Grade description
X2NiCoMo18-12 is an 18% nickel cobalt-molybdenum maraging steel. It belongs to the 18Ni(350) family, the strongest of the four classic 18Ni maraging grades: 200, 250, 300 and 350.
Unlike a quenched and tempered alloy steel, it holds almost no carbon, 0.03% maximum. Its strength comes from nanometre-scale intermetallic precipitates, chiefly Ni3Mo, Ni3Ti and Fe2Mo, that form inside a tough nickel martensite during a low-temperature ageing treatment. Four consequences follow from that:
- Tensile strength reaches roughly 2,410 MPa, with fracture toughness well above that of a carbon steel at the same strength level.
- Both heat treatment steps are air cooled. There is no quench and no quench crack risk, and parts are commonly finish machined before ageing.
- With no carbon there is no hard, crack-prone heat-affected zone, so the steel welds more easily than most materials at this strength level.
- Heavy sections age uniformly through the thickness, which is why the grade suits large open-die work.
There are costs to that. Cobalt and molybdenum make the alloy expensive. It has no corrosion resistance and needs plating or coating in wet service. It over-ages above about 450 °C.
Equivalent grades and specifications
Buyers reach this grade under several different names. The designations below normally refer to the same 18Ni(350) material.
| System | Designation |
|---|---|
| EN / DIN, short form | X2NiCoMo18-12 |
| EN / DIN, full form | X2NiCoMoTi18-12-4 |
| Werkstoff number | 1.6356 |
| US aerospace spec | AMS 6515 (bar, forgings) |
| US military, cancelled 2024 | MIL-S-46850D, grade 350 |
| Trade names | Maraging 350, C350, Vascomax C350, 18Ni-350, MAR 350 |
| Nearest lower grades | Maraging 300 (X2NiCoMo18-9-5), Maraging 250 (X2NiCoMo18-8-5, 1.6359) |
Werkstoff numbering for maraging steels is inconsistent between references, and some sources also apply 1.6354 and 1.6358 to grades in this family. Order against the specification and revision printed on your drawing rather than against a trade name. If your drawing calls for something not listed here, send it to us and we will confirm in writing whether we can meet it before you place an order.
X2NiCoMo18-12 chemical composition
Composition limits for the 18Ni(350) grade, as defined in MIL-S-46850D and carried into AMS 6515.
| Element | Specified range | Typical |
|---|---|---|
| Nickel (Ni) | 18.0 – 19.0 | 18.5 |
| Cobalt (Co) | 11.5 – 12.5 | 12.0 |
| Molybdenum (Mo) | 4.6 – 5.2 | 4.8 |
| Titanium (Ti) | 1.3 – 1.6 | 1.40 |
| Aluminium (Al) | 0.05 – 0.15 | 0.10 |
| Carbon (C) | 0.03 max | < 0.01 |
| Silicon (Si) | 0.10 max | 0.05 |
| Manganese (Mn) | 0.10 max | 0.05 |
| Phosphorus (P) | 0.01 max | 0.005 |
| Sulphur (S) | 0.01 max | 0.005 |
| Boron (B), Zirconium (Zr), Calcium (Ca) | grain-refining additions | B 0.003, Zr 0.01 |
| Iron (Fe) | balance | balance |
Nickel, cobalt and titanium contents are what separate the 350 grade from 250 and 300. Titanium has the largest single effect on peak aged strength.
