What is Vascomax C350?
Vascomax C350 is the highest-strength member of the cobalt-strengthened, 18% nickel maraging steel family. The name is a contraction of martensitic and aging. The alloy is cooled to a soft iron-nickel martensite, machined in that soft state, then hardened by a low-temperature aging step instead of a conventional quench.
Carbon is held below 0.03%. Strength comes from intermetallic precipitates, principally Ni3(Mo,Ti) and Fe2Mo, that form during aging. Cobalt does not precipitate on its own. It lowers the solubility of molybdenum in the martensite matrix, and that produces a much denser precipitate population. Raising cobalt from 9% in C300 to 12% in C350, with titanium going from 0.6% to 1.4%, lifts yield strength from roughly 2,000 MPa to roughly 2,320 MPa.
Two things follow from the carbon-free chemistry, and both matter more to a buyer than the strength figure. Hardening happens at 480 °C rather than through an 850 °C quench, so the part changes size by a predictable 0.05% instead of warping, and can be machined nearly to finished size before it is hardened. There is also no hard heat-affected zone, so the alloy welds without preheat.
Also known as
The same material reaches us under many names. All of the following refer to grade 350 cobalt-strengthened maraging steel and are quoted identically by our sales desk.
| System | Designation | Notes |
|---|---|---|
| UNS | K93120 | Unified Numbering System, USA |
| SAE / AMS | AMS 6515 | Bar, forgings and tubing, consumable-electrode vacuum melted |
| US military | MIL-S-46850D | 350 ksi grade, high quality |
| ASTM | A538 Grade C | Pressure-vessel plate, 18Ni maraging |
| Werkstoff (DIN/EN) | 1.6358 | X2NiCoMoTi 18-12-4 |
| Generic | Maraging 350, 18Ni(350), 18Ni-350 | Strength level in ksi |
| Trade names | Vascomax® C350, ATI C-350®, Marage 350 | Vascomax is a registered trademark of ATI / Allvac. We forge the equivalent grade, not ATI-branded product |
| Common misspellings | Vascomax C-350, Vascomax 350, Vasco Max C350 | All refer to the same grade |
Trademarks are the property of their respective owners and are used here only to identify the grade.
Chemical composition of Vascomax C350
The figure missing from most published composition tables is iron. It is the balance, at roughly 63%. Vascomax C350 is a steel rather than a nickel-base superalloy, although most distributors catalogue it under nickel alloys.
| Element | Symbol | Weight % | Role in the alloy |
|---|---|---|---|
| Iron | Fe | Balance, about 63 | Matrix |
| Nickel | Ni | 18.50 | Forms the soft, tough Fe-Ni martensite |
| Cobalt | Co | 12.0 | Lowers Mo solubility, intensifying precipitation |
| Molybdenum | Mo | 4.8 | Forms Ni3Mo and Fe2Mo strengthening phases |
| Titanium | Ti | 1.40 | Forms Ni3Ti, the main strength lever over C300 |
| Aluminium | Al | 0.10 | Deoxidiser, minor age-hardening contribution |
| Carbon | C | 0.03 max | Held low on purpose, since carbides would embrittle |
| Manganese | Mn | 0.10 max | Residual |
| Silicon | Si | 0.10 max | Residual |
| Phosphorus | P | 0.01 max | Impurity, restricted for toughness |
| Sulphur | S | 0.01 max | Impurity, restricted for toughness |
| Boron | B | 0.003 | Grain-boundary refinement |
| Zirconium | Zr | 0.01 | Grain-boundary refinement |
Nominal aim values to AMS 6515 and MIL-S-46850D. Actual heat analysis is reported on the EN 10204 3.1 certificate supplied with every forging.
Mechanical properties, annealed and aged
Vascomax C350 is delivered soft and hardened after machining, either by the customer or by us. A single set of properties would be misleading, so both conditions are listed below.
| Property | Solution annealed (as delivered) | Aged 480 °C, 3 to 6 h |
|---|---|---|
| 0.2% proof stress | about 830 MPa (120 ksi) | 2,320 MPa min (337 ksi) |
| Ultimate tensile strength | about 1,140 MPa (165 ksi) | 2,413 MPa min (350 ksi) |
| Elongation, 4D | about 18% | 6% min, typically 7% |
| Reduction of area | about 60% | 25% min, typically 30% |
| Hardness | 30–35 HRC | 54–58 HRC |
| Young's modulus | about 190 GPa | about 186 GPa |
| Machinability | Comparable to 4340 at equal hardness | Grinding, or rigid setups with very sharp tooling |
Service temperature
Vascomax C350 holds most of its strength to about 450 °C. Above that the aging precipitates coarsen and the alloy over-ages. Notch toughness is maintained down to minus 50 °C and below, which is why it appears in high-altitude and cold-climate driveline work.
