Quick answer
Which maraging grade should you specify?
Specify Maraging 300 (1.6358, AMS 6514) for fracture-critical, fatigue-driven, thick-section or welded forgings — it reaches 1,930–2,050 MPa with a fracture toughness of 80–110 MPa√m. Specify Maraging 350 (1.6356 / X2NiCoMoTi18-12-4, AMS 6515) only when mass or envelope is the binding constraint — it reaches 2,340–2,450 MPa but its fracture toughness falls to 30–50 MPa√m and delivered cost rises 25–40%.
- Strength gain from C300 to C350: about +20% tensile, +21% yield.
- Toughness loss: about −55% KIC, roughly halving Charpy energy.
- Tolerable crack at 80% of yield: 0.93 mm in C300 versus 0.13 mm in C350.
- Delivered cost: C350 typically 25–40% above C300 for the same geometry.
- Neither grade is stainless. Both contain zero chromium and require corrosion protection.
The number that decides most projects
Tolerable flaw size in Maraging 300 and Maraging 350, drawn to the same scale
Both bars show the largest surface crack each grade can carry at 80% of its own yield strength before fast fracture. Same scale, same assumptions, same page.
Maraging 300
1.6358 · AMS 6514
18Ni(300)
0.93mm critical flaw
Larger than the reference reflectors used in premium aerospace ultrasonic inspection. The flaw can be found before it finds the part.
Maraging 350
1.6356 / 1.6355 · AMS 6515
18Ni(350)
0.13mm critical flaw
Below the smallest routine UT reference reflector. Integrity has to be built in at the melt shop, not proven at the inspection bench.
Assumptions: ac = (1/π) × [KIC / (Y × σ)]², with Y = 1.12 and σ = 0.8 × yield. Maraging 300: KIC 90 MPa√m, yield 1,860 MPa. Maraging 350: KIC 40 MPa√m, yield 2,250 MPa. Illustrative only — use programme-specific KIC data and a validated stress intensity solution for design.
Almost every enquiry Jiangsu Liangyi receives for Maraging 350 forged parts in 1.6356 / X2NiCoMoTi18-12-4 arrives with the same unspoken question attached: should this actually have been Maraging 300? It is a fair question. Maraging 300 and Maraging 350 look nearly identical on a chemistry sheet, they are forged on the same presses, and they arrive in the same soft, machinable delivery condition. The two grades only diverge after aging — and by then the forging is bought.
This guide sets out the differences that change a purchase decision, from the perspective of the plant that has to forge, age and inspect the part.
Maraging 350 vs Maraging 300 at a glance
The table below summarises the six differences that most often decide the grade. Full property data follows in the sections beneath it.
| Criterion | Maraging 300 (1.6358) | Maraging 350 (1.6356) |
|---|---|---|
| Tensile strength, aged | 1,930–2,050 MPa (280–297 ksi) | 2,340–2,450 MPa (340–355 ksi) |
| Fracture toughness KIC | 80–110 MPa√m | 30–50 MPa√m |
| Cobalt content | 8.5–9.5% | 11.5–12.5% |
| Governing specification | AMS 6514, MIL-S-46850 | AMS 6515, MIL-S-46850 |
| Relative delivered cost | 100 (baseline) | 125–140 |
| Best suited to | Fracture-critical, fatigue, welded, heavy section | Mass-critical, proof-tested, small section |
Swipe the table sideways — the first column stays fixed.
Values compiled from published data in SAE AMS 6514, SAE AMS 6515, MIL-S-46850 and ASM International handbooks. Ranges are indicative; confirm against the applicable specification revision for design.
What actually separates the two grades
Maraging 300 and Maraging 350 both belong to the 18% nickel maraging steel family. Both are essentially carbon-free — under 0.03% C — so both harden by precipitating intermetallic compounds (Ni3Mo, Ni3Ti and Fe2Mo) inside a soft iron-nickel martensite matrix, not by carbon supersaturation. That shared mechanism is why the two grades behave so similarly during forging and so differently after aging.