Mechanical and physical properties
X2NiCoMo18-12 is bought and sold in two very different conditions. Machine in the annealed condition, then age.
| Property | Solution annealed | Aged at 480 °C |
|---|---|---|
| Tensile strength, Rm | 1,000 – 1,170 MPa | 2,340 – 2,480 MPa |
| 0.2% proof strength, Rp0.2 | 800 – 900 MPa | 2,200 – 2,340 MPa |
| Elongation, A (4D) | 10 – 15% | 6 – 8% |
| Reduction of area | 55 – 70% | 30 – 40% |
| Hardness | 30 – 35 HRC | 56 – 59 HRC |
| Fracture toughness, KIC | not normally quoted | ~35 – 50 MPa·m1/2 |
| Property | Value |
|---|---|
| Density | ~8.1 g/cm³ |
| Modulus of elasticity | ~186 GPa |
| Melting range | ~1,410 – 1,440 °C |
| Magnetic response | Ferromagnetic |
| Maximum service temperature | ~400 – 450 °C |
Values on this page are typical published figures for the 18Ni(350) grade, given for design and enquiry purposes. They are not a guarantee of the properties of any particular forging. Acceptance values for your order are those written on the approved drawing or specification, and actual results are recorded on the EN 10204 certificate supplied with the parts.
X2NiCoMo18-12 heat treatment
Two stages, both air cooled. There is no quench in the cycle, which is why maraging steels distort so little.
Stage 1, solution anneal
Heat to 815–830 °C, hold roughly one hour per 25 mm of ruling section, air cool. The alloying elements go into solution and the steel transforms to a soft nickel martensite at about 30–35 HRC. This is the condition to machine, drill, straighten and weld in.
Stage 2, age
Heat to 480–500 °C, hold 3 to 8 hours. For the 350 grade, 480 °C for 6 hours is the common starting point. Air cool. Longer times or higher temperatures over-age the steel: strength falls and reverted austenite begins to form.
Heat treatment notes
- Machine before ageing. At 56–59 HRC the aged material is a grinding job rather than a turning job.
- Allow for shrinkage. Ageing produces a small, repeatable linear contraction of roughly 0.05 to 0.10%. On close-tolerance parts, prove the figure on a sample from the same heat before committing production.
- Degrease before ageing. Surface contamination and scale during the age can locally alter the response.
- Do not quench. Water or oil quenching gains nothing and introduces residual stress.
- Decarburisation is not the concern it would be in a carbon steel, since there is almost no carbon to lose. The risk in this grade is titanium nitride formation in a nitrogen-rich atmosphere.
X2NiCoMo18-12 forged shapes
Everything is made to order from your drawing or size list. We do not hold X2NiCoMo18-12 in stock, because the grade is almost always ordered to a specific part.
- Forged rings and seamless rolled rings, plain or stepped section
- Solid discs, pierced discs and gear blanks
- Straight, stepped and eccentric shafts, spindles and torsion bars
- Sleeves and bushings, trepanned or pierced
- Flanges and tube sheets
- Forged round bar, square and rectangular blocks
- Hollow bar and forged tube
- Contour-forged near-net blanks, which cut machining loss on an expensive alloy
Send the drawing and we will confirm whether the size falls within our forging and ring rolling capacity.
Manufacturing route
- Melting. EAF for the primary melt, VOD for decarburisation and degassing, then ESR to refine the ingot structure and reduce inclusion content. Cleanliness matters more in this grade than in most, because coarse titanium nitrides are the usual fatigue initiation sites.
- Ingot to billet. Homogenise, then cog down under the press with controlled reduction to break up the as-cast structure.
- Forging. Open-die forging or ring rolling to the required shape, with the forging reduction ratio agreed in advance where the specification calls for one.
- Solution annealing, giving a machinable 30–35 HRC.
- Rough machining. Optional, and usually worth doing, since it exposes the material for a cleaner ultrasonic inspection and cuts freight cost on an expensive alloy.
- Ageing, where the order calls for delivery in the aged condition.
- Testing and certification, as set out below.
