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Maraging 300 / (1.6358) / UNS K93120 / AMS 6514 / AMS 6521

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Maraging 300 (1.6358 – 1.6354), also widely known by its designation UNS K93120, is a special alloy steel formed essentially from nickel, molybdenum, cobalt and titanium. Grade 300 is the most widely used quality among the maraging materials. Including this one, there are four commonly known types of maraging steel: Maraging 200, 250, 300 and 350, and maraging steels are also known as 18Ni maraging steels. The numbers 200, 250 and 300 that follow the name indicate the ultimate tensile strength of the material in lbf/in², so the higher the number the stronger the material. Maraging materials contain little carbon, and that low carbon content is what makes them easy to machine. They can also, before ageing, be cold formed to a large degree without generating cracks. Maraging materials may undergo a small amount of dimensional change after heat treatment, so final precision dimensions on these materials are best machined after heat treatment. Thanks to their high alloy content, this material can be taken to high hardness after heat treatment. Maraging materials generally have moderate corrosion resistance, and where corrosion resistance has to be increased, cadmium plating or various other coatings can be used to raise it.

The most widely used of the maraging materials, this is an alloy steel of very high mechanical strength and is generally chosen where very high mechanical strength is required. It is frequently used in aircraft landing gear parts, in aircraft gear components that have to carry mechanical load, and in structural applications that are permanently under load and need very high mechanical strength.​‌​​‌​

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Standards by Product Form

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Maraging 300 (18Ni-300 / UNS K93120 / C300) is an iron-nickel alloy that hardens not through carbon but through intermetallic precipitation. The 300 in the name denotes the nominal 300 ksi (~2070 MPa) tensile class in the aged condition. Two things must be understood from the outset: it is not stainless, and it is an export-controlled dual-use material.

WARNING — catalogue traps. (1) AMS 6514 and AMS 6521 must not be conflated: 6514 covers bar, billet and forging stock; 6521 covers sheet, strip and plate. Many reseller pages list every product form under “AMS 6514”. (2) ASTM A538 should not be presented as an active standard. (3) Some supplier pages sell the product as “Maraging 300 stainless steel” — metallurgically wrong; the alloy contains no significant chromium. (4) The ageing cycle is frequently quoted without the solution-treatment step; a customer who applies ageing alone to non-annealed stock will not obtain the quoted properties.

Composition: C ≤0.03% · Ni 18.0–19.0% · Co 8.0–9.5% · Mo 4.6–5.2% · Ti 0.50–0.80% · Al 0.05–0.15% · Si ≤0.10% · Mn ≤0.10% · S ≤0.010% · P ≤0.010%. Why the very low carbon matters: strength does not come from the hardness of a carbon martensite as it does in conventional steels. Carbon is deliberately held below 0.03%, so the as-cooled structure is not a hard, brittle carbon martensite but a soft, ductile iron-nickel martensite. All of the strength is added afterwards, in a separate ageing step, by nanoscale Ni₃Mo, Ni₃Ti and Fe₂Mo intermetallic precipitates forming on dislocations.​‌​​‌​

Mechanical Properties and Heat Treatment

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Heat treatment​‌​​‌​

The mechanism is age hardening of a low-carbon iron-nickel martensite, with the strength coming from Ni₃Mo / Ni₃Ti / Fe₂Mo intermetallics rather than carbides. Solution annealing: about 816 °C (1500 °F), typically 1 hour (roughly 1 hour per inch for heavy sections), air cool → ~30–35 HRC. Ageing: 482 °C (900 °F) for 3–6 hours (sources give 3 or 6 hours depending on section size), air cool → ~52 HRC and ~2000 MPa tensile. Modified cycle for die-casting tooling: anneal at 816–829 °C for 1 hour per inch, then age at 527–538 °C for 6 hours. Dimensional stability: ageing produces roughly 0.05% linear contraction — with no quench involved, distortion is far below that of conventionally hardened steels, so parts can be machined close to final size and aged afterwards. Standard delivery is the solution-annealed condition.

Welding​‌​​‌​

Weldability is good for the strength class. The common statement in mill and service-centre datasheets is that no preheat or pre-heating is required — a genuine distinguishing feature against conventional high-strength alloy steels, and a direct consequence of the very low carbon (no brittle carbon martensite and no hydrogen-cracking risk in the HAZ). Repair weldability is also good. Post-weld ageing is required: welding is normally done in the soft, solution-annealed condition and the whole part is aged afterwards. Welding already-aged material still requires ageing, and the HAZ toughness will not fully match the base metal. Note: one general materials database states that preheat and post-weld heat treatment are required, which conflicts with the mill-datasheet consensus — for critical work your welding engineer should confirm this against the specific mill’s WPS.

Machining​‌​​‌​

In the annealed condition, machinability is comparable to AISI 4340 at the same hardness level. The standard practice is to rough and usually finish machine in the soft, solution-annealed condition and age afterwards — because distortion on ageing is so small, most finish machining can be completed before hardening, which is the alloy’s principal commercial advantage over conventional tool steels that distort and need post-hardening grinding. Machining in the aged condition (52+ HRC) requires rigid equipment, very sharp (usually carbide) tooling and abundant coolant. Nitriding can be applied for wear resistance (single-sourced).

Service Limits and Comparison​‌​​‌​

Frequently Asked Questions

What does “maraging” mean, and why is there almost no carbon?​‌​​‌​

“Maraging” is a contraction of martensitic ageing. Conventional alloy steels take their hardness from carbon locked into a martensitic structure by quenching. Maraging steel instead keeps carbon deliberately very low (≤0.03%). The result, straight from the mill or after a simple air-cooled solution anneal, is a soft, tough iron-nickel martensite at around 30–35 HRC, easily machined or formed. All of the strength is added afterwards in a separate low-temperature ageing step (roughly 482 °C for a few hours), during which nanoscale nickel-molybdenum and nickel-titanium intermetallics precipitate throughout the matrix, raising hardness to 50–55 HRC and tensile strength to roughly 2000 MPa. Because none of this depends on carbon, the alloy avoids the brittleness and cracking risk that high-carbon steels show at equivalent strength.

Why are parts machined before ageing, and how much do they move?​‌​​‌​

Maraging 300 is normally supplied and machined in its soft, solution-annealed condition (~30–35 HRC), where it cuts much like a mild alloy steel. The ageing treatment that follows is a simple, low-temperature (~482 °C) air-cooled cycle with no quench, so it produces only a very small, predictable dimensional change — mill data for C300 puts it at roughly 0.05% linear contraction. That is far less movement than a conventional tool steel sees during a hardening quench, so shops routinely finish-machine tight-tolerance features before ageing, cutting or eliminating the post-hardening grinding pass that other high-strength steels need. This is the material’s main commercial advantage for precision tooling and aerospace hardware.

Is Maraging 300 stainless, and does it need corrosion protection?​‌​​‌​

No — despite the name, and despite some catalogues mislabelling it, Maraging 300 is not a stainless steel. It contains essentially no chromium; its corrosion resistance comes only incidentally from its nickel content and is broadly similar to an ordinary low-alloy martensitic steel, not to a 400- or 300-series stainless. Bare parts will rust in humid, marine or chemically aggressive environments. For any service outside a dry, controlled environment, plan on conventional protection — plating, phosphating, paint or an oil or grease film — the same as you would specify for 4340 or a comparable alloy steel. Confirm plating and coating compatibility with the customer’s ageing cycle, since some coatings must be applied after the final age.

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