The compositional gap between Maraging 300 and Maraging 350 is narrow and almost entirely in two elements:
Maraging 300
18Ni(300) · C300 · 1.6358 · AMS 6514
- Nickel
- 18.0–19.0%
- Cobalt
- 8.5–9.5%
- Molybdenum
- 4.6–5.2%
- Titanium
- 0.50–0.80%
- Aluminium
- 0.05–0.15%
- Carbon
- ≤ 0.03%
- Chromium
- None
Maraging 350
18Ni(350) · C350 · 1.6356 / 1.6355 · AMS 6515
- Nickel
- 18.0–19.0%
- Cobalt
- 11.5–12.5%
- Molybdenum
- 4.6–5.2%
- Titanium
- 1.30–1.60%
- Aluminium
- 0.05–0.15%
- Carbon
- ≤ 0.03%
- Chromium
- None
Two changes, two consequences. Cobalt does not form precipitates itself; cobalt lowers the solubility of molybdenum in the martensite, which forces more Ni3Mo out of solution during aging. Raising cobalt from 9% to 12% therefore multiplies the precipitate count without adding a single new phase. Titanium is the blunter instrument: roughly doubling titanium to 1.3–1.6% produces a much denser population of Ni3Ti particles, and it is titanium — not cobalt — that carries most of the blame for the toughness loss and the hot-working difficulty that come with Maraging 350.
A note on designations. The EN description X2NiCoMoTi18-12-4 rounds the nominal nickel, cobalt and molybdenum contents of Maraging 350. The "4" in that name should not be read as a molybdenum specification limit — always work from the actual AMS 6515 or purchase-order range, which the product datasheet lists in full.
Properties in the annealed and aged conditions
In the as-delivered solution annealed condition, Maraging 300 and Maraging 350 are close to indistinguishable — which is exactly the trap. A soft maraging forging cannot be identified by hardness testing alone, and mixed stock has caused more than one programme a very expensive recall.
| Property | Maraging 300 | Maraging 350 | Unit |
|---|---|---|---|
| Tensile strength, solution annealed | ~1,000 | ~1,050 | MPa |
| Hardness, solution annealed | 30–35 | 32–37 | HRC |
| Tensile strength, aged | 1,930–2,050 | 2,340–2,450 | MPa |
| Tensile strength, aged | 280–297 | 340–355 | ksi |
| Yield strength 0.2%, aged | 1,860–1,970 | 2,200–2,400 | MPa |
| Elongation, aged | 6–10 | 4–7 | % |
| Reduction of area, aged | 40–55 | 25–40 | % |
| Hardness, aged | 50–54 | 56–59 | HRC |
| Fracture toughness KIC | 80–110 | 30–50 | MPa√m |
| Charpy V-notch, aged | 24–34 | 11–20 | J |
| Density | 8.0 | 8.1 | g/cm³ |
| Elastic modulus, aged | ~190 | ~193 | GPa |
| Solution anneal | 815–830 °C / 1 h | 815–830 °C / 1 h | — |
| Aging cycle | 480 °C / 3–6 h | 480–500 °C / 6–12 h | — |
| Linear contraction on aging | 0.04–0.08 | 0.08–0.12 | % |
Swipe the table sideways — the first column stays fixed.
Indicative ranges compiled from published data in AMS 6514, AMS 6515, MIL-S-46850 and ASM International handbooks. Actual values depend on section size, forging reduction ratio and aging practice, and should be confirmed by test on the supplied material.
Read Table 2 from the bottom up rather than the top down. The strength rows flatter Maraging 350. The fracture toughness and reduction-of-area rows are where the engineering actually happens.
Why fracture toughness decides most projects
Datasheets sell strength because strength is a single number. Fracture toughness is the number that decides whether a forged part survives a scratch, an inclusion or a handling knock — and between Maraging 300 and Maraging 350 it moves in the opposite direction to strength.
Worked example
How big a crack can each grade live with?
Linear elastic fracture mechanics gives the critical flaw size directly:
ac = (1/π) × [ KIC / (Y × σ) ]²Take a semi-circular surface flaw (Y = 1.12) in a part working at 80% of its own yield strength — a normal design point for a proof-tested structure.
Maraging 300: KIC 90 MPa√m, σ = 1,488 MPa → ac ≈ 0.93 mm
Maraging 350: KIC 40 MPa√m, σ = 1,800 MPa → ac ≈ 0.13 mm
Working the stronger grade harder does not just erase the toughness advantage — it compounds the loss. The tolerable defect shrinks by a factor of about seven.
The practical consequence of Maraging 350's low toughness is an inspection problem. Premium aerospace ultrasonic inspection classes are written around reference reflectors down to a 1/64 in (0.40 mm) flat-bottom hole. A 0.13 mm critical flaw sits below that detection floor. A Maraging 350 forging cannot be inspected into compliance; it has to be melted and forged clean in the first place. That is why double vacuum melting (VIM followed by VAR), a minimum 4:1 forging reduction ratio and full-volume ultrasonic coverage are not optional extras on this grade, and why any supplier quoting Maraging 350 from a single-melt route should be treated with caution.
