Maraging steel
Encyclopedia
Maraging steels are steel
s (iron
alloys) which are known for possessing superior strength and toughness without losing malleability, although they cannot hold a good cutting edge. Aging refers to the extended heat-treatment process. These steels are a special class of low-carbon
ultra-high-strength steels which derive their strength not from carbon, but from precipitation of inter-metallic compounds. The principal alloying element is 15 to 25% nickel
. Secondary alloying elements are added to produce intermetallic precipitates, which include cobalt
, molybdenum
, and titanium
. Original development was carried out on 20 and 25% Ni steels to which small additions of Al, Ti, and Nb were made.
The common, non-stainless grades contain 17–19% nickel, 8–12% cobalt, 3–5% molybdenum, and 0.2–1.6% 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
when heat treated, while lower-nickel steels can transform to martensite.
. Prior to aging, they may also be cold rolled to as much as 80–90% without cracking. Maraging steels offer good weldability
, but must be aged afterward to restore the properties of heat affected zone.
When heat-treated 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
to increase case hardness, and polished to a fine surface finish.
Non-stainless varieties of maraging steel are moderately corrosion
-resistant, and resist stress corrosion and hydrogen embrittlement
. Corrosion-resistance can be increased by cadmium plating or phosphating.
at approximately 820 °C (1,508 °F) for 15–30 minutes for thin sections and for 1 hour per 25 mm thickness for heavy sections, to ensure formation of a fully austenitized structure. This is followed by air cooling to room temperature to form a soft, heavily-dislocated iron-nickel lath (untwinned) martensite. Subsequent aging (precipitation hardening) of the more common alloys for approximately 3 hours at a temperature of 480 to 500 °C produces a fine dispersion
of Ni3(X,Y) intermetallic phases along dislocations left by martensitic transformation, where X and Y are solute
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 phase
s 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).
s and have maximum service temperatures of over 400 °C (752 °F). 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
s and dies
. Other ultra-high-strength steels, such as Aermet alloys, are not as machinable because of their carbide content.
In the sport of fencing
, blades used in competitions run under the auspices of the Fédération Internationale d'Escrime
are often made with maraging steel. Maraging blades are required in foil and épée because crack propagation in maraging steel is 10 times slower than in carbon steel, resulting in less blade breakage and fewer injuries. The notion that such blades break flat is a fencing urban legend
: testing has shown that the blade-breakage patterns in carbon steel and maraging steel blades are identical . Stainless maraging steel is used in bicycle
frames and golf
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.
Maraging steel production, import, and export by certain states, such as the United States, is closely monitored by international authorities because it is particularly suited for use in gas centrifuges for uranium enrichment; lack of maraging steel significantly hampers this process. Older centrifuges used aluminum tubes; modern ones, carbon fiber composite.
Steel
Steel 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...
s (iron
Iron
Iron 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...
alloys) which are known for possessing superior strength and toughness without losing malleability, although they cannot hold a good cutting edge. Aging refers to the extended heat-treatment process. These steels are a special class of low-carbon
Carbon
Carbon 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...
ultra-high-strength steels which derive their strength not from carbon, but from precipitation of inter-metallic compounds. The principal alloying element is 15 to 25% nickel
Nickel
Nickel 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...
. Secondary alloying elements are added to produce intermetallic precipitates, which include cobalt
Cobalt
Cobalt 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....
, molybdenum
Molybdenum
Molybdenum , 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...
, and titanium
Titanium
Titanium 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....
. Original development was carried out on 20 and 25% Ni steels to which small additions of Al, Ti, and Nb were made.
The common, non-stainless grades contain 17–19% nickel, 8–12% cobalt, 3–5% molybdenum, and 0.2–1.6% titanium. Addition of chromium produces stainless grades resistant to corrosion. This also indirectly increases hardenability
Hardenability
The 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,...
as they require less nickel: high-chromium, high-nickel steels are generally austenitic
Austenite
Austenite, also known as gamma phase iron, is a metallic non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of ; other alloys of steel have different eutectoid temperatures...
and unable to transform to martensite
Martensite
Martensite, named after the German metallurgist Adolf Martens , most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by displacive transformation. It includes a class of hard minerals occurring as lath- or...
when heat treated, while lower-nickel steels can transform to martensite.
Properties
Due to the low carbon content maraging steels have good machinabilityMachinability
The term machinability refers to the ease with which a metal can be machined to an acceptable surface finish. Materials with good machinability require little power to cut, can be cut quickly, easily obtain a good finish, and do not wear the tooling much; such materials are said to be free machining...
. Prior to aging, they may also be cold rolled to as much as 80–90% without cracking. Maraging steels offer good weldability
Weldability
The 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...
, but must be aged afterward to restore the properties of heat affected zone.
When heat-treated 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
Nitridization
Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case hardened surface. It is predominantly used on steel, but also titanium, aluminum and molybdenum....
to increase case hardness, and polished to a fine surface finish.
