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Maraging 250 / (1.6359) / UNS K92890 / AMS 6512

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Maraging 250 is the C250 class of an iron-nickel based, age-hardened family of ultra high strength steels. Within the alloy steel group it belongs to a special set in which strength comes not from carbon but from intermetallic precipitates; its UNS designation is K92890 and its material number 1.6359. It is also known as Udimar 250 and Vascomax C250.

The distinguishing feature of maraging steels is that they machine easily in the solution treated (soft) condition and are then taken above 1750 MPa by a simple low temperature ageing treatment. Because ageing is carried out at around 480 °C, distortion and dimensional change are very low, which is a major advantage in parts held to tight tolerances.​‌​​‌​

Strength comes from the fine intermetallic precipitates formed by 17-19% nickel together with 7-8.5% cobalt and 4.6-5.2% molybdenum. The carbon content is held below 0.03%. The material shows high resistance to crack propagation and its transverse properties are also good.

It is used in missile and launch systems, in wing slat tracks and in drive shafts. It is supplied as bar, billet, forging stock and in powder metallurgy forms.​‌​​‌​

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What Maraging 250 Is — and Exactly Where It Sits in the 18Ni Family

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Maraging 250 (18Ni-250 / UNS K92890 / W.Nr. 1.6359 / EN X2NiCoMo18-8-5) is an iron-nickel-cobalt-molybdenum alloy that hardens not through carbon but through intermetallic precipitation. The 250 in the name denotes the nominal 250 ksi (1724 MPa) yield class in the aged condition — the yield, not the tensile; within this family the number names the yield strength. Trade names include Vascomax C250, Marval 18, BÖHLER V250, ATI C-250 and Marvac 250. Three things must be understood from the outset: it is not stainless, it is not an ASME pressure-vessel code material, and it belongs to an export-controlled, dual-use material family.

The alloy’s distinguishing property in one sentence: it is the only common class of engineering steel that reaches a 1700 MPa yield with no quench, no preheat and essentially no distortion. It is machined, welded and cold formed soft; a simple air-cooled furnace cycle of a few hours at 480 °C then doubles its strength. What you buy is not the strength — it is the way the strength is obtained.​‌​​‌​

The 18Ni family: what actually changes as the number rises

The family has four steps and the design logic is visible in a single line: cobalt and titanium rise together, molybdenum stays nearly constant, and carbon is close to zero at every step. Titanium is the direct fuel for the Ni₃Ti precipitate; cobalt forms no precipitate of its own — it lowers the solubility of molybdenum in the matrix and therefore forces more Mo to precipitate as Ni₃Mo and Fe₂Mo. That is why cobalt and titanium are raised together, and why every step buys strength with toughness.​‌​​‌​

The toughness ladder — publish this as prominently as the strength table​‌​​‌​

When all four steps are measured in the same laboratory, in the same programme, the ladder is unambiguous. The figures below come from a single defence research report and are therefore mutually comparable (they are K_Q values, i.e. provisional fracture-toughness results whose validity conditions were not separately confirmed — read them as a ratio between grades, not as absolute design values):

Why there is no carbon — and why that explains everything else

A conventional alloy steel takes its hardness from carbon trapped in a martensite lattice by quenching. The carbon atom distorts the lattice, pins dislocations and gives hardness — and at the same time brings brittleness, quench cracking, hydrogen cracking and HAZ hardening. In maraging steel, carbon is deliberately held at ≤0.03% (≤0.010% at some mills). The result: the structure that cools from high temperature is not a hard carbon martensite but a soft, ductile, massive iron-nickel martensite — roughly 30–35 HRC, about 870 MPa yield, 14% elongation. In that condition it is machined, welded and cold rolled by up to 90%.​‌​​‌​

All of the strength is added afterwards, in a separate step. A few hours at 480 °C precipitate nanoscale Ni₃Mo, Ni₃Ti and Fe₂Mo intermetallics on the high dislocation density of the martensite itself. These are not carbides; they introduce no carbon into the lattice and do not weaken grain boundaries. That is why 9–12% elongation and 45–53% reduction of area survive at a 1700 MPa yield — a carbon steel tempered to the same hardness would be effectively without ductility. Cobalt’s role becomes clear here: it forms no precipitate of its own but lowers the solubility of molybdenum, forcing Mo out of solution. Cobalt and molybdenum work together, not separately.

