Alloy steel

Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight, typically to improve its mechanical properties.

Types

Alloy steels divide into two groups: low and high alloy steels. The difference between the two is disputed. Smith and Hashemi define the difference at 4.0%, while Degarmo, et al., define it at 8.0%.^[1]^[2] Most alloy steels are low-alloy steels.

The simplest steels are iron (Fe) alloyed with (0.1% to 1%) carbon (C) and nothing else (excepting slight impurities); these are called carbon steels. However, "alloy steel" refers to steels with additional alloying elements. Common alloyants include manganese (Mn) (the most common), nickel (Ni), chromium (Cr), molybdenum (Mo), vanadium (V), silicon (Si), and boron (B). Less common alloyants include aluminium (Al), cobalt (Co), copper (Cu), cerium (Ce), niobium (Nb), titanium (Ti), tungsten (W), tin (Sn), zinc (Zn), lead (Pb), and zirconium (Zr).

Properties

Various alloy steels improve strength, hardness, toughness, wear resistance, corrosion resistance, hardenability, and hot hardness. To achieve these improved properties the metal may require specific heat treating, combined with strict cooling protocols.

Although alloy steels have been made for centuries, their metallurgy was not well understood until the advancing chemical science of the nineteenth century revealed their compositions. Alloy steels from earlier times were expensive luxuries made on the model of "secret recipes" and forged into tools such as knives and swords. Machine age alloy steels were developed as improved tool steels and as newly available stainless steels. Alloy steels serve many applications, from hand tools and flatware to turbine blades of jet engines and in nuclear reactors.

Because of iron's ferromagnetic properties, some alloys find important applications where their responses to magnetism are very important, including in electric motors and in transformers.

Low-alloy steels

**Principal 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%, and added boron
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%, and added boron
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%, and added boron
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. The alloying elements can change and personalize properties—their flexibility, strength, formability, and hardenability.^[4] 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, sulfur and phosphorus 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.
Sulfur (in the form of manganese sulfide), lead, bismuth, selenium, and tellurium increase machinability.^[5]

The alloying elements tend to form either solid solutions or 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 W₂C and Mo₂C, respectively.
Vanadium, titanium, and niobium are strong carbide forming elements, forming vanadium carbide, titanium carbide, and niobium carbide, 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	(Boron steel) A powerful hardenability agent
Chromium	0.5–2	Increases hardenability
Chromium	4–18	Increases corrosion resistance
Copper	0.1–0.4	Corrosion resistance
Lead	—	Improved machinability
Manganese	0.25–0.40	Combines with sulfur and with phosphorus to reduce the brittleness. Also helps to remove excess oxygen from molten steel.
Manganese	>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.
Nickel	2–5	Toughener
Nickel	12–20	Increases corrosion resistance
Silicon	0.2–0.7	Increases strength
	2.0	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	—	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

References

^ Smith, p. 393.
^ ^a ^b Degarmo, p. 112.
^ Smith, p. 394.
^ "What Are the Different Types of Steel? | Metal Exponents Blog". Metal Exponents. 2020-08-18. Retrieved 2021-01-29.
^ Degarmo, p. 113.
^ Smith, pp. 394–395.
^ Smith, pp. 395–396.
^ Degarmo, p. 144.

Bibliography

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

[1] Smith, p. 393.

[degarmo112-2] Degarmo, p. 112.

[3] Smith, p. 394.

[4] "What Are the Different Types of Steel? | Metal Exponents Blog". Metal Exponents. 2020-08-18. Retrieved 2021-01-29.

[5] Degarmo, p. 113.

[6] Smith, pp. 394–395.

[7] Smith, pp. 395–396.

[8] Degarmo, p. 144.

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