Steel Company: Maraging steel

Maraging steel

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% nickel.[1] 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.

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 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

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

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 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. 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.

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 3500 MPa (500,000 psi)
Elongation at break: up to 15%
KIC fracture toughness: up to 175 MPa-m?
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)