Maraging steel

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Maraging steels (from Martensitic Aging) are iron alloys which are known for possessing superior strength without losing malleability. The iron base is alloyed principally with a large percentage of nickel to produce a very specific heat-treatment product. Other alloying elements include molybdenum, aluminum, copper and titanium and are added to produce intermetallic precipitates. Cobalt is added in percentages up to 12% to accelerate the precipitation reactions and ensure profuse and uniform precipitates. Maraging steel is essentially free of carbon, which distinguishes it from most other types of steel. The result is a steel which:

• Possesses high strength and toughness
• Allows for easy machining with minimal distortion
• Has uniform, predictable shrinkage during heat treatment
• Can be easily nitrided
• Resists corrosion and crack propagation
• Can be finely polished

A special class of low carbon ultra-high strength steels which derive their strength not from carbon but from precipitation of inter-metallic compounds. 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.

Contents 1 Salient features 2 Heat treatment cycle 3 Uses 4 Physical properties 5 External links

[edit] Salient features

• Offer the best available combination of ultra-high yield and tensile strength, ductility and fracture toughness of any ferrous materials.
• Can retain strength at least up to 350 °C.
• Being a very low-carbon martensite, the structure is soft. Readily machinable. May be cold rolled to as much as 80 - 90% without cracking before aging treatment. Hot deformation is also possible.
• Possible to finish-machine before aging. Very little dimensional change after aging treatment.
• Parts uniformly harden throughout the entire section because they have high hardenability.
• Exhibit good weldability. Properties of heat affected zone (HAZ) can be restored by a post-weld aging treatment.
• Since ductile FeNi martensites are formed upon cooling, cracks are non-existent or negligible.
• Can be surface hardened by nitriding.
• Good corrosion, stress corrosion and hydrogen embrittlement characteristics.
• Protection can be provided by cadmium plating or phosphating.

[edit] Heat treatment cycle

The steel is first annealed, at ~820 °C 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 (hence the "mar-" in mar-aging). Subsequent aging (precipitation hardening) of the more common commercially used alloys for ~3 hours in 480-500 °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, eg; Mo, Ti, Al, Cu, Si. 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.

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.

[edit] 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 room for payload while still possessing sufficient strength for the application. Maraging steels have very stable microstructural properties, and even after overaging due to excessive temperature only soften very sluggishly. These alloys retain their properties at mildly elevated operating temperatures and have maximum service temperatures of over 400 °C. They are suited to engine component applications such as crankshafts and gears, that work at 'warm' temperatures, and the firing pins of automatic weapons that cycle from hot to cool repeatedly while under substantial loads and impacts. Their uniform expansion and easy machinability, carried out before aging makes maraging steel useful in high wear portions of assembly lines, as well as in the manufacture of dies. Other ultra-high strength steels, such as the secondary hardening 'Aermet' family are not so amenable to processing because of their ever-present carbide particle dispersion.

In the sport of fencing, blades used in competitions run under the auspicies of the Fédération Internationale d'Escrime (International Fencing Federation, or FIE) are often made with maraging steel (although there are some épée blades that are approved for use at FIE events which are not made from maraging steel). Maraging blades are required in foil and épée (there is no such requirement for sabre blades) 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 - thus in the words of one equipment catalogue, "doing your opponent a great favor" - is actually a fencing urban legend. Testing has shown that the blade breakage patterns in carbon steel and maraging steel blades are identical.

Maraging steels are appearing more and more in sporting use. Reynolds cycles, of the UK and USA have recently introduced a new stainless maraging tubeset for the construction of bicycle frames. Stainless maraging steels have been used in golf club heads and in surgical components and hypodermic syringes. They are not suitable for scalpel blades, as their virtually zero carbon content prevents holding a good cutting edge.

Maraging steel production, import, and export by certain states is closely monitored by international authorities. Maraging steel is used in creating gas centrifuges for uranium enrichment due to its extremely high strength, good machineability, and excellent dimensional stability. Very few other materials will work for this task, and maraging steel’s other uses are very specialized. For example, Iran's supply of maraging steel is a controversial subject, with Iran's difficulties in producing maraging steel regarded as a potentially major setback to large-scale uranium enrichment.

[edit] 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, 1686 K
• Thermal conductivity: 25.5 W/(m·K)
• Mean Coefficient of Thermal Expansion: 11.3×10^-6
• Yield tensile strength: typically 1400-2300 MPa (200,000-330,000 PSI)
• Ultimate 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%
• K_IC fracture toughness: up to 175MPam^½
• Young's modulus: 195 GPa
• Shear modulus: 77 GPa
• Bulk modulus: 140 GPa

[edit] External links

• Key To Steel
• Thin cold rolled strips in maraging steels