AMS 6512 vs 6514 vs 6515 vs 6520 The complete 18Ni maraging steel specification map

1. Why the specification number quietly misleads you

Line up the four wrought specifications in numerical order and something convenient happens. AMS 6511 is the 200 grade, 6512 is the 250, 6514 is the 300, 6515 is the 350. Ascending number, ascending strength. Engineers notice this within about ten minutes of working with the family, and it quietly becomes a mental shortcut.

The pattern is a coincidence, and it fails in three places.

It fails at AMS 6520 and 6521, which carry higher numbers than every wrought specification but sit back at the 250 and 300 chemistry. It fails at 6513, which is not a maraging bar specification at all, so the sequence is not continuous. And it fails hardest in the direction people actually use it — reading a number off a legacy drawing and inferring a grade instead of looking it up.

2. Two axes, not one list

The useful mental model is a grid, not a ranking.

The vertical axis is the strength grade, set by cobalt, molybdenum and above all titanium. Titanium drives Ni₃Ti precipitation during aging, so it is the element that moves you up the ladder: 0.20% at the 200 grade, 0.40% at 250, 0.65% at 300, and 1.40% at 350. Nickel barely changes across the whole family. Titanium changes by a factor of seven.

The horizontal axis is the product form, and it determines which specification number you are permitted to write. Bars, forgings, mechanical tubing, flash welded rings and forging stock live under 6511, 6512, 6514 and 6515. Sheet, strip and plate live under 6520 and 6521. No specification covers both. The delivery condition differs across that axis too: the wrought specifications are supplied annealed, the flat-product specifications are supplied solution heat treated.

Once the grid is in your head, most of the confusion in this family dissolves. A drawing that says AMS 6520 beside a forging callout is not a tolerance question — it is a specification that does not cover