Physical properties
| Property | Value |
|---|---|
| Density | 8.02 to 8.08 g/cm³ (0.290 to 0.292 lb/in³) |
| Melting range, liquidus | about 1,470 °C (2,680 °F) |
| Poisson's ratio | 0.30 |
| Shear modulus | about 75 GPa |
| Magnetic response | Ferromagnetic |
| Corrosion resistance | Not stainless, no chromium. Moderate general resistance, with good resistance to stress-corrosion cracking and hydrogen embrittlement for its strength level |
| Surface hardening | Nitridable, case to about 65 HRC without embrittling the core |
How Vascomax C350 is heat treated
The commercial case for this alloy rests on the sequence below. Because hardening happens at 480 °C rather than through a quench, the part changes size predictably and does not warp.
- Solution anneal, 815 to 830 °C (1500 to 1525 °F), one hour per 25 mm of section, air cool. Result: soft Fe-Ni martensite at 30–35 HRC. No protective atmosphere is required, since the alloy is essentially carbon free.
- Machine. Rough and finish machine in this condition, close to final dimensions.
- Age, 480 to 485 °C (900 °F, plus or minus 10 °F), 3 to 6 hours, air cool. Result: 2,413 MPa tensile at 54–58 HRC. Time matters less than temperature, so a furnace survey to AMS 2750 is worth having.
Allow for the contraction. Aging shrinks the part by roughly 0.05%, about 0.05 mm on a 100 mm diameter, uniformly in all directions. On close-tolerance bores and journals, size the annealed part 0.05% oversize or leave grind stock.
Die-casting dies are an exception. For die-casting and hot-work tooling, age at 527 to 538 °C (980 to 1000 °F) for 6 hours instead. This trades peak hardness for thermal-fatigue resistance and gives longer die life.
Jiangyin Jiangnan Metal Co., Ltd. can supply Vascomax C350 forgings in the solution annealed condition for you to age after machining, or fully aged and finish-machined to drawing. State which you need on the enquiry, because it changes both the price and the tolerance we can hold.
Our Vascomax C350 forging capability
These are the working limits of our plant at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin. If your part falls outside them, say so on the enquiry and we will tell you whether we can make it.
| Parameter | Range |
|---|---|
| Seamless rolled ring, outside diameter | 200 to 3,000 mm |
| Ring height or width | 50 to 1,000 mm |
| Ring wall thickness | 20 to 400 mm |
| Forged disc or blank diameter | 100 to 2,000 mm |
| Forged shaft length | up to 6,000 mm |
| Round bar diameter | 30 to 800 mm |
| Single-piece forged weight | 3 kg to 8,000 kg |
| Forging temperature | Start 1,150 to 1,260 °C, finish above 815 °C, air cool |
| Minimum forging reduction ratio | 4:1, higher on request for aerospace work |
| Melt routes supplied | VIM + VAR for AMS 6515 aerospace work, EAF + VOD + ESR for industrial grades |
| Minimum order | 1 piece |
| Typical lead time | 25 to 45 days depending on size and heat treatment |
| Quotation turnaround | Within 24 hours of receiving a drawing |
Forms we forge in Vascomax C350
We also forge Vascomax C350 tube sheets, valve bodies and stems, valve seat rings, gear blanks, spindles, nozzles, eccentric shafts, crankshafts, wheels, manifolds and rolls. Anything that can be made by open-die forging or ring rolling within the size limits above.
