Maraging steel

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Maraging steels (a portmanteau of "martensitic" and "aging") are steels (iron alloys) that 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 that derive their strength not from carbon, but from precipitation of intermetallic compounds. The principal alloying element is 15 to 25 wt.% nickel.[1] Secondary alloying elements are added to produce intermetallic precipitates, which include cobalt, molybdenum, and titanium.[1] Original development was carried out on 20 and 25 wt.% Ni steels to which small additions of Al, Ti, and Nb were made.

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 when heat treated, while lower-nickel steels can transform to martensite. Alternative variants of Ni-reduced maraging steels are based on alloys of Fe and Mn plus minor additions of Al, Ni, and Ti where compositions between Fe-9wt.% Mn to Fe-15wt.% Mn have been used.[2] The Mn has a similar effect as Ni, i.e. it stabilizes the austenite phase. Hence, depending on their Mn content, Fe-Mn maraging steels can be fully martensitic after quenching them from the high temperature austenite phase or they can contain retained austenite.[3] The latter effect enables the design of maraging-TRIP steels where TRIP stands for Transformation-Induced-Plasticity.[4]

Properties[edit]

Due to the low carbon content 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.[1]

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.

Maraging steel 250 is an 18 percent nickel steel that has been strengthened with cobalt. Maraging 250, like all maraging steels, goes through an aging process that forces the metal to cool from its molten state to its solid state over an artificially long time. This process results in tempered steel that has both high levels of strength and hardness. It will also resist certain stresses and maintain its structure in environments that would cause irreparable changes to many other steels. The properties that make Maraging 250 particularly appealing to many industries is its workability. This allows Maraging 250 to be more versatile than many other alloys in its class. However, it is still the alloy’s strength and resistance to extreme temperatures that make it a truly effective material in a wide range of atmospheres. After Maraging 250 has undergone heat treatment, it demonstrates excellent mechanical properties. It will reach a yield strength of 240 ksi and a fracture toughness of 75 k1c. These properties have made Maraging 250 effective in the construction of missile and rocket motor cases, landing and takeoff gear, and high performance shafting.

Maraging steel 300 is an iron-nickel steel alloy that, as with all maraging steels, exhibits high levels of strength and hardness. However, Maraging 300 also possesses an extreme resistance to crack propagation, even in the most extreme environments. Maraging 300 is often used in applications where high fracture toughness is required or where dimensional changes have to remain at a minimal level. The unique properties of Maraging 300 have made it an integral part of the aircraft and aerospace industries. It is often used in rocket motor casings and the landing gear for certain planes. Maraging 300 is also effective in the design of power shafts and low-temperature cooling systems.

Maraging steel 350 refers to crystalline tempered steel. Martensite, which is created through an aging process. When aging is used, steel is forced to cool from its molten state to its solid state over a prolonged period of time. The result is a metal that is harder and stronger than it would be had the steel been allowed to cool at a natural rate. Maraging 350 is an alloy that has become an integral material in the airplane and aerospace industries. Due to its strength and its ability to withstand extreme conditions including frequent and sudden changes in speed and temperature, Maraging 350 is used in the production of rocket motor cases, takeoff and landing gear, and certain munitions created by defense companies. Maraging 350 also has uses in less drastic applications such as die casting and high performance shafting.

Maraging steel 362 is known for their exceptional strength and hardness. Their ability to resist various forms of stress in extreme environments has made maraging steels commonplace in the aerospace and aircraft industries. Each maraging alloy has its own unique qualities, but many of them are used in similar applications. Maraging 362, like other maraging steels, undergoes an artificial aging process. This process leads to the material’s added strength and hardness. The results of the aging procedure has recently caught the eye of golf club designers and manufacturers who have begun to use maraging alloys on the faces of their clubs in the hope that players will see increased power and that clubs will not corrode over time. Maraging steel gives you an elevated level of strength, hardness, and ductility. These steels are created through an aging process that results in the development of a hard, brittle crystalline called martensite. The term “maraging” is, in fact, a simple combination of martensite and aging. The construction of maraging steel allows it to withstand atmospheres that would quite simply destroy most standard steel. The aging process instils maraging steel with the ability to withstand sudden changes in speed and temperature, even at extreme levels. This quality has made maraging steel alloys an important component of many of the air and spacecraft used today.

Heat treatment cycle[edit]

The steel is first annealed at approximately 820 °C (1,510 °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 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[edit]

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.[5] 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 temperatures and have maximum service temperatures of over 400 °C (752 °F).[citation needed] 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 lines 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 usually made with maraging steel. Maraging blades are superior for 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.[6][7] 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,[8] 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.[citation needed]

Physical properties[edit]

References[edit]

  1. ^ a b c Degarmo, E. Paul; Black, J. T.; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, p. 119, ISBN 0-471-65653-4 
  2. ^ Raabe, D.; Sandlöbes, S.; Millan, J. J.; Ponge, D.; Assadi, H.; Herbig, M.; Choi, P.P. (2013), Segregation engineering enables nanoscale martensite to austenite phase transformation at grain boundaries: A pathway to ductile martensite, 61(16), Acta Materialia, p. 6132-6152 .
  3. ^ Dmitrieva, O.; Ponge, D.; Inden, G.; Millan, J.; Choi, P.; Sietsma, J.; Raabe, D. (2011), Chemical gradients across phase boundaries between martensite and austenite in steel studied by atom probe tomography and simulation 59, Acta Materialia, p. 364, doi:10.1016/j.actamat.2010.09.042, ISSN 1359-6454 
  4. ^ Raabe, D.; Ponge, D.; Dmitrieva, O.; Sander, B. (2009), "Nano-precipitate hardened 1.5 GPa steels with unexpected high ductility", Scripta Materialia 60: 1141, doi:10.1016/j.scriptamat.2009.02.062 
  5. ^ Joby Warrick (2012-08-11). "Nuclear ruse: Posing as toymaker, Chinese merchant allegedly sought U.S. technology for Iran". The Washington Post. Retrieved 2014-02-21. 
  6. ^ However, the notion that maraging steel blades break flat is a fencing urban legend. Testing has shown that the blade-breakage patterns in carbon steel and maraging steel are identical due to the similarity in the loading mode during bending. Additionally, a crack is likely to start at the same point and propagate along the same path (although much more slowly), as crack propagation in fatigue is a plastic phenomenon rather than microstructural.
  7. ^ Juvinall, Robert C.; Marshek, Kurt M. (2006). Fundamentals of Machine Component Design (Fourth ed.). John Wiley & Sons, Inc. p. 69. ISBN 978-0-471-66177-1. 
  8. ^ Part 110--export and import of nuclear equipment and material, retrieved 2009-11-11. 
  9. ^ http://www.imoa.info/
  10. ^ Ohue, Yuji; Matsumoto, Koji (10 September 2007). "Sliding–rolling contact fatigue and wear of maraging steel roller with ion-nitriding and fine particle shot-peening". Wear 263 (1-6): 782–789. doi:10.1016/j.wear.2007.01.055. 

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