Alloy steel

Alloy steel is steel alloyed with other elements in amounts of between 1 and 50% by weight to improve its mechanical properties. Alloy steels are broken down into two groups: low alloy steels and high alloy steels. Low alloy steels are defined as having an alloy contents between 1 and 4% and high alloy steels have 4 to 50% alloying contents. [Smith, p. 393.] However, most commonly alloy steel refers to low alloy steel.

These steels have greater strength, hardness, hot hardness, wear resistance, hardenability, or toughness compared to carbon steel. However, they may require heat treatment in order to achieve such properties. Common alloying elements are molybdenum, manganese, nickel, chromium, vanadium, silicon and boron.

Low alloy steel

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. [Citation | title = Classification of Carbon and Low-Alloy Steel | url = http://www.key-to-steel.com/Articles/Art62.htm | accessdate = 2008-09-25.]

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:

*D6AC
*300M

Material science

Alloying elements are added to achieve certain properties in the material. The alloying elements tend to either form 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•M_xO_y. 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 the carbides V₃C₃, TiC, and NiC, respectively. [Smith, pp. 394-395.]

Alloying elements also have an affect on the eutectoid temperature of the steel. Manganese and nick 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". [Smith, pp. 395-396]

ee also

*HSLA steel
*SAE steel grades

References

Notes

Bibliography

*Groover, M. P., 2007, p. 105-106, "Fundamentals of Modern Manufacturing: Materials, Processes and Systems", 3rd ed, John Wiley & Sons, Inc., Hoboken, NJ, ISBN-13 978-0-471-74485-6.
*Citation | last = Smith | first = William F. | last2 = Hashemi | first2 = Javad | title = Foundations of Material Science and Engineering | publisher = McGraw-Hill | page = 394 | year = 2001 | edition = 4th | isbn = 0-07-295358-6