Non-stainless varieties of maraging steel are moderately corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
-resistant, and resist stress corrosion and hydrogen embrittlement
Hydrogen embrittlement
Hydrogen embrittlement is the process by which various metals, most importantly high-strength steel, become brittle and fracture following exposure to hydrogen...
. Corrosion-resistance can be increased by cadmium plating or phosphating.
Heat treatment cycle
The steel is first annealedAnnealing (metallurgy)
Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the recrystallization temperature, maintaining a suitable temperature, and...
at approximately 820 °C (1,508 °F) for 15–30 minutes for thin sections and for 1 hour per 25 mm thickness for heavy sections, to ensure formation of a fully austenitized structure. This is followed by air cooling to room temperature to form a soft, heavily-dislocated iron-nickel lath (untwinned) martensite. Subsequent aging (precipitation hardening) of the more common alloys for approximately 3 hours at a temperature of 480 to 500 °C produces a fine dispersion
Dispersion (chemistry)
A dispersion is a system in which particles are dispersed in a continuous phase of a different composition . See also emulsion. A dispersion is classified in a number of different ways, including how large the particles are in relation to the particles of the continuous phase, whether or not...
of Ni3(X,Y) intermetallic phases along dislocations left by martensitic transformation, where X and Y are solute
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
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 phase
Laves phase
Laves phases are intermetallic phases that have composition AB2 and are named for Fritz Laves who first described them. The phases are classified on the basis of geometry alone. There are three different classification classes: cubic, hexagonal MgZn2, and hexagonal MgNi2. The latter two classes...
s 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 temperatureOperating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
s and have maximum service temperatures of over 400 °C (752 °F). 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
Assembly line
An assembly line is a manufacturing process in which parts are added to a product in a sequential manner using optimally planned logistics to create a finished product much faster than with handcrafting-type methods...
s and dies
Die (manufacturing)
A die is a specialized tool used in manufacturing industries to cut or shape material using a press. Like molds, dies are generally customized to the item they are used to create...
. Other ultra-high-strength steels, such as Aermet alloys, are not as machinable because of their carbide content.
In the sport of fencing
Fencing
Fencing, which is also known as modern fencing to distinguish it from historical fencing, is a family of combat sports using bladed weapons.Fencing is one of four sports which have been featured at every one of the modern Olympic Games...
, blades used in competitions run under the auspices of the Fédération Internationale d'Escrime
Fédération Internationale d'Escrime
Fédération Internationale d'Escrime is the international governing body of Olympic fencing. It was founded on November 29, 1913 in Paris, France. Today, its head office is in Lausanne, Switzerland...
are often made with maraging steel. Maraging blades are required in foil and épée because crack propagation in maraging steel is 10 times slower than in carbon steel, resulting in less blade breakage and fewer injuries. The notion that such blades break flat is a fencing urban legend
Urban legend
An urban legend, urban myth, urban tale, or contemporary legend, is a form of modern folklore consisting of stories that may or may not have been believed by their tellers to be true...
: testing has shown that the blade-breakage patterns in carbon steel and maraging steel blades are identical . Stainless maraging steel is used in bicycle
Bicycle
A bicycle, also known as a bike, pushbike or cycle, is a human-powered, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A person who rides a bicycle is called a cyclist, or bicyclist....
frames and golf
Golf
Golf is a precision club and ball sport, in which competing players use many types of clubs to hit balls into a series of holes on a golf course using the fewest number of strokes....
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.
Maraging steel production, import, and export by certain states, such as the United States, is closely monitored by international authorities because it is particularly suited for use in gas centrifuges for uranium enrichment; lack of maraging steel significantly hampers this process. Older centrifuges used aluminum tubes; modern ones, carbon fiber composite.
Physical properties
- DensityDensityThe mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
: 8.1 g/cm³ (0.29 lb/in³) - Specific heat, mean for 0–100 °C (32–212 °F): 813 J/(kg·K) (0.108 Btu/(lb·°F))
- Melting pointMelting pointThe melting point of a solid 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 depends on pressure and is usually specified at standard atmospheric pressure...
: 2575 °F, 1413 °C - Thermal conductivityThermal conductivityIn physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
: 25.5 W·m/(m²·K) - Mean coefficient of thermal expansion: 11.3×10−6
- Yield tensile strength: typically 1030–2420 MPa (150,000–350,000 psi)
- Ultimate tensile strengthTensile strengthUltimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...
: typically 1600–2500 MPa (230,000–360,000 psi). Grades exist up to 3.5 GPa (500,000 psi) - Elongation at break: up to 15%
- KIC fracture toughness: up to 175 MPa-m½
- Young's modulusYoung's modulusYoung's modulus is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds. In solid mechanics, the slope of the stress-strain...
: 210 GPa - Shear modulus: 77 GPa
- Bulk modulusBulk modulusThe bulk modulus of a substance measures the substance's resistance to uniform compression. It is defined as the pressure increase needed to decrease the volume by a factor of 1/e...
: 140 GPa - Hardness (aged): 50 HRC (grade 250); 54 HRC (grade 300); 58 HRC (grade 350)