Alloy steel is
steelSteel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
that is
alloyAn alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
ed with a variety of
elementsA chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 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
alloyAn alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
, but not all steels are called "alloy steels". Even the simplest steels are
ironIron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
(Fe) (about 99%) alloyed with
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
(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
manganeseManganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
(the most common one),
nickelNickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
,
chromiumChromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
,
molybdenumMolybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
,
vanadiumVanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
,
siliconSilicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, and
boronBoron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
. Less common alloyants include aluminum,
cobaltCobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
,
copperCopper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
,
ceriumCerium is a chemical element with the symbol Ce and atomic number 58. It is a soft, silvery, ductile metal which easily oxidizes in air. Cerium was named after the dwarf planet . Cerium is the most abundant of the rare earth elements, making up about 0.0046% of the Earth's crust by weight...
,
niobiumNiobium or columbium , is a chemical element with the symbol Nb and atomic number 41. It's a soft, grey, ductile transition metal, which is often found in the pyrochlore mineral, the main commercial source for niobium, and columbite...
,
titaniumTitanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
,
tungstenTungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
,
tinTin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
,
zincZinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
,
leadLead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
, and
zirconiumZirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
.
The following is a range of improved properties in alloy steels (as compared to carbon steels):
strengthIn materials science, the strength of a material is its ability to withstand an applied stress without failure. The applied stress may be tensile, compressive, or shear. Strength of materials is a subject which deals with loads, deformations and the forces acting on a material. A load applied to a...
, hardness,
toughnessIn materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing; Material toughness is defined as the amount of energy per volume that a material can absorb before rupturing...
, wear resistance,
hardenabilityThe hardenability of a metal alloy is its capability to be hardened by heat treatment. It should not be confused with hardness, which is a measure of a sample's resistance to indentation or scratching. It is an important property for welding, since it is inversely proportional to weldability,...
, 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 engineA jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...
s, in
spacecraftA spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....
, and in
nuclear reactorA nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
s. Because of the ferromagnetic properties of
ironIron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, some steel alloys find important applications where their responses to
magnetismMagnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...
are very important, including in
electric motorAn electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
s and in
transformerA transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s.
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.
With medium to high
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
levels, low-alloy steel is difficult to
weldWeld most commonly refers to a joint formed by welding.Weld may also refer to:-People:* Weld family, an extended family of New England** Theodore Dwight Weld** Tuesday Weld* Weld-Blundell family* Cecil Weld-Forester, 1st Baron Forester...
. Lowering the carbon content to the range of 0.10% to 0.30%, along with some reduction in alloying elements, increases the
weldabilityThe weldability, also known as joinability, of a material refers to its ability to be welded. Many metals and thermoplastics can be welded, but some are easier to weld than others...
and
formabilityFormability is the ability of a given metal workpiece to undergo plastic deformation without being damaged. The plastic deformation capacity of metallic materials, however, is limited to a certain extent....
of the steel while maintaining its strength. Such a metal is classed as a
high-strength low-alloy steelHigh-strength low-alloy steel is a type of alloy steel that provides better mechanical properties or greater resistance to corrosion than carbon steel. HSLA steels vary from other steels in that they are not made to meet a specific chemical composition but rather to specific mechanical properties...
.
Some common low alloy steels are:
Principal low-alloy steels
| SAE designation | Composition |
| 13xx |
Mn 1.75% |
| 40xx |
Mo 0.20% or 0.25% or 0.25% Mo & 0.042% S |
| 41xx 41xx steel is a family of SAE steel grades, as specified by the Society of Automotive Engineers . Alloying elements include chromium and molybdenum, and as a result these materials are often referred to as chromoly steel or CRMO...
|
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% |
| ES-1 Eglin steel is a high-strength, high-performance, low-alloy, low-cost steel, developed for new generation of bunker buster type bombs, eg. the Massive Ordnance Penetrator and the improved version of the GBU-28 bomb known as EGBU-28...
|
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, or aluminium are added during the
steelmakingSteelmaking is the second step in producing steel from iron ore. In this stage, impurities such as sulfur, phosphorus, and excess carbon are removed from the raw iron, and alloying elements such as manganese, nickel, chromium and vanadium are added to produce the exact steel required.-Older...
process to remove dissolved
oxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
,
sulfurSulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
and
phosphorusPhosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
from the
meltMelt is the term used to describe the working material in the steelmaking process, in making glass, and when forming thermoplastics. For thermoplastics, the term specifically refers to the plastic in its forming temperature, which can vary depending on how it is being used. For steelmaking, it...
.
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
carbideIn chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by chemical bonding type as follows: salt-like, covalent compounds, interstitial compounds, and "intermediate" transition metal carbides...
s. 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 Ni
3Al. Aluminium dissolves in the ferrite and forms the compounds Al
2O
3 and AlN. Silicon is also very soluble and usually forms the compound SiO
2•M
xO
y. Manganese mostly dissolves in ferrite forming the compounds MnS, MnO•SiO
2, 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,Cr
3)C, Cr
7C
3, and Cr
23C
6. 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 &
niobiumNiobium or columbium , is a chemical element with the symbol Nb and atomic number 41. It's a soft, grey, ductile transition metal, which is often found in the pyrochlore mineral, the main commercial source for niobium, and columbite...
), they form the carbides Mo
2C and W
2C, respectively. Vanadium, titanium, and niobium are strong carbide forming elements, forming
vanadium carbideVanadium carbide, VC, is an extremely hard refractory ceramic material. It is commercially used in tool bits cutting tools. It has the appearance of gray metallic powder with cubic crystal structure....
,
titanium carbideTitanium carbide, TiC, is an extremely hard refractory ceramic material, similar to tungsten carbide.It is commercially used in tool bits. It has the appearance of black powder with NaCl-type face centered cubic crystal structure...
, and
niobium carbideNiobium carbide is an extremely hard refractory ceramic material, commercially used in tool bits for cutting tools. It is usually processed by sintering and is a frequent additive in cemented carbides. It has the appearance of a brown-gray metallic powder with purple lustre...
, 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.
Principal effects of major alloying elements for steel
| Element | Percentage | Primary function |
| Aluminium |
0.95–1.30 |
Alloying element in nitriding steels |
| Bismuth |
- |
Improves machinability |
| Boron |
0.001–0.003 |
A powerful hardenability agent |
| Chromium |
0.5–2 |
Increases hardenability |
| 4–18 |
Increases corrosion resistance |
| Copper |
0.1–0.4 |
Corrosion resistance |
| Lead |
- |
Improved machinability |
| Manganese |
0.25–0.40 |
Combines with sulfur Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
and with phosphorusPhosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
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 carbideIn chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by chemical bonding type as follows: salt-like, covalent compounds, interstitial compounds, and "intermediate" transition metal carbides...
s; inhibits grain growth. Increases the toughness of steel, thus making molybdenum a very valuable alloy metal for making the cutting parts of machine toolA machine tool is a machine, typically powered other than by human muscle , used to make manufactured parts in various ways that include cutting or certain other kinds of deformation...
s and also the turbineA turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...
blades of turbojet engines. Also used in rocket motors. |
| Nickel |
2–5 |
Toughener |
| 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 |