Maraging 250 vs 300 vs 350: how to choose the right grade for forged components

Section 01Why do maraging 250, 300 and 350 exist as separate grades?

They exist to offer three different titanium levels — 0.4%, 0.65% and 1.4% — within one otherwise identical alloy. Titanium controls how much Ni₃Ti precipitate forms during aging, and that precipitate density sets the strength.

The 18% nickel maraging steels are not hardened by carbon. Carbon is a contaminant here, held below 0.03%, and the base structure is a soft, ductile iron–nickel lath martensite that forms on air cooling regardless of section thickness. Strength arrives later, during a low-temperature aging treatment, when intermetallic compounds — principally Ni₃Ti, Ni₃Mo and Fe₂Mo — precipitate inside that matrix.

That mechanism is why the grade numbers exist at all. Grades 250, 300 and 350 are named for their approximate aged yield strength in ksi, and they reach those levels almost entirely by changing how much titanium is available to form precipitates.

Cobalt is the quieter half of the story. Cobalt does not form precipitates itself. It lowers the solubility of molybdenum in the martensite, which forces more molybdenum out of solution during aging and multiplies the number of Mo-bearing particles. That is why cobalt tracks upward alongside titanium — about 8%, 9% and 12% — even though it never appears in the strengthening phase.

Section 02How do the chemical compositions compare?

Nickel, molybdenum and aluminium are effectively constant across all three grades. Only titanium and cobalt change materially, so grade selection is in practice a titanium selection.

The table below gives typical published composition ranges for the three grades, consistent with the SAE AMS and MIL specifications commonly cited on drawings. Iron is the balance in all three.

Typical published values, consistent with SAE AMS 6512, AMS 6514, AMS 6515 and MIL-S-46850. This table is a summary for engineering orientation and is not a reproduction of, or a substitute for, the specifications themselves — order the current revision from SAE International before writing a purchase specification. Incoming material at Jiangsu Liangyi is verified by optical emission spectrometry against the chemistry stated on the melter's certificate.

Section 03What are the mechanical properties of maraging 250, 300 and 350?

In the aged condition, tensile strength runs 1,720–1,860 MPa for grade 250, 1,930–2,050 MPa for grade 300 and 2,300–2,450 MPa for grade 350. Plane-strain fracture toughness runs in the opposite direction: 88–110, 66–88 and 33–55 MPa·m^0.5 respectively.

The values below are representative of double vacuum melted (VIM + VAR) material in the solution-treated and peak-aged condition, tested on longitudinal specimens taken from forged bar.

Typical published engineering values. Fracture toughness is determined per ASTM E399 test practice. Transverse properties in heavy forged sections typically run 5–15% below longitudinal; state the required test direction and acceptance limits on your drawing. These figures are for orientation only and are not guaranteed values — contractual properties are those stated on the order and confirmed on the mill test certificate.

Why the critical flaw size matters more than the strength number

Going from grade 250 to grade 350 raises yield strength by about 33%. It reduces K_IC by roughly 55%. Because the tolerable flaw size scales with the square of the ratio of toughness to applied stress, the practical consequence is severe: at their respective design stresses, a grade 250 forging can tolerate an internal defect several times larger than a grade 350 forging before fast fracture becomes a risk.

In other words, moving up the family does not simply make the part stronger. It transfers the burden onto your ultrasonic inspection, your surface finish and your material cleanliness. A grade 350 component with a grinding burn, a sharp fillet or a 2 mm inclusion is not a strong part — it is a fracture waiting for a load cycle.

Section 04How does grade choice change the forging process?

All three grades forge between roughly 1,150 °C and 900 °C and air cool, but grade 350 needs a higher reduction ratio than grade 250 — commonly 6:1 against 4:1 — because its 1.4% titanium segregates more severely during ingot solidification.

Maraging steels are more forgiving to forge than most high-alloy steels because there is no carbide network to break up. But the differences between grades are real, and they compound with section size.