How the two grades behave in the forge shop
From a distance Maraging 300 and Maraging 350 are forged the same way. Up close, Maraging 350 removes most of the margin for error. The six differences below are the ones that show up on a production traveller.
Hot working window
Narrower, and less forgiving
Both grades are worked from about 1,150–1,250 °C. The higher titanium in Maraging 350 raises its sensitivity to hot shortness near the top of that range and to cracking if forging continues too cold, so the usable window is tighter at both ends and reheats are more frequent.
Easier: C300Segregation control
Homogenisation matters more
Titanium and molybdenum segregate in the ingot. At 1.5% titanium the resulting banding produces measurable scatter in aged properties across a heavy section, so Maraging 350 ingots need a longer high-temperature soak before breakdown and tighter top-and-bottom sampling.
Easier: C300Section size
Practical ceiling is lower
Aging is diffusion-driven and largely section-independent, but segregation and residual stress are not. Very heavy Maraging 350 forgings show more property scatter core-to-surface, so Maraging 300 is the safer choice for the largest cross-sections unless the design genuinely needs 2,300 MPa throughout.
Easier: C300Aging and distortion
Longer cycle, more movement
Maraging 300 ages in 3–6 h at 480 °C and contracts about 0.04–0.08%. Maraging 350 needs 6–12 h and contracts about 0.08–0.12% because of its higher precipitate volume fraction. On close-tolerance parts that difference has to be designed into the machining allowance.
Watch: C350Welding
Both weldable, one riskier
Maraging steels are welded soft, without preheat, then aged. The higher titanium in Maraging 350 raises hot-cracking risk and produces a wider soft reverted-austenite band in the heat affected zone. For welded assemblies, Maraging 300 is the default and Maraging 350 the exception.
Easier: C300Machining
Comparable soft, very different hard
Both grades machine readily at 30–35 HRC and both should be finished before aging. After aging, Maraging 350 at 56–59 HRC is effectively a grinding-only material, while Maraging 300 at 50–54 HRC still accepts light carbide finishing.
Easier: C300None of this makes Maraging 350 unworkable. It does mean the two grades are not interchangeable line items, and a quotation that prices them the same is not accounting for the process.
Where the cost difference comes from
Buyers usually assume the Maraging 350 premium is all cobalt. It is roughly half cobalt.
| Cost element | Maraging 300 | Maraging 350 | Driver |
|---|---|---|---|
| Raw material | 100 | 115–125 | +3% Co, +0.8% Ti |
| Melting (VIM+VAR) | 100 | 105–110 | Tighter cleanliness targets |
| Forging | 100 | 110–120 | Narrower window, more reheats, higher scrap |
| Heat treatment | 100 | 115–130 | Longer age, furnace occupancy |
| Inspection and NDT | 100 | 120–140 | Finer UT sensitivity, more sampling |
| Typical delivered total | 100 | 125–140 | — |
Swipe the table sideways — the first column stays fixed.
Indicative modelling based on published cobalt and nickel pricing and typical process cost structure for these grades. Indices move with the metals markets; cobalt is the largest single variable and has historically swung by more than a factor of two within a single year. Figures are for guidance only and are not a quotation.
There is a second cost that never appears on a quotation: rework exposure. A Maraging 350 forging rejected at final ultrasonic inspection has already absorbed a long aging cycle and, usually, most of its machining. The value destroyed by that rejection is a great deal higher than for a Maraging 300 part rejected at the same stage.
Grade selection by component type
The matrix below reflects common industry practice for these two grades. It is a starting point for discussion, not a substitute for your own qualification data.
| Component | Grade | Governing reason |
|---|---|---|
| Rocket and launch vehicle motor cases | C300 | Damage tolerance and proof-test logic; flaw size must be detectable |
| Landing gear fittings, axle beams | C300 | Fatigue and impact loading; competes with 300M and AISI 4340 |
| High-load drive shafts, torsion bars | C350 | Torsional strength per unit mass; low flaw exposure in service |
| Aerospace fasteners, studs, pins | C350 | Strength governs; small sections, fully inspectable |
| Precision gears and splines | C350 | Surface hardness after nitriding; dimensional stability |
| Seamless rolled rings, large structural | C300 | Section size, welding, cost per kilogram |
| Aluminium die-casting dies and inserts | C300 | Thermal fatigue resistance; toughness beats peak hardness |
| Cold extrusion punches and mandrels | C350 | Compressive yield strength governs tool life |
| Cryogenic structural parts | C300 | Retains useful toughness at low temperature; C350 does not |
| Welded assemblies of any kind | C300 | Lower hot-cracking risk, better HAZ recovery after aging |
Swipe the table sideways — the first column stays fixed.