What Vascomax C350 is used for
Vascomax C350 is specified where strength-to-weight governs the design, and where the part is too complex or too tightly toleranced to survive a conventional quench. It is an expensive alloy, so it appears only where both of those conditions hold.
| Industry | Typical forged components | Why C350 |
|---|---|---|
| Aerospace | Landing gear cylinders and pistons, structural fittings, actuator bodies, hydraulic hollows | Highest strength-to-weight of any weldable steel, AMS 6515 approved |
| Rotorcraft and driveline | Helicopter drive shafts, high-torque couplings, splined shafts | Fatigue strength and toughness down to minus 50 °C |
| Tooling | Die-casting dies, extrusion dies and containers, cold-heading tooling | 0.05% distortion allows finish machining before hardening |
| Power transmission | Gear blanks, pinion shafts, gearbox rings, spindles | Nitridable surface over an ultra-strong core |
| Precision instruments | Load cells, torsion springs, flexures | High elastic limit and dimensional stability |
| Motorsport | Driveshafts, gearbox internals, turbocharger shafts | Weight reduction at fixed torque capacity |
| Oil, gas and process | Valve stems, seat rings, valve blocks, high-pressure fittings | Strength with resistance to hydrogen embrittlement |
Vascomax C250, C300 and C350 compared
All three are 18% nickel cobalt-strengthened maraging steels, heat treated the same way. They differ in cobalt and titanium, and therefore in the trade between strength and toughness. Strength rises across the family and fracture toughness falls.
| Grade | Co % | Mo % | Ti % | 0.2% proof | Tensile | Hardness | Spec |
|---|---|---|---|---|---|---|---|
| C200 | 8.5 | 3.25 | 0.20 | 1,400 MPa | 1,450 MPa | 44 HRC | — |
| C250 | 8.5 | 4.8 | 0.40 | 1,725 MPa | 1,800 MPa | 50 HRC | AMS 6512 |
| C300 | 9.0 | 4.8 | 0.60 | 2,000 MPa | 2,050 MPa | 54 HRC | AMS 6514 |
| C350 | 12.0 | 4.8 | 1.40 | 2,320 MPa | 2,413 MPa | 54–58 HRC | AMS 6515 |
Approximate typical values in the aged condition, for grade selection only.
Rule of thumb. If the design is strength-limited and the loading is static or well characterised, specify C350. If fracture toughness, damage tolerance or crack-growth life governs, as it does on most primary aircraft structure, C250 or C300 is usually the right choice despite the lower strength. We forge all three, so ask us and we will discuss the trade rather than simply quote what you asked for.
Testing, inspection and certification
Every Vascomax C350 forging leaving Jiangyin Jiangnan Metal Co., Ltd. is traceable to a heat number and ships with documentation.
- Ultrasonic testing to EN 10228-3, SEP 1921 or ASTM A388, to the acceptance class stated on your order.
- Chemical analysis by optical emission spectrometry on the finished forging, not only on the ingot.
- Mechanical testing, tensile, hardness and Charpy impact on test coupons heat treated with the production parts.
- Dimensional report against your drawing.
- Magnetic particle inspection to ASTM E709 where specified.
- EN 10204 3.1 certificate as standard, covering heat number, analysis, heat treatment record and test results.
- EN 10204 3.2 certificate witnessed by TUV, SGS, BV, Lloyd's Register or your own inspector, on request.
Machining and welding notes
Machining
Machine in the solution annealed condition wherever possible. At 30–35 HRC, machinability is comparable to 4340 at the same hardness. In the aged condition at 54–58 HRC machining is still possible, but it demands rigid setups, very sharp carbide or CBN tooling, low feed rates and flood coolant. Grinding is usually the better answer for finish features.
Welding
Vascomax C350 welds well for a steel at this strength level. The low carbon content prevents a hard heat-affected zone, so no preheat is required in either condition. GTAW with matching filler and electron-beam welding are both routine. Age the assembly after welding to restore full strength in the weld and the heat-affected zone. Keep the joint clean, since maraging steels pick up hydrogen and oxide readily.
Surface treatment
The alloy contains no chromium and is not stainless. For exposed service, nitride, cadmium plate, phosphate or paint. Nitriding produces a case around 65 HRC over an already strong core.
Frequently asked questions about Vascomax C350
Is Vascomax C350 a nickel alloy or a steel?
It is a steel. Vascomax C350 is iron-base, with iron making up roughly 63% of the alloy, plus about 18% nickel and 12% cobalt. Distributors file it under nickel alloys because of the nickel content, but metallurgically it is a low-carbon martensitic steel rather than a nickel-base superalloy like Inconel 718.
Why is Vascomax C350 so expensive?
There are three reasons. The alloy carries 12% cobalt and 4.8% molybdenum, both expensive. Aerospace grades are double vacuum melted by VIM and VAR, a slow process with low yield. The two-stage heat treatment adds furnace time. Against that, near-net-shape machining and weldability often bring the total part cost below the alternatives once scrap and rework are counted.