Overageing and reverted austenite. If the ageing temperature or time is exceeded, two things happen: the precipitates coarsen and — more importantly — austenite re-forms (reverted austenite) in nickel-rich regions. This is a soft phase that lowers strength while, for a while, raising ductility; with prolonged overageing the coarsened Ti-Ni intermetallics become crack initiation sites and severe embrittlement follows. The practical consequence: temperature control of the ageing furnace is a quality parameter in this alloy, not a convenience. The same mechanism caps continuous service at roughly 400–450 °C — above that the alloy overages itself.​‌​​‌​

Standards by Product Form

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The standards landscape for this alloy is aerospace-led (AMS), not ASTM/ASME-led. The B-series ASTM product specifications you are used to on stainless and nickel alloys largely do not exist here; AMS numbers and one military specification take their place. This is the single most common disappointment on the buying side.

ASME Code Acceptance and Pressure-Equipment Status

The answer here is short and it bears directly on the sales conversation: Maraging 250 is not an ASME pressure-vessel code material. The “SB-xxx, Section VIII Div. 1, maximum 427 °C” table you are used to on nickel alloys has no counterpart here — and that is the material’s real position, not a gap in the research.​‌​​‌​

Product Forms With NO Standard — the Commercially Valuable Section​‌​​‌​

This is the section your sales engineers should memorise. The list of forms that can honestly be sold “to a standard” for Maraging 250 is in fact short: bar, forgings, rings, tubing, sheet, strip, plate. Everything else is sold to a mill specification and a customer drawing.

Chemical Composition

The band below is the common wording of the AMS 6512 / MIL-S-46850 / ASTM family. The nominal mill composition is 18.5Ni – 7.5Co – 4.8Mo – 0.4Ti – 0.1Al, balance iron.​‌​​‌​

Three lines that actually matter on a certificate. (1) Carbon. 0.03% is not merely a ceiling, it is a design parameter; a heat close to 0.03% is not the same material as one at 0.005% — higher carbon forms titanium carbonitrides and carbides at grain boundaries and lowers fracture toughness. (2) Titanium. The top of the band means higher strength, the bottom means higher toughness; narrow the band for toughness-critical work. (3) Melt route. AMS 6512 requires CEVM (consumable-electrode vacuum melting); most mills run VIM + VAR. Air melting is not acceptable in this alloy — titanium and aluminium oxidise, inclusion cleanliness collapses and toughness goes with it.​‌​​‌​

Mechanical Properties

Never mix the two sets of numbers in this section: specification minima (what you are buying) and typical mill values (what you will usually get). And every figure must state its condition — solution annealed or aged.​‌​​‌​

Physical Properties

The physical properties differ between the two conditions, and electrical resistivity in particular changes markedly on ageing — which makes it a usable shop-floor check.​‌​​‌​

Heat Treatment and Thermal Stability​‌​​‌​

The heat treatment of Maraging 250 is two steps and both are simple — this is precisely where the alloy’s commercial strength lies. There is no quench, a protective atmosphere is not mandatory (with no carbon there is no decarburisation risk), and distortion is very small.

Welding

Welding is maraging steel’s single greatest advantage, and the reason is directly the absence of carbon. In a conventional high-strength alloy steel the heat-affected zone transforms on rapid cooling into a hard, brittle carbon martensite, which is why preheat, interpass control, low-hydrogen consumables and post-weld stress relief are mandatory. In Maraging 250 what forms in the HAZ is again a soft, tough iron-nickel martensite — so no preheat is needed and the risk of hydrogen cracking is practically absent.​‌​​‌​

Machining​‌​​‌​

The rule in one sentence: rough and, where possible, finish machine in the solution-annealed (soft) condition, then age. Because the dimensional change on ageing is of the order of 0.05–0.09% and is predictable, most tight-tolerance detail can be finished before hardening. This is the alloy’s principal commercial advantage over conventional tool steels — there is no quench distortion, and the post-hardening grinding pass is reduced or eliminated.