Testing, inspection and certification
| Test | Standard | Notes |
|---|---|---|
| Chemical analysis | Spectrographic, per heat | Ladle and, on request, product analysis |
| Tensile test | EN ISO 6892-1 / ASTM A370 | In the delivery condition |
| Hardness | ASTM E18 (HRC) | Both conditions on request |
| Ultrasonic testing | EN 10228-3, SEP 1921, ASTM A388 | Quality class agreed at order |
| Magnetic particle, dye penetrant | ASTM E1444 / ASTM E165 | On request |
| Grain size, cleanliness | ASTM E112 / ASTM E45 | On request |
| Certification | EN 10204 3.1 | 3.2 with third-party witness on request |
Additional tests and customer-specific inspection plans can be built into the order. Send your ITP with the enquiry and we will price against it.
X2NiCoMo18-12 applications
The grade is chosen when a part has to carry an extreme load in a small envelope, and when heat treatment distortion would be unacceptable.
- Aerospace and defence structures, including rocket motor cases, missile bodies, landing gear components and high-load fittings
- High-performance shafting: drive shafts, torsion bars, couplings and splined shafts
- Heavily loaded gears and pinions, with surface treatment applied after ageing
- Tooling: aluminium die-casting dies, extrusion tooling, punches, cold-forming and hydroforming tools, plastic mould inserts
- Centrifuge and flywheel components and other high-speed rotor parts
- Load cells, flexures and wind-tunnel model components, where the very high elastic limit is the reason for the choice
- Motorsport components and high-load fasteners
Limitations
X2NiCoMo18-12 is not a high-temperature alloy. Above roughly 450 °C it over-ages and loses strength permanently, so it does not substitute for a nickel-base superalloy in a gas turbine hot section. Use Waspaloy or a comparable alloy for that duty. Corrosion resistance is poor, and parts need plating, coating or a controlled environment in wet service. For corrosive duty a precipitation-hardening stainless such as PH13-8Mo is usually the better answer.
Machining, welding and handling
Machining
Machine in the solution annealed condition. At 30–35 HRC the material cuts like a tough alloy steel. It work-hardens, so keep feeds positive, use rigid setups, sharp tooling and generous coolant, and avoid dwelling. Once aged, plan on grinding or EDM only.
Welding
Weld in the annealed condition using GTAW or electron beam with matching filler. No preheat is needed. Keep interpass temperature and heat input low, keep the joint clean and free of grease and moisture, and age the whole assembly afterwards so weld and parent metal reach strength together. Welding an already-aged part locally over-ages the heat-affected zone and leaves a soft band.
Surface treatment and storage
The grade rusts readily. Oil or VCI-wrap parts for shipment, and specify nitriding, plating or coating where the part will see moisture in service. Nitriding after ageing is a common way to add wear resistance without a second high-temperature cycle.
Export control
Maraging steel capable of an ultimate tensile strength of 1,950 MPa or more at 20 °C is a listed dual-use item, entry 1C216 on the EU, UK, Australian and other national control lists. Forms whose linear dimensions are all 75 mm or less are exempt. X2NiCoMo18-12 is capable of about 2,410 MPa, so it falls inside that entry whether it ships annealed or aged.
In practice an end-use and end-user statement is normally required, and depending on destination and application an export licence may be needed. We ask for this at the enquiry stage rather than after an order is placed, so that nobody loses time. If your compliance team needs the material declaration in a particular format, send us the template.
Requesting a quotation
Send these six items and we can normally return a price and lead time within one working day:
- Drawing, or rough dimensions. A PDF or STEP file is ideal, and a sketch with OD, ID, length and tolerances works too.
- Quantity, and whether repeat orders are likely.
- Delivery condition: as forged, solution annealed, aged, or rough machined.
- Governing specification: AMS 6515, a customer spec, or the EN designation.
- NDT and certificate level: UT class, MT or PT, EN 10204 3.1 or 3.2.
- Destination port and the Incoterm you work on.
Email sales@steelforgepieces.com or call +86 189 2135 9659. We quote in USD, ex-works or FOB Shanghai as you prefer.
X2NiCoMo18-12 frequently asked questions
Is X2NiCoMo18-12 the same as Maraging 350?