General engineering guidance based on published material behaviour. Always confirm against your own qualification data and the applicable design standard.
How to specify the forging correctly
Grade choice is only half the specification. On maraging steel the process is the property, and a purchase order that names only the alloy leaves the important decisions to whoever is cheapest. A complete maraging steel forging enquiry should state all eight of the following:
- Grade and specification. Not just "maraging 350" — cite AMS 6515 or MIL-S-46850 for Maraging 350, or AMS 6514 for Maraging 300, plus the EN material number (1.6356, 1.6358) if you work to European standards.
- Melt route. VIM + VAR should be stated explicitly. For flight hardware, ask for the remelt ingot record.
- Delivery condition. Solution annealed is normal, so that you machine soft and age last. If the mill is to age the part, say so and agree who owns the dimensional movement.
- Minimum forging reduction ratio. 4:1 as a floor; 6:1 or higher for fatigue-critical parts. Ask for it to be recorded on the traveller.
- Test direction and location. Transverse properties are the meaningful ones on a heavy forging. Specify prolongation location and whether testing is per heat or per piece.
- NDT class. Ultrasonic acceptance level, magnetic particle or penetrant requirement, and — for Maraging 350 — the reference reflector size you expect.
- Certification. EN 10204 3.1 as standard; EN 10204 3.2 witnessed by an independent accredited third-party inspection agency where the end customer requires it.
- Export documentation. Aged maraging steels exceed the 1,950 MPa dual-use threshold, so end-use and end-user statements should be settled by both parties before production starts, not at the shipping dock.
The full property tables, international equivalents, forging size envelope and inspection scope for the 350 grade are set out on the Maraging 350 (1.6356 / 1.6355) forged parts and rolled rings page. If you are still weighing the two grades, send the drawing and the load case — that is usually a faster route to an answer than another datasheet.
Frequently asked questions
Is Maraging 350 always stronger than Maraging 300?
In the fully aged condition, yes — Maraging 350 reaches roughly 2,340–2,450 MPa against 1,930–2,050 MPa for Maraging 300. In the solution annealed delivery condition the two grades are nearly identical at about 1,000 MPa and 30–35 HRC. That is precisely why the delivery condition must be stated on the purchase order, and why soft stock of the two grades must never share a rack without positive material identification.
What is the difference between 1.6356 and 1.6358?
1.6356 (X2NiCoMoTi18-12-4, also listed as 1.6355 by some mills) is the European material number for 18Ni Maraging 350, covered in the United States by AMS 6515 and MIL-S-46850. 1.6358 corresponds to 18Ni Maraging 300, covered by AMS 6514 for bar and forgings. The compositional difference is approximately 3% cobalt and 0.8% titanium.
Which maraging grade should be used for a rocket motor case?
Maraging 300 is the more common choice for rocket motor cases. Motor case design is governed by fracture mechanics and proof-test logic rather than tensile strength alone, and Maraging 300's KIC of 80–110 MPa√m gives a tolerable flaw size that ultrasonic inspection can actually resolve. Maraging 350 is used where the mass budget forces the higher strength and the programme can support the tighter melt cleanliness, inspection and handling discipline that follow.
How much more does Maraging 350 cost than Maraging 300?
Raw material for Maraging 350 typically runs 15–25% higher. Once the narrower forging window, longer aging cycle, higher scrap exposure and tighter inspection are included, delivered cost for a finished forging is commonly 25–40% higher. Cobalt price volatility is the largest single variable and can move the number substantially within one quarter.
Do Maraging 300 and Maraging 350 shrink during aging?
Yes, both grades contract. Maraging 300 typically contracts about 0.04–0.08% linearly during a 480 °C age; Maraging 350 typically contracts about 0.08–0.12% because of its higher precipitate volume fraction. On close-tolerance parts, set the machining allowance from a measured value for the actual heat rather than a handbook average. A sacrificial coupon run through the same furnace charge is the usual way to establish that figure.
Can Maraging 300 and Maraging 350 be welded?