In the annealed condition (30–35 HRC) machinability is compared to AISI 4340 at the same hardness. Machining after ageing (48–52 HRC) is possible but demands rigid machines, very sharp carbide tooling, short tool overhang and abundant coolant. The parameters below are the mill datasheet’s annealed-condition starting points.​‌​​‌​

Nitriding​‌​​‌​

Maraging 250 can be nitrided; mill documents list nitriding explicitly as a surface treatment. The metallurgically convenient point: nitriding temperatures overlap the alloy’s ageing band, so nitriding and ageing can be combined in a single cycle with the core ageing while the case forms. Honest limit: no verified case hardness or case depth value was found that could be published here — academic work on ion nitriding of 18Ni-250 and on the fatigue strength of nitrided specimens exists, but no catalogue figure. Case data must be obtained from the treatment house on a part-by-part basis.

Corrosion — Where It Is Good, WHERE IT FAILS​‌​​‌​

This section must be published at least as prominently as the strength section, because most field failures come from here.

The basic fact: it is not stainless​‌​​‌​

Maraging 250 is not a stainless steel. Chromium is not an alloying addition but a residual capped at 0.50% (0.25% at some mills). There is no passive oxide film. Its corrosion resistance is comparable to that of an ordinary low-alloy martensitic steel — not to a 300- or 400-series stainless. Bare parts will rust in humid, marine and industrial atmospheres. For any service outside a dry, controlled environment, plan on conventional protection: plating, phosphating, paint, dry-film lubricant or an oil or grease film. Supplier pages that sell the product as “maraging stainless steel” are metallurgically wrong.

Where it is relatively good: atmospheric and general corrosion rate​‌​​‌​

Despite carrying no chromium, the general atmospheric corrosion rate of maraging steel is no worse than that of conventional low-alloy steels at the same strength level, and mill documents report better pitting and corrosion resistance than common tool steels. Rust advances uniformly, as in conventional steels — localised penetrating attack is not typical. It also takes an excellent polish, which matters commercially for tooling surfaces.

WHERE IT FAILS — stress-corrosion cracking​‌​​‌​

The widely repeated sentence “maraging steels resist stress corrosion and hydrogen embrittlement” is misleading when used on its own. The correct statement is: maraging steels are more resistant than conventional high-strength steels AT THE SAME STRENGTH LEVEL — but they are not immune, and susceptibility rises sharply with the strength grade. An independent NASA study tested all four grades side by side:

Hydrogen embrittlement

Maraging 250 is susceptible to hydrogen embrittlement, and — as in every steel at a 1700 MPa yield — that susceptibility rises with strength. The practical consequences bear directly on your manufacturing route: acid pickling, electroplating (cadmium and zinc especially), electro-polishing and cathodic protection all charge hydrogen into the part. Plated parts require a post-plating hydrogen bake-out, planned at a temperature that does not disturb the aged condition — the classical bake-out range of 190–200 °C sits far below the 480 °C ageing temperature and is safe in that respect. No verified numerical hydrogen threshold (critical hydrogen concentration) was found in this research — the plating specification should be written jointly with the customer.​‌​​‌​

Where it must not be used

(1) Any acid service, any chemical-process duty, because there is no chromium. This is not a corrosion alloy; it is a high-strength structural alloy. (2) Bare in permanent salt-spray or marine immersion service. (3) H₂S (sour) service — it is not a listed material under NACE MR0175 / ISO 15156 and must never be offered for that duty. (4) Anywhere that must be non-magnetic — it is ferromagnetic. (5) Continuous service above ~450 °C — it overages.​‌​​‌​

Frequently Asked Questions

Should we buy Maraging 250 or 300? Is the difference just price and strength?​‌​​‌​

No — the real difference is toughness and stress-corrosion behaviour, and it is not small.
On strength the difference is what you would expect: the 250 grade gives ~1724 MPa yield, the 300 grade ~2068 MPa yield — about 20% more. What you pay for it shows up in the chemistry: the 300 grade carries more cobalt and roughly twice the titanium of the 250 (0.50–0.80% against 0.30–0.50%).
The real price is paid in toughness. Measured in the same laboratory in the same programme: fracture toughness 115–124 MPa√m for the 250 against ~77 MPa√m for the 300; Charpy energy 33–41 J against ~24 J. You are giving up roughly a third of the toughness for 20% more strength.
And on the corrosion side the gap is sharper still. In an independent NASA study the 250 grade did not fail in salt water at up to 90% of its yield strength; the 300 grade failed at 75%. So the material is not only more brittle — the fraction of yield you may safely apply also drops.
The practical decision rule: if your design is stress-limited and the part is thin, notch-free, dry and well protected, the 300 grade saves weight. If your design is governed by crack tolerance, damage tolerance, fatigue life or a corrosive environment — aerospace structural parts, fasteners, welded cases — the 250 is usually the correct engineering choice, and it is cheaper. A bigger number is not automatically an improvement.

