Alloy steel.html

Iron alloy phases v • d • e
Ferrite (α-iron, δ-iron; soft) Austenite (γ-iron; harder) Spheroidite Pearlite (88% ferrite, 12% cementite) Bainite Martensite Ledeburite (ferrite-cementite eutectic, 4.3% carbon) Cementite (iron carbide, Fe₃C; hardest)
Steel classes
Carbon steel (≤2.1% carbon; low alloy) Stainless steel (+chromium) Maraging steel (+nickel) Alloy steel (hard) Tool steel (harder)
Other iron-based materials
Cast iron (>2.1% carbon) Ductile iron Wrought iron (contains slag)

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.¹ 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.

1 Low alloy steel
2 Material science
3 See also
4 References
- 4.1 Notes
- 4.2 Bibliography

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.²

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

**Principle low alloy steels**³
SAE designation	Composition
13xx	Mn 1.75%
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
51Bxx	Cr 0.80%
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%

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.⁴

Alloying elements also have an affect 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.⁵

References

Notes

^ Smith, p. 393.
^ Classification of Carbon and Low-Alloy Steel, http://www.key-to-steel.com/Articles/Art62.htm, retrieved on 25 September 2008 .
^ Smith, p. 394.
^ Smith, pp. 394-395.
^ Smith, pp. 395-396

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.
Smith, William F.; Hashemi, Javad (2001), Foundations of Material Science and Engineering (4th ed.), McGraw-Hill, p. 394, ISBN 0-07-295358-6