• Segregation risk. Titanium is the element most prone to interdendritic segregation during vacuum arc remelting. At 1.4% Ti, grade 350 ingots require slower, tightly controlled remelt rates and longer homogenisation at the melter. Grade 250 at 0.4% Ti is comparatively forgiving.
• Reduction ratio. A 4:1 minimum is normal practice for grade 250 and 6:1 for grade 350 at the same finished geometry, because the higher-titanium grade needs more mechanical work to disperse segregation bands and Ti(C,N) stringers. We record the achieved reduction ratio on the forging traveller for every order and can report it with the inspection documents on request.
• Hot ductility. Grade 350's narrower hot working window makes it less tolerant of long transfer times and slow presses. Complex upset geometries are noticeably easier in grades 250 and 300.
• Section size ceiling. Because no quench is involved, all three grades are metallurgically indifferent to thickness. The practical ceiling on grade 350 is set by input material quality, not by hardenability.

The consequence for procurement is straightforward: for a forging above roughly 300 mm controlling section, grade 250 will deliver more consistent transverse properties, lower scrap and a shorter lead time than grade 350 at any price. Size range, tolerances and inspection scope for that grade are set out on our custom maraging 250 forgings page, and press and ring mill capacity is listed under equipment and facilities.

Section 05Do the three grades need different heat treatments?

No — the route is identical. Solution anneal at 815–830 °C, air cool, machine, then age at 480 °C and air cool. Only the aging hold differs: 3–6 hours for grades 250 and 300, and 6–12 hours for grade 350.

There is no quench medium, no protective atmosphere requirement during aging, and no tempering ladder to negotiate. This is one of the reasons the family is so attractive to manufacturers of large or complex parts.

Aging times are for peak strength. Under-aging at 455–470 °C trades 5–8% strength for measurably better toughness, and is a legitimate option on grade 300 and 350 parts that are marginal on damage tolerance. Where the drawing calls up a specific specification, we follow the cycle stated in that specification rather than the typical values above.

Two cautions apply across the family. Aging above about 510 °C for extended periods produces reverted austenite at lath boundaries, which softens the part and cannot be corrected by further aging — only by a full re-solution treatment. And on grade 350 the longer aging hold makes furnace uniformity more important, not less: a ±15 °C spread across a large load will show up as a 3 HRC spread across your parts.

Section 06How do machining, welding and dimensional change differ?

Machining

All maraging steels are machined in the annealed condition at 30–36 HRC and aged afterwards. Grade 350 removes material roughly 15–20% more slowly than grade 250 at equal tool life.

The 2–4 HRC difference between grades in the annealed state sounds trivial but is not: grade 350 at 36 HRC cuts noticeably slower than grade 250 at 31 HRC, and the higher titanium content increases built-up edge on carbide tooling.

After aging, none of the three grades is realistically machinable by conventional turning or milling. Finish operations are grinding, EDM or honing. This is a manufacturing sequencing constraint rather than a grade selection criterion — but it hurts more on grade 350, where any post-age correction means grinding at 58 HRC with a real risk of grinding burn.

Welding

All three grades weld without preheat because the matrix is virtually carbon free, and the weldment is aged at 480 °C afterwards. Weld metal toughness, however, falls sharply with titanium content.

• Grade 250 — weld metal toughness close to base metal. Routinely used for welded shells, fabricated frames and repair work.
• Grade 300 — well established in aerospace fabrication, with a modest toughness penalty in the fusion zone.
• Grade 350 — weld deposits are markedly less tough and more sensitive to restraint cracking. Design grade 350 parts as one-piece forgings and fasten them mechanically.

Dimensional change

Aging causes a uniform, predictable contraction. At 0.04–0.06%, a 1,000 mm grade 250 part shortens by 0.4–0.6 mm. Grade 350 can move nearly twice as far. For close-tolerance tooling this is compensated in the pre-age machining dimensions, which is why we ask for the intended aged condition to be stated at enquiry stage rather than after machining has started.

Section 07How much more do the higher grades cost?

On alloy cost per kilogram, grade 300 typically runs 8–15% above grade 250 and grade 350 runs 35–50% above. Cobalt is the dominant driver, rising from 8% to 12% across the family.

The melt route — vacuum induction melting followed by vacuum arc remelting — is the same for all three, but grade 350 carries a higher remelt reject rate at the melter and consumes more downstream inspection and machining hours.

Indicative ratios for engineering planning only. They are not a price list and do not constitute an offer. Absolute prices move with London Metal Exchange cobalt and nickel benchmarks, order quantity, geometry and inspection level, and are quoted per enquiry.

