Maraging steel

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Maraging steels (a portmanteau of "martensitic" and "aging") are steels (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 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. 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.[2] The latter effect enables the design of maraging-TRIP steels where TRIP stands for Transformation-Induced-Plasticity.[3]

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.

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.[4] 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.[5][6] 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,[7] 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. ^ 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 
  3. ^ Raabe, D.; Ponge, D.; Dmitrieva, O.; Sander, B. (2009), Nano-precipitate hardened 1.5 GPa steels with unexpected high ductility 60, Scripta Materialia, p. 1141, doi:10.1016/j.bbr.2011.03.031 
  4. ^ 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. 
  5. ^ 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.
  6. ^ Juvinall, Robert C.; Marshek, Kurt M. (2006). Fundamentals of Machine Component Design (Fourth edition ed.). John Wiley & Sons, Inc. p. 69. ISBN 978-0-471-66177-1. 
  7. ^ Part 110--export and import of nuclear equipment and material, retrieved 2009-11-11. 
  8. ^ ASM Materials Information Online
  9. ^ 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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