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Iron alloy phases

Ferrite (α-iron, δ-iron)
Austenite (γ-iron)
Pearlite (88% ferrite, 12% cementite)
Bainite
Martensite
Ledeburite (ferrite-cementite eutectic, 4.3% carbon)
Cementite (iron carbide, Fe3C)

Steel classes

Carbon steel (≤2.1% carbon; low alloy)
Crucible steel
Alloy steel (contains non-carbon elements)

Maraging steel (contains nickel)
Stainless steel (contains chromium)
Tool steel (alloy steel for tools)
Other iron-based materials

Cast iron (>2.1% carbon)

Ductile iron
Gray iron
Malleable iron
White iron

Wrought iron (contains slag)

Alloy steel is steel alloyed with a variety of 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. The differentiation between the two is somewhat arbitrary; Smith and Hashemi define the difference at 4%, while Degarmo, et al., define it at 8%.[1][2] 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 to achieve such properties. Common alloying elements are molybdenum, manganese, nickel, chromium, vanadium, silicon and boron.

Contents

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.[3]

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
  • 256A
Principal low alloy steels[4]
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. 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.[2]

Manganese, silicon, or aluminium are added during the steelmaking process to remove dissolved oxygen from the melt. 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.[5]

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•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 the carbides V3C3, TiC, and NiC, respectively.[6]

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.[7]

Principal effects of major alloying elements for steel[8]
Element Percentage Primary function
Aluminium 0.95–1.30 Alloying element in nitriding steels
Bismuth - Improves machinability
Boron 0.001–0.003 Powerful hardenability agent
Chromium 0.5–2 Increases hardenability
4–18 Corrosion resistance
Copper 0.1–0.4 Corrosion resistance
Lead - Improves machinability
Manganese 0.25–0.40 Combines with sulfur to prevent brittleness
>1 Increases hardenability by lowering transformation points and causing transformations to be sluggish
Molybdenum 0.2–5 Stable carbides; inhibits grain growth
Nickel 2–5 Toughener
12–20 Corrosion resistance
Silicon 0.2–0.7 Increases strength
2 Spring steels
Higher percentages Improves magnetic properties
Sulfur 0.08–0.15 Free-machining properties
Titanium - Fixes carbon in inert particles; reduces martensitic hardness in chromium steels
Tungsten - Hardness at high temperatures
Vanadium 0.15 Stable carbides; increases strength while retaining ductility; promotes fine grain structure

See also

References

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Notes

  1. ^ Smith, p. 393.
  2. ^ a b Degarmo, p. 112.
  3. ^ Classification of Carbon and Low-Alloy Steel, http://www.key-to-steel.com/Articles/Art62.htm, retrieved 2008-09-25 .
  4. ^ Smith, p. 394.
  5. ^ Degarmo, p. 113.
  6. ^ Smith, pp. 394-395.
  7. ^ Smith, pp. 395-396
  8. ^ Degarmo, p. 114.

Bibliography

  • Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, ISBN 0-471-65653-4 .
  • 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 

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