Should we machine before or after ageing, and how much does the part move?​‌​​‌​

Machine first, age afterwards — that is the main reason to buy this alloy.
Maraging 250 arrives solution annealed at 30–35 HRC. In that state it cuts like a mild alloy steel; turning with carbide runs around 145 m/min, and slotting and drilling are unproblematic. The ageing that follows is a simple furnace cycle at 480 °C with air cooling and no quench.
The movement is very small, one-directional (contraction) and predictable: European mill data give ~0.05%, US mill data 0.0009 in/in ≈ 0.09%. We state plainly that these conflict — if you are holding micron-level tolerances, run a trial piece from your own heat, measure the contraction and build it into the machining size. Both figures are far below the quench distortion of a conventional tool steel.
The physical reason: the martensite transformation is already complete during air cooling from the solution anneal (starting at about 230 °C). Ageing performs no new phase transformation; it only forms nanoscale precipitates inside the existing martensite. No quench, no transformation stress, no distortion.
Practical shop recipe: rough machine → repeat the solution anneal if stress relief is needed → finish machine (leaving the contraction allowance) → age → grind only the critical surfaces if required. On most parts the final grinding pass disappears entirely.

We are building a welded case. Is preheat really unnecessary, and what happens after welding?​‌​​‌​

It really is unnecessary — and the reason is metallurgical, not a convenience claim.
In a conventional high-strength steel, preheat exists because the HAZ transforms on rapid cooling into a hard, brittle carbon martensite that is open to hydrogen cracking. In Maraging 250 carbon is ≤0.03%; what forms in the HAZ is again a soft, tough iron-nickel martensite. There is no carbon to drive hardening, so preheat, controlled cooling and post-weld stress-relief annealing are not required. Mill datasheets put it in one line: “good weldability without preheating or post heating”.
The route is this: weld the parts in the solution-annealed (soft) condition. Then age the whole structure together: 480 °C, 3–6 hours, air cool. The critical detail most people do not know: the weld bead hardens on the same ageing cycle as the parent metal, with no separate solution treatment of the assembly. Not having to take a large rocket motor case or a welded frame back up to 815 °C is the real commercial value of this material.
Three things still need attention. (1) Keep heat input low — high heat input produces reverted austenite and a coarsened structure in the HAZ, which does not fully recover on ageing. (2) HAZ toughness does not fully match the parent metal; if fracture toughness is contractual, take the specimen from the weld zone. (3) Surface cleanliness: this is a titanium- and aluminium-bearing alloy, and oxide or oil leaves inclusions in the bead and costs toughness.
One final note of honesty: a general materials database states that this alloy requires preheat and post-weld heat treatment, which conflicts with the mill documents. The mill position is far better supported, but on critical work have your own WPS validated against the relevant mill’s recommendation.

Should we be using AerMet 100 or 300M instead of Maraging 250?​‌​​‌​

All three are sold under the heading “1700–1900 MPa class ultra-high-strength steel”, but they are not interchangeable. The dividing lines are carbon, distortion and stress-corrosion cracking.
AerMet 100 (0.23C – 13.4Co – 11.1Ni – 3.1Cr – 1.2Mo) is a carbon-bearing, secondary-hardening steel. Its typical values start at almost exactly the same yield as Maraging 250 (1724 MPa yield / 1965 MPa tensile) but deliver 14% elongation, 65% reduction of area and K_Ic 126 MPa√m. The decisive difference is K_ISCC: 88 MPa√m for AerMet 100 against ≈44 MPa√m for Maraging 250 — roughly twice the crack tolerance in a salt environment. The price: it needs a conventional quench plus a deep-freeze step (−73 °C) plus 5 hours at 482 °C, so it distorts; and its carbide content makes it harder to machine. Where damage tolerance in a marine or landing-gear environment governs, AerMet 100 is the right answer.
300M (0.40C – 1.6Si – 0.8Cr – 1.8Ni – 0.4Mo – 0.07V) is the classic quenched-and-tempered landing-gear steel: the AMS 6417 minima are 1862 MPa tensile / 1517 MPa yield, 8% elongation, 30% reduction of area, ≥52 HRC. Higher tensile, lower yield and markedly lower ductility than Maraging 250; being oil quenched it is the most distortion-prone and the most hydrogen-sensitive option, and its high carbon means welding needs preheat. In return it is far cheaper. 300M is a cost-performance choice.
Decision rule: for welded structures, tight tolerances, distortion-free hardening and a simple heat treatment, choose Maraging 250. For maximum damage tolerance in a salt environment, AerMet 100. Where budget dominates and the part is a solid, machined, plated forging, 300M. If you need more strength, look at Maraging 350 — but read the toughness penalty on that page first.

