Maraging steels (a

portmanteau A portmanteau word, or portmanteau (, ) is a blend of wordsmartensitic" and "aging") are

steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistan ...

s that are known for possessing superior strength and toughness without losing

ductility Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile str ...

. ''Aging'' refers to the extended heat-treatment process. These steels are a special class of very-low-

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

ultra-high-strength steels that derive their strength not from carbon, but from

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...

intermetallic An intermetallic (also called an intermetallic compound, intermetallic alloy, ordered intermetallic alloy, and a long-range-ordered alloy) is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic eleme ...

compounds. The principal alloying element is 15 to 25

wt% In chemistry, the mass fraction of a substance within a mixture is the ratio w_i (alternatively denoted Y_i) of the mass m_i of that substance to the total mass m_\text of the mixture. Expressed as a formula, the mass fraction is: : w_i = \frac ...

nickel Nickel is a chemical element with 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 slow t ...

. Secondary alloying elements, which include

cobalt Cobalt is a chemical element with the 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. The free element, p ...

molybdenum Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lead ...

and

titanium Titanium is a chemical element with the 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 i ...

, are added to produce intermetallic precipitates. Original development (by Bieber of Inco in the late 1950s) was carried out on 20 and 25 wt% Ni steels to which small additions of

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

, titanium, and

niobium Niobium is a chemical element with chemical symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to pure titanium, and it has s ...

were made; a rise in the price of cobalt in the late 1970s led to the development of cobalt-free maraging steels. The common, non-stainless grades contain 17–19 wt% nickel, 8–12 wt% cobalt, 3–5 wt% molybdenum and 0.2–1.6 wt% titanium. Addition of chromium produces stainless grades resistant to corrosion. This also indirectly increases hardenability as they require less nickel; high-chromium, high-nickel steels are generally austenitic and unable to transform to

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 M ...

when heat treated, while lower-nickel steels can transform to martensite. Alternative variants of nickel-reduced maraging steels are based on alloys of iron and manganese plus minor additions of aluminium, nickel and titanium where compositions between Fe-9wt% Mn to Fe-15wt% Mn have been used.. The manganese has a similar effect as nickel, i.e. it stabilizes the austenite phase. Hence, depending on their manganese content, Fe-Mn maraging steels can be fully martensitic after quenching them from the high temperature austenite phase or they can contain retained austenite. The latter effect enables the design of maraging-TRIP steels where TRIP stands for Transformation-Induced-Plasticity.

Properties

Due to the low carbon content (less than 0.03%) maraging steels have good machinability. Prior to aging, they may also be cold rolled to as much as 90% without cracking. Maraging steels offer good weldability, but must be aged afterward to restore the original properties to the

heat affected zone In fusion welding, the heat-affected zone (HAZ) is the area of base material, either a metal or a thermoplastic, which is not melted but has had its microstructure and properties altered by welding or heat intensive cutting operations. The heat ...

. When

heat-treated 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 als ...

the alloy has very little dimensional change, so it is often machined to its final dimensions. Due to the high alloy content maraging steels have a high hardenability. Since ductile FeNi martensites are formed upon cooling, cracks are non-existent or negligible. The steels can be

nitrided Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface. These processes are most commonly used on low-alloy steels. They are also used on titanium, aluminium and molybdenum. Ty ...

to increase case hardness and polished to a fine surface finish. Non-stainless varieties of maraging steel are moderately

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...

-resistant and resist stress corrosion and

hydrogen embrittlement Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbe ...

. Corrosion-resistance can be increased by cadmium plating or phosphating.