Can you forge Vascomax C350 rings larger than 3,000 mm?
Not on our ring mill. Above 3,000 mm outside diameter we would need to sub-contract, and we would tell you so rather than accept the order. Within 200 to 3,000 mm OD and up to 8,000 kg single-piece weight, we forge in-house at our Jiangyin plant.
What tolerance can you hold on an aged C350 forging?
As-forged, we work to standard open-die forging tolerances with machining allowance. If you need finished dimensions, we finish machine after aging and hold to your drawing. The 0.05% aging contraction is uniform and predictable, so it is compensated in the pre-age machining rather than corrected afterwards.
Do you supply annealed or aged material?
Both. Most customers take solution annealed forgings with machining allowance and age in-house after machining, which is the standard route. We also supply fully aged and finish-machined parts. Specify which you want at enquiry stage, because it changes the price, the lead time and the tolerance we can hold.
Does Vascomax C350 rust?
Yes, it will. There is no chromium in the alloy, so it is not stainless. It does resist stress-corrosion cracking and hydrogen embrittlement better than quenched-and-tempered steels at the same strength, but exposed parts need nitriding, cadmium plating, phosphating or paint. We oil and VCI-wrap all forgings for shipment.
Can it be nitrided?
Yes. Gas or ion nitriding produces a case of roughly 65 HRC. Because nitriding temperatures overlap the aging range, the cycle can often be combined with the aging treatment. Discuss it with us before you fix the process route.
What is the minimum order quantity?
One piece. We regularly forge single prototype rings and shafts in Vascomax C350 for customers qualifying a design, and we quote them on the same basis as production quantities.
How long does delivery take?
Typically 25 to 45 days from order, depending on size, whether we hold suitable stock material, and whether heat treatment and finish machining are in scope. Quotations go out within 24 hours of receiving a drawing.
Which countries do you ship to?
Worldwide, from Shanghai and Zhangjiagang ports. Our regular Vascomax C350 destinations include the United States, Germany, Italy, the United Kingdom, Turkey, India, Japan, Korea, Brazil and the UAE. We quote EXW, FOB, CIF or DDP as you prefer.
Request a quotation for Vascomax C350 forgings
Send a drawing and we will come back within 24 hours. If you do not have a drawing yet, the following six items are enough for a firm price.
- Form and dimensions. Ring OD by ID by height, disc diameter by thickness, shaft length by diameter, or a drawing.
- Quantity, and whether this is a prototype or repeat production.
- Specification. AMS 6515, MIL-S-46850D, or your own material standard.
- Delivery condition. Solution annealed with machining allowance, or aged and finish machined.
- Certification and NDT level. EN 10204 3.1 or 3.2, UT class to EN 10228-3, SEP 1921 or ASTM A388.
- Destination port and required delivery date.
About the manufacturer
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. We produce seamless rolled rings, forged discs, shafts, flanges, sleeves, bushings, tube sheets and bars in carbon steel, alloy steel, tool steel, stainless steel, maraging steel and nickel-base superalloys, from 3 kg to 8,000 kg per piece.
Vascomax C350 is one of roughly 200 grades we forge to order. Enquiries are answered by our engineers, and we will tell you when a grade or a size is the wrong choice for the application.
Jiangyin Jiangnan Metal Co., Ltd.No.1 Chengxiqiao Road, Zhouzhuang Town
Jiangyin City, Jiangsu Province 214423, China
Phone: +86 189 2135 9659
Email: sales@steelforgepieces.com
Web: www.steelforgepieces.com
Cite this datasheet
This page is maintained by the engineering team at Jiangyin Jiangnan Metal Co., Ltd. and was last reviewed on . If you are quoting these figures in a specification, report or technical document, please attribute them as follows.
Jiangyin Jiangnan Metal Co., Ltd. (2026). Vascomax C350 Forgings, AMS 6515 / UNS K93120 Maraging Steel Datasheet. Jiangyin, Jiangsu, China. https://www.steelforgepieces.com/Nickel-Alloy/Vascomax-C350.html
Composition and property values are compiled to AMS 6515 and MIL-S-46850D and cross-checked against published mill data for the grade. Forging size limits, lead times and certification options are our own plant data. Certified values for any individual forging appear on its EN 10204 3.1 certificate.