W 1%, Mn 0.85%, Mo 0.55%, Cu 0.5%, Cr 0.40%, C 0.2%, V 0.1%|
Material science 
Alloying elements are added to achieve certain properties in the material. As a guideline, alloying elements are added in lower percentages (less than 5%) to increase strength or hardenability, or in larger percentages (over 5%) to achieve special properties, such as corrosion resistance or extreme temperature stability.
Manganese, silicon, nickel, and copper are added to increase strength by forming solid solutions in ferrite. Chromium, vanadium, molybdenum, and tungsten increase strength by forming second-phase carbides. Nickel and copper improve corrosion resistance in small quantities. Molybdenum helps to resist embrittlement. Zirconium, cerium, and calcium increase toughness by controlling the shape of inclusions. Manganese sulfide, lead, bismuth, selenium, and tellurium increase machinability.
The alloying elements tend to form either compounds or carbides. Nickel is very soluble in ferrite; therefore, it forms compounds, usually Ni3Al. Aluminium dissolves in the ferrite and forms the compounds Al2O3 and AlN. Silicon is also very soluble and usually forms the compound SiO2•MxOy. Manganese mostly dissolves in ferrite forming the compounds MnS, MnO•SiO2, but will also form carbides in the form of (Fe,Mn)3C. Chromium forms partitions between the ferrite and carbide phases in steel, forming (Fe,Cr3)C, Cr7C3, and Cr23C6. The type of carbide that chromium forms depends on the amount of carbon and other types of alloying elements present. Tungsten and molybdenum form carbides if there is enough carbon and an absence of stronger carbide forming elements (i.e., titanium & niobium), they form the carbides Mo2C and W2C, respectively. Vanadium, titanium, and niobium are strong carbide forming elements, forming vanadium carbide, titanium carbide, and niobium carbide, respectively.
Alloying elements also have an effect on the eutectoid temperature of the steel. Manganese and nickel lower the eutectoid temperature and are known as austenite stabilizing elements. With enough of these elements the austenitic structure may be obtained at room temperature. Carbide-forming elements raise the eutectoid temperature; these elements are known as ferrite stabilizing elements.
|Aluminium||0.95–1.30||Alloying element in nitriding steels|
|Boron||0.001–0.003||A powerful hardenability agent|
|4–18||Increases corrosion resistance|
|Manganese||0.25–0.40||Combines with sulfur and with phosphorus to reduce the brittleness. Also helps to remove excess oxygen from molten steel.|
|>1||Increases hardenability by lowering transformation points and causing transformations to be sluggish|
|Molybdenum||0.2–5||Stable carbides; inhibits grain growth. Increases the toughness of steel, thus making molybdenum a very valuable alloy metal for making the cutting parts of machine tools and also the turbine blades of turbojet engines. Also used in rocket motors.|
|12–20||Increases corrosion resistance|
|Higher percentages||Improves magnetic properties|
|Titanium||-||Fixes carbon in inert particles; reduces martensitic hardness in chromium steels|
|Tungsten||-||Also increases the melting point.|
|Vanadium||0.15||Stable carbides; increases strength while retaining ductility; promotes fine grain structure. Increases the toughness at high temperatures|
See also 
Resources The Iron and Steel Dictionary - Alloy Steel
- Smith, p. 393.
- Degarmo, p. 112.
- Classification of Carbon and Low-Alloy Steel, retrieved 2008-09-25.
- Smith, p. 394.
- Degarmo, p. 113.
- Smith, pp. 394-395.
- Smith, pp. 395-396
- Degarmo, p. 144.
- Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2007), Materials and Processes in Manufacturing (10th ed.), Wiley, ISBN 978-0-470-05512-0.
- Groover, M. P., 2007, p. 105-106, Fundamentals of Modern Manufacturing: Materials, Processes and Systems, 3rd ed, John Wiley & Sons, Inc., Hoboken, NJ, ISBN 978-0-471-74485-6.
- Smith, William F.; Hashemi, Javad (2001), Foundations of Material Science and Engineering (4th ed.), McGraw-Hill, p. 394, ISBN 0-07-295358-6