In practice, yes. X2NiCoMo18-12 is the European short designation for the 18Ni(350) grade that the American market calls Maraging 350, C350 or Vascomax C350 and specifies under AMS 6515. The full EN designation is normally written X2NiCoMoTi18-12-4. Designations vary between standards, so confirm the exact specification and revision on your drawing before ordering.
How strong is X2NiCoMo18-12?
About 2,410 MPa (350 ksi) tensile and 2,275 MPa (330 ksi) 0.2% proof strength after ageing, at 56–59 HRC, with 6–8% elongation. Before ageing, in the solution annealed condition, it is a much softer 1,000–1,170 MPa at 30–35 HRC, which is the condition you machine in.
What is the difference between Maraging 250, 300 and 350?
The number is the approximate tensile strength in ksi, and the difference comes mainly from cobalt and titanium. Maraging 250 runs about 7–8.5% Co and 0.3–0.5% Ti. Maraging 300 runs about 8.5–9.5% Co and 0.5–0.8% Ti. X2NiCoMo18-12 (Maraging 350) runs about 11.5–12.5% Co and 1.3–1.6% Ti. Higher strength costs toughness and money, so specify the lowest grade that carries your load.
Can X2NiCoMo18-12 be welded?
Yes, and unusually well for a 2,400 MPa material. With carbon under 0.03% there is no hard, crack-prone heat-affected zone. Weld in the solution annealed condition with matching filler and no preheat, keep heat input low, then age the whole assembly so weld and parent reach strength together.
Does X2NiCoMo18-12 rust?
Yes. It has no meaningful corrosion resistance, because there is no chromium in the alloy. Parts need oil or VCI protection in storage and transit, and plating, nitriding or coating for service in a wet environment. For a high-strength part that must also resist corrosion, a precipitation-hardening stainless such as PH13-8Mo or 17-4PH is usually the better choice.
How much does X2NiCoMo18-12 shrink during ageing?
A small, repeatable linear contraction of roughly 0.05 to 0.10%. It is consistent enough to compensate for in the machining allowance, but it varies with section and heat, so prove it on a sample from the same heat before running close-tolerance production.
What is the minimum order quantity?
There is no fixed piece count. Because X2NiCoMo18-12 is melted and forged to order, what governs is the ingot size needed for your part. Single prototype pieces are possible, and the price per kilogram simply reflects the small quantity. Send the drawing and we will tell you whether a one-off makes commercial sense.
Who supplies X2NiCoMo18-12 forgings?
Jiangyin Jiangnan Metal Co., Ltd., an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, forges X2NiCoMo18-12 into rings, seamless rolled rings, discs, shafts, sleeves, bushings, flanges, blocks and bars to customer drawing, with EAF+VOD+ESR melting, in-house heat treatment, ultrasonic testing to EN 10228-3, SEP 1921 or ASTM A388, and EN 10204 3.1 or 3.2 certification. Contact sales@steelforgepieces.com or +86 189 2135 9659.
Is X2NiCoMo18-12 export controlled?
It can be. Maraging steel capable of 1,950 MPa or more is dual-use entry 1C216 on the EU, UK and other national control lists, exempting forms whose linear dimensions are all 75 mm or less. Since this grade reaches about 2,410 MPa, shipments above that size threshold normally require an end-use statement and may require an export licence depending on destination.
What do you need in order to quote?
Drawing or rough dimensions, quantity, delivery condition, governing specification, NDT and certificate level, and destination port. With those six items a price and lead time normally come back within one working day.
Data sources
Composition limits are those of the 18Ni(350) grade defined in MIL-S-46850D and carried into SAE AMS 6515. Mechanical and physical values are typical published figures for the grade, compiled from mill and distributor datasheets. Export control wording follows dual-use entry 1C216 as published in national control lists.
This page is maintained by the sales engineering team at Jiangyin Jiangnan Metal Co., Ltd. Last reviewed . If you find an error, or need a value not published here, write to sales@steelforgepieces.com.