Yes. Maraging steels are welded in the solution annealed condition without preheat, because the martensite is low in carbon and tough, and the joint is aged afterwards. Maraging 350 is the more crack-sensitive of the two: its higher titanium increases hot-cracking risk and produces a wider soft reverted-austenite band in the heat affected zone. For welded assemblies, Maraging 300 is the safer default.
What is the aging temperature for Maraging 350?
Maraging 350 is aged at 480–500 °C for 6–12 hours, followed by air cooling. Maraging 300 is aged at 480 °C for 3–6 hours. Exceeding either the temperature or the time allows reverted austenite to form, which softens the material and reduces strength. Both grades are solution annealed at 815–830 °C for about one hour before aging.
Is Maraging 350 corrosion resistant?
No. Maraging 350 and Maraging 300 contain no chromium and are not stainless steels. Both rust in humid air and require protective coating, plating or oiling in service. Where corrosion resistance is required alongside very high strength, precipitation-hardening stainless grades such as 17-4PH should be considered instead, accepting the lower strength ceiling.
Which grade can be forged in heavier sections?
Section size limits the two grades differently. Maraging 300 tolerates heavier cross-sections because segregation and residual stress are easier to control at 9% cobalt and 0.6% titanium. Maraging 350 shows more property scatter core-to-surface as section thickness grows, so its practical ceiling is lower even when press capacity is not the constraint. Above roughly 200 mm controlling section, treat Maraging 300 as the default and justify Maraging 350 on mass grounds alone.
Are maraging steels export controlled?
Maraging steels capable of an ultimate tensile strength of 1,950 MPa or more are listed as dual-use items in several jurisdictions, including Nuclear Suppliers Group Part 2 controls and the corresponding EU and US control lists. Both Maraging 300 and Maraging 350 exceed that threshold in the aged condition. Exporter and buyer should each confirm end-use documentation and licensing requirements with their own compliance function before an order is placed.
Glossary
- Maraging
- A contraction of martensitic and aging. Maraging steels harden by precipitating intermetallic compounds in an iron-nickel martensite matrix rather than by carbon supersaturation, which is why they can be machined soft and hardened afterwards with almost no distortion.
- Critical flaw size (ac)
- The largest crack a component can carry at its design stress before fast fracture occurs, calculated from fracture toughness and applied stress using linear elastic fracture mechanics. For Maraging 300 at 80% of yield it is approximately 0.93 mm; for Maraging 350 approximately 0.13 mm.
- Reverted austenite
- Austenite that re-forms from martensite when maraging steel is aged too hot or too long. Reverted austenite is soft and reduces strength, and it is the main reason aging temperature must be held within a narrow band.
- VIM + VAR
- Vacuum induction melting followed by vacuum arc remelting. The double vacuum route required for maraging steel to control non-metallic inclusions, dissolved hydrogen and oxygen. Single-melt maraging steel should not be accepted for structural forgings.
- Forging reduction ratio
- The ratio of ingot cross-sectional area to final forging cross-sectional area. A minimum of 4:1 is required to break down the as-cast dendritic structure; 6:1 or higher is normal for fatigue-critical parts.
References
- SAE AMS 6514 — Steel, Maraging, Bars, Forgings, Tubing and Rings, 18Ni – 8.0Co – 5.0Mo (Grade 300).
- SAE AMS 6515 — Steel, Maraging, Bars, Forgings, Tubing and Rings, 18Ni – 12Co – 4.8Mo (Grade 350).
- MIL-S-46850 — Steel, Bar, Plate, Sheet, Strip, Forgings and Extrusions, 18 Percent Nickel Alloy, Maraging.
- ASM International, Metals Handbook Vol. 1: Properties and Selection — Irons, Steels and High-Performance Alloys, 10th edition.
- ASM International, Metals Handbook Vol. 4: Heat Treating, 10th edition.
- ASM International, Metals Handbook Vol. 19: Fatigue and Fracture.
- ASTM E399 — Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness KIC of Metallic Materials.
- ASTM A388 and SAE AMS 2154 — Ultrasonic examination of steel forgings and wrought products.
- EN 10204:2004 — Metallic Products: Types of Inspection Documents.
- Nuclear Suppliers Group Guidelines, Part 2 (Dual-Use List), maraging steel entry.
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Property values, cost indices and application guidance on this page are indicative and provided in good faith for general engineering information. They do not form part of any contract, do not constitute a quotation, and carry no warranty of accuracy or fitness for any particular purpose.