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Maraging steel
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Maraging steels (a portmanteau of martensitic and aging) are iron alloys which are known for possessing superior strength and toughness without losing malleability. 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.
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Encyclopedia
Maraging steels (a portmanteau of martensitic and aging) are iron alloys which are known for possessing superior strength and toughness without losing malleability. 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. Stainless grades rely on chromium not only to prevent their rusting, but to augment the hardenability of the alloy as their nickel content is substantially reduced. This is to ensure they can transform to martensite when heat treated, as high-chromium, high-nickel steels are generally austenitic, and unable to undergo such a transition.
Properties
Due to the low carbon content maraging steels have good machinability. 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 the alloys have a high hardenability. Since ductile FeNi martensites are formed upon cooling, cracks are non-existent or negligible. They can also be nitrided to increase case hardness. They can be polished to a fine surface finish.
Non-stainless varieties of maraging steels are moderately corrosion resistant and resist stress corrosion and hydrogen embrittlement. More corrosion protection can be gained by cadmium plating or phosphating.
Heat treatment cycle
The steel is first annealed at approximately 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 in the 480 to 500 °C range 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 - usually simplified to Ni50M50.
Uses
Maraging steel's strength and malleability in the pre-aged stage allows it to be formed into thinner rocket and missile skins, allowing more weight for payload while still possessing sufficient strength for the application. 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 . They are suited to engine components, such as crankshafts and gears, and the firing pins of automatic weapons that cycle from hot to cool repeatedly while under substantial loads. Their uniform expansion and easy machinability, carried out before aging makes 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 often made with maraging steel. Maraging blades are required in foil and épée because the crack propagation in maraging steel is 10 times slower than in carbon steel. This results in less blade breakage and fewer injuries. The thought that such blades break flat is actually 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, however it 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 is closely monitored by international authorities because of their use in gas centrifuges for uranium enrichment. Very few other materials will work for this task, and its other uses are very specialized.
Physical properties
- Density: 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 point: 2575 °F, 1413 °C
- Thermal conductivity: 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 strength: 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 modulus: 210 GPa
- Shear modulus: 77 GPa
- Bulk modulus: 140 GPa
- Hardness (aged): 50 HRC (grade 250); 54 HRC (grade 300); 58 HRC (grade 350)
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