Alloy steel is

steel Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...

that is alloyed with a variety of elements in 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. The boundary between the two is disputed. Smith and Hashemi define the difference at 4.0%, while Degarmo, ''et al.'', define it at 8.0%. Most alloy steels are low-alloy. The simplest steels are

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

(Fe) alloyed with (0.1% to 1%)

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

(C) and nothing else (excepting slight impurities); these are called carbon steels. However, alloy steel encompasses steels with additional (metal) alloying elements. Common alloyants include

manganese Manganese is a chemical element; it has Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese was first isolated in the 1770s. It is a transition m ...

(Mn) (the most common),

nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...

(Ni),

chromium Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal. Chromium ...

(Cr),

molybdenum Molybdenum is a chemical element; it has Symbol (chemistry), symbol Mo (from Neo-Latin ''molybdaenum'') and atomic number 42. The name derived from Ancient Greek ', meaning lead, since its ores were confused with lead ores. Molybdenum minerals hav ...

(Mo),

vanadium Vanadium is a chemical element; it has Symbol (chemistry), symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an ...

(V),

silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

(Si), and

boron Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three ...

(B). Less common alloyants include

Aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

(Al),

cobalt Cobalt is a chemical element; it has Symbol (chemistry), symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. ...

(Co),

copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

(Cu),

cerium Cerium is a chemical element; it has Chemical symbol, symbol Ce and atomic number 58. It is a hardness, soft, ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it ...

(Ce),

niobium Niobium is a chemical element; it has chemical symbol, symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and Ductility, ductile transition metal. Pure niobium has a Mohs scale of mineral hardness, Mohs h ...

(Nb),

titanium Titanium is a chemical element; it has symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...

(Ti),

tungsten Tungsten (also called wolfram) is a chemical element; it has symbol W and atomic number 74. It is a metal found naturally on Earth almost exclusively in compounds with other elements. It was identified as a distinct element in 1781 and first ...

(W), tin (Sn),

zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...

(Zn),

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

(Pb), and zirconium (Zr).

Properties

Alloy steels variously improve strength,

hardness In materials science, hardness (antonym: softness) is a measure of the resistance to plastic deformation, such as an indentation (over an area) or a scratch (linear), induced mechanically either by Pressing (metalworking), pressing or abrasion ...

toughness In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing.wear resistance, corrosion resistance, hardenability, and hot hardness. To achieve these improved properties the metal may require specific

heat treating Heat treating (or heat treatment) is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are al ...

, combined with strict cooling protocols. Although alloy steels have been made for centuries, their

metallurgy Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys. Metallurgy encompasses both the ...

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 tool steels and

stainless steels Stainless steel, also known as inox, corrosion-resistant steel (CRES), or rustless steel, is an iron-based alloy that contains chromium, making it resistant to rust and corrosion. Stainless steel's resistance to corrosion comes from its chr ...

. Because of iron's

ferromagnetic Ferromagnetism is a property of certain materials (such as iron) that results in a significant, observable magnetic permeability, and in many cases, a significant magnetic coercivity, allowing the material to form a permanent magnet. Ferromagne ...

properties, some alloys find important applications where their responses to magnetism are valued, including in electric motors and in transformers.

Low-alloy steels

Material science

Alloying elements enable specific properties. 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 improve corrosion resistance or temperature stability. The alloying elements tend to form either solid solutions, compounds or carbides. * Nickel is soluble in ferrite; therefore, it usually forms Ni₃Al. * Aluminum dissolves in ferrite and forms Al₂O₃ and AlN. Silicon is also soluble and usually forms SiO₂•M_xO_y. * Manganese mostly dissolves in ferrite forming MnS, MnO•SiO₂, but also carbides: (Fe,Mn)₃C. * Chromium forms partitions between the ferrite and carbide phases in steel, forming (Fe,Cr₃)C, Cr₇C₃, and Cr₂₃C₆. The type of c#arbide that chromium forms depends on the amount of carbon and other alloying elements present. * Tungsten and molybdenum form carbides given enough carbon and an absence of stronger carbide forming elements (i.e.,

and

), 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.

Eutectoid temperature

Alloying elements can have an effect on the eutectoid temperature. * Manganese and nickel lower the eutectoid temperature and are known as austenite stabilizing elements. With enough of these elements the austenitic structure may form at room temperature. * Carbide-forming elements raise the eutectoid temperature and stabilize ferrites.

Microstructure

The properties of steel depend on its microstructure: the arrangement of different phases, some harder, some with greater

ductility Ductility refers to the ability of a material to sustain significant plastic Deformation (engineering), deformation before fracture. Plastic deformation is the permanent distortion of a material under applied stress, as opposed to elastic def ...

. At the atomic level, the four phases of auto steel include

martensite Martensite is a very hard form of steel crystalline structure. It is named after German metallurgist Adolf Martens. By analogy the term can also refer to any crystal structure that is formed by diffusionless transformation. Properties Mar ...

(the hardest yet most brittle), bainite (less hard), ferrite (more ductile), and austenite (the most ductile). The phases are arranged by steelmakers by manipulating intervals (sometimes by seconds only) and temperatures of the heating and cooling process.

Transformation-induced plasticity

TRIP steels transform from relatively ductile to relatively hard under deformation such as in a car crash. Deformation transforms austenitic microstructure to martensitic microstructure. TRIP steels use relatively high carbon content to create the austenitic microstructure. Relatively high silicon/aluminum content suppresses

carbide In chemistry, a carbide usually describes a compound composed of carbon and a metal. In metallurgy, carbiding or carburizing is the process for producing carbide coatings on a metal piece. Interstitial / Metallic carbides The carbides of th ...

precipitation in the bainite region and helps accelerate ferrite/bainite formation. This helps retain carbon to support austenite at room temperature. A specific cooling process reduces the austenite/martensite transformation during forming. TRIP steels typically require an isothermal hold at an intermediate temperature during cooling, which produces some bainite. The additional silicon/carbon requires weld cycle modification, such as the use of pulsating welding or dilution welding. In one approach steel is heated to a high temperature, cooled somewhat, held stable for an interval and then quenched. This produces islands of austenite surrounded by a matrix of softer ferrite, with regions of harder bainite and martensite. The resulting product can absorb energy without fracturing, making it useful for auto parts such as bumpers and pillars. Three generations of advanced, high-strength steel are available. The first was created in the 1990s, increasing strength and ductility. A second generation used new alloys to further increase ductility, but were expensive and difficult to manufacture. The third generation is emerging. Refined heating and cooling patterns increase strength at some cost in ductility (vs 2nd generation). These steels are claimed to approach nearly ten times the strength of earlier steels; and are much cheaper to manufacture.

Intermetallics

Researches created an alloy with the strength of steel and the lightness of titanium alloy. It combined iron, aluminum, carbon, manganese, and nickel. The other ingredient was uniformly distributed nanometer-sized B2 intermetallic (two metals with equal numbers of atoms) particles. The use of nickel team avoided problems with earlier attempts to use B2, while increasing ductility.

References

External links

* {{Authority control Steels