Grades of maraging steel

Maraging steels are usually described by a number (e.g., SAE steel grades 200, 250, 300 or 350), which indicates the approximate nominal tensile strength in thousands of pounds per square inch (ksi); the compositions and required properties are defined in US military standard MIL-S-46850D.Military Specification 46850D: STEEL : BAR, PLATE, SHEET, STRIP, FORGINGS, AND EXTRUSIONS, 18 PERCENT NICKEL ALLOY, MARAGING, 200 KSI, 250 KSI, 300 KSI, AND 350 KSI, HIGH QUALITY, available from http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-S/MIL-S-46850D_19899/ The higher grades have more cobalt and titanium in the alloy; the compositions below are taken from table 1 of MIL-S-46850D: That family is known as the 18Ni maraging steels, from its nickel percentage. There is also a family of cobalt-free maraging steels which are cheaper but not quite as strong; one example is Fe-18.9Ni-4.1Mo-1.9Ti. There has been Russian and Japanese research in Fe-Ni-Mn maraging alloys.

Heat treatment cycle

The steel is first annealed at approximately for 15–30 minutes for thin sections and for 1 hour per thickness for heavy sections, to ensure formation of a fully austenitized structure. This is followed by

air cooling Air cooling is a method of dissipating heat. It works by expanding the surface area or increasing the flow of air over the object to be cooled, or both. An example of the former is to add cooling fins to the surface of the object, either by maki ...

or quenching to room temperature to form a soft, heavily dislocated iron-nickel lath (untwinned) martensite. Subsequent aging (

precipitation hardening Precipitation hardening, also called age hardening or particle hardening, is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel, titanium, and ...

) of the more common alloys for approximately 3 hours at a temperature of produces a fine

dispersion Dispersion may refer to: Economics and finance *Dispersion (finance), a measure for the statistical distribution of portfolio returns *Price dispersion, a variation in prices across sellers of the same item *Wage dispersion, the amount of variatio ...

of Ni₃(X,Y) intermetallic phases along dislocations left by martensitic transformation, where X and Y are

solute In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solvent ...

elements added for such precipitation. Overaging leads to a reduction in stability of the primary, metastable, coherent precipitates, leading to their dissolution and replacement with semi-coherent Laves phases such as Fe₂Ni/Fe₂Mo. Further excessive heat-treatment brings about the decomposition of the martensite and reversion to austenite. Newer compositions of maraging steels have revealed other intermetallic stoichiometries and crystallographic relationships with the parent martensite, including rhombohedral and massive complex Ni₅₀(X,Y,Z)₅₀ (Ni₅₀M₅₀ in simplified notation).

Uses

Maraging steel's strength and malleability in the pre-aged stage allows it to be formed into thinner rocket and missile skins than other steels, reducing weight for a given strength. Maraging steels have very stable properties and, even after overaging due to excessive temperature, only soften slightly. These alloys retain their properties at mildly elevated

operating temperature An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...

s and have maximum service temperatures of over . They are suitable for engine components, such as crankshafts and gears, and the firing pins of automatic weapons that cycle from hot to cool repeatedly while under substantial load. Their uniform expansion and easy machinability before aging make maraging steel useful in high-wear components of

assembly line An assembly line is a manufacturing process (often called a ''progressive assembly'') in which parts (usually interchangeable parts) are added as the semi-finished assembly moves from workstation to workstation where the parts are added in se ...

s and

dies Dies may refer to: * Dies (deity), the Roman counterpart of the Greek goddess Hemera, the personification of day, daughter of Nox (Night) and Erebus (Darkness). * Albert Christoph Dies (1755–1822), German painter, composer, and biographer * Jos ...

. Other ultra-high-strength steels, such as AerMet alloys, are not as machinable because of their carbide content. In the sport of

fencing Fencing is a group of three related combat sports. The three disciplines in modern fencing are the foil, the épée, and the sabre (also ''saber''); winning points are made through the weapon's contact with an opponent. A fourth discipline, ...