Common Datasheet Errors and Traps​‌​​‌​

1. “X3NiCoMoTi18-9-5” or “1.2709” given as Maraging 250 — WRONG. 1.2709 / X3NiCoMoTi18-9-5 is the tool-steel and additive-manufacturing grade (MS1) with titanium around 1%, corresponding roughly to the 300 grade. The correct European equivalent of Maraging 250 is 1.6359 / X2NiCoMo18-8-5. This is the single most frequent error in the trade and it leads to the wrong powder or the wrong bar being ordered.
2. AMS 6512 and AMS 6520 are conflated. AMS 6512 = bar, forgings, TUBING and rings (CEVM, annealed). AMS 6520 = sheet, strip and plate (CE, solution treated). Many reseller pages list every product form under “AMS 6512”. And the fact that AMS 6512 covers tubing is omitted on most pages — which is a sellable advantage.
3. AMS 6514 / AMS 6521 quoted as Maraging 250 — WRONG, those are the 300 grade. 6514 = Maraging 300 bar/billet/forging stock, 6521 = Maraging 300 sheet/strip/plate. The numbers look alike, and ordering errors are common.
4. ASTM A579 presented as a current standard — WRONG. A579/A579M was withdrawn in May 2024 with no replacement. Do not write “forgings to ASTM A579 Grade 72” into new contracts; cite AMS 6512.
5. ASTM A538 presented as an active standard. A538/A538M Grade B (“MAR-18-250”) is an old document that is no longer active; valuable as a historical reference, not as the governing current standard.
6. Sold as “Maraging 250 stainless steel” — metallurgically WRONG. Chromium is not an alloying addition but a residual capped at 0.50%; there is no passive film. Some suppliers carry “maraging stainless steel” even in their product-page URLs.
7. “Maraging steels resist stress corrosion and hydrogen embrittlement” published without qualification. The correct statement: more resistant than conventional steels at the same strength level, but not immune. For the 250 grade K_ISCC ≈ 44 MPa√m, only 40% of the toughness in air.
8. The ageing cycle quoted without the solution-anneal step. A customer who applies ageing alone to unannealed or wrongly annealed stock will not obtain the quoted properties. In the same defence report one heat finished below the 240 ksi minimum, at 232 ksi yield.
9. Ageing time published as a single number. Published cycles diverge across 480 °C/3 h, 480 °C/4 h, 482–496 °C/6 h and 482 °C/4–6 h. They differ by section thickness and by the intended toughness-strength balance; the “right” cycle is the one the customer specification names.
10. Dimensional change given as a single number. There is a genuine conflict between 0.05% (European mill) and 0.09% (US mill). Measure it on your own material for tight tolerances.
11. Claiming cobalt rises from the 200 to the 250 grade — WRONG. The 200 grade carries 8–9% Co and the 250 grade 7.0–8.5% Co; cobalt actually falls slightly. The jump in strength is made by molybdenum (3.0–3.5% → 4.6–5.2%) and titanium.
12. Talking as though ASME code acceptance exists. No confirmed ASME Section VIII / B31.3 acceptance and no P-number were found for 18Ni maraging. Answer a “code vessel” request honestly.
13. Export control skipped. Maraging steels are dual-use materials; the gas-centrifuge control text explicitly names “maraging steel capable of an ultimate tensile strength of 1.95 GPa or more”. The typical aged tensile of Maraging 250 (~1.75–1.87 GPa) falls below that threshold, while the 300 and 350 grades are above it — but thresholds vary between regimes, and some lists apply lower thresholds to sheet, plate and tube forms (the exact figure could not be independently verified in this research). Always have end-use statements and licence status checked for international shipments.

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