The honest conclusion: the delivered cost gap between grade 250 and grade 350 is wider than the alloy price difference suggests, because the higher-titanium grade also consumes more inspection, more machining hours and more scrap. Specify upward only when the design genuinely requires it.

Section 08Which maraging grade suits which component?

Grade 250 suits large, welded and damage-tolerance-critical parts. Grade 300 suits small, fully inspectable parts limited by strength-to-mass. Grade 350 suits compressive tooling.

General engineering orientation only. Final grade selection must be made by the design authority against the applicable qualification specification, and is not a recommendation for any specific safety-critical application.

Section 09How do the three grades map across EN, UNS and AMS?

The three grades run in parallel through every major standard system, separated only by their cobalt and titanium levels. Reading across a single row of the table below gives you the matching designation for all three at once.

Use this to check that a drawing calling up one system is asking for the grade you think it is. If you have settled on a single grade and need its full specification, tolerances and testing scope rather than a cross-reference, the individual material page is the better starting point.

Cross-references are approximate and are given for identification only. Composition limits under DIN, EN, AMS and MIL are close but not always identical, particularly on titanium and residual elements. Always verify against the current revision of the specification cited on your drawing before treating two designations as interchangeable. Proprietary producer brand names for these grades are deliberately not listed here; if your drawing calls up a brand name, send it to us and we will confirm the generic equivalent.

Section 10Five questions that settle the choice

Work through these five questions in order. The first one that returns a hard constraint decides the grade.

1. Is the part damage tolerance qualified?

   If a fracture mechanics assessment forms part of the qualification, grade 250 almost always wins on the design allowable, even though it is nominally the weakest of the three.

2. Will it be welded?

   Any welded joint carrying primary load points to grade 250. Grade 300 is acceptable with qualified procedures. Grade 350 should be treated as effectively non-weldable for structural joints.

3. What is the controlling section thickness?

   Above roughly 300 mm, choose grade 250 or 300. Grade 350 ingot segregation and Ti(C,N) clustering become difficult to control at that scale even with a 6:1 reduction ratio.

4. Is the loading tensile or compressive?

   Predominantly compressive service — punches, dies, bearing surfaces — is where grade 350 is genuinely the right answer, because its low toughness is not exercised by the load path.

5. Is mass or envelope the real constraint?

   If nothing above binds and you are chasing strength per kilogram in a small, fully inspectable part, step up to grade 300. Reserve grade 350 for the cases where grade 300 is still not enough.

Section 11Glossary

Maraging steel

A family of ultra-high-strength iron–nickel alloys containing about 18% nickel and almost no carbon, strengthened by precipitating intermetallic compounds during a low-temperature aging treatment rather than by carbon martensite. The name combines martensitic and aging.

Aging

The strengthening heat treatment, typically 480 °C for 3 to 12 hours followed by air cooling, during which Ni₃Ti, Ni₃Mo and Fe₂Mo particles precipitate inside the martensite matrix.

Plane-strain fracture toughness (K_IC)

The critical stress intensity factor at which a crack propagates unstably under plane-strain conditions, determined per ASTM E399 and expressed in MPa·m^0.5. For maraging steel it falls from about 100 in grade 250 to about 44 in grade 350.

Critical flaw size

The largest internal or surface defect a component can contain before fast fracture occurs at the design stress. It scales with the square of the ratio of fracture toughness to applied stress, which is why lower-toughness grades demand more sensitive inspection.

VIM + VAR

Vacuum induction melting followed by vacuum arc remelting. The standard double-melt route for aerospace-quality maraging steel, used to control dissolved gases, non-metallic inclusions and ingot structure. Performed by specialty melters, not by forging shops.

Reverted austenite

Austenite that re-forms at martensite lath boundaries when maraging steel is aged above roughly 510 °C or held too long. It softens the part and cannot be corrected by further aging, only by a full re-solution treatment.

Forging reduction ratio

The ratio of starting billet or ingot cross-sectional area to final forging cross-sectional area, used as a measure of how thoroughly the as-cast structure has been mechanically worked.

Controlling section

The greatest thickness through which heat must travel during heat treatment, used to set soak times. For maraging steels the usual basis is one hour of solution anneal soak per 25 mm of controlling section.

Section 12Frequently asked questions