, blades used in competitions run under the auspices of the

Fédération Internationale d'Escrime The ''Fédération Internationale d'Escrime'' ( en, International Fencing Federation), commonly known by the acronym FIE, is the international governing body of Olympic fencing. Today, its head office is at the Maison du Sport International i ...

are usually made with maraging steel. Maraging blades are superior for

foil Foil may refer to: Materials * Foil (metal), a quite thin sheet of metal, usually manufactured with a rolling mill machine * Metal leaf, a very thin sheet of decorative metal * Aluminium foil, a type of wrapping for food * Tin foil, metal foil ...

and

épée The ( or , ), sometimes spelled epee in English, is the largest and heaviest of the three weapons used in the sport of fencing. The modern derives from the 19th-century , a weapon which itself derives from the French small sword. This contain ...

because crack propagation in maraging steel is 10 times slower than in carbon steel, resulting in less frequent breaking of the blade and fewer injuries. Stainless maraging steel is used in

bicycle A bicycle, also called a pedal cycle, bike or cycle, is a human-powered or motor-powered assisted, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A is called a cyclist, or bicyclist. B ...

frames (e.g. Reynolds 953) and

golf Golf is a club-and-ball sport in which players use various clubs to hit balls into a series of holes on a course in as few strokes as possible. Golf, unlike most ball games, cannot and does not use a standardized playing area, and coping wi ...

club heads. It is also used in surgical components and hypodermic syringes, but is not suitable for scalpel blades because the lack of carbon prevents it from holding a good cutting edge. In

cycling Cycling, also, when on a two-wheeled bicycle, called bicycling or biking, is the use of cycles for transport, recreation, exercise or sport. People engaged in cycling are referred to as "cyclists", "bicyclists", or "bikers". Apart from ...

Reynolds introduced a form of maraging steel, 953, which has excellent welding and cold working properties while having extreme stiffness which means lighter tubing. American musical instrument string producer Ernie Ball has made a specialist type of

electric guitar An electric guitar is a guitar that requires external amplification in order to be heard at typical performance volumes, unlike a standard acoustic guitar (however combinations of the two - a semi-acoustic guitar and an electric acoustic gu ...

string out of maraging steel, claiming that this alloy provides more output and enhanced tonal response. The production, import, and export of maraging steels by certain entities, such as the United States, is closely monitored by international authorities because it is particularly suited for use in

gas centrifuge A gas centrifuge is a device that performs isotope separation of gases. A centrifuge relies on the principles of centrifugal force accelerating molecules so that particles of different masses are physically separated in a gradient along the radiu ...

s for

uranium enrichment Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238 ...

; lack of maraging steel significantly hampers the uranium-enrichment process. Older centrifuges used aluminum tubes, while modern ones use carbon fiber composite.

Physical properties

Density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...

: 8.1 g/cm³ (0.29 lb/in³) *

Specific heat In thermodynamics, the specific heat capacity (symbol ) of a substance is the heat capacity of a sample of the substance divided by the mass of the sample, also sometimes referred to as massic heat capacity. Informally, it is the amount of heat t ...

, mean for 0–100 °C (32–212 °F): 452 J/kg·K (0.108 Btu/lb·°F) *

Melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depen ...

: *

Thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

: 25.5 W/m·K * Mean

coefficient of thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kineti ...

: 11.3×10⁻⁶ K⁻¹ (20.3×10⁻⁶ °F⁻¹) * Yield tensile strength: typically * Ultimate

tensile strength Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials ...

: typically . Grades exist up to * Elongation at break: up to 15% * K_IC fracture toughness: up to 175 MPa·m *

Young's modulus Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied ...

: *

Shear modulus In materials science, shear modulus or modulus of rigidity, denoted by ''G'', or sometimes ''S'' or ''μ'', is a measure of the elastic shear stiffness of a material and is defined as the ratio of shear stress to the shear strain: :G \ \stackre ...

: *

Bulk modulus The bulk modulus (K or B) of a substance is a measure of how resistant to compression the substance is. It is defined as the ratio of the infinitesimal pressure increase to the resulting ''relative'' decrease of the volume. Other moduli descri ...

: *

Hardness In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example hard ...

(aged): 50 HRC (grade 250); 54 HRC (grade 300); 58 HRC (grade 350)

References

External links

Maraging steel data sheets
{{Webarchive, url=https://web.archive.org/web/20160815131638/http://www.matthey.ch/en/alloys/maraging-steels-durnico-durimphy-ultrafort-durinox-phynox , date=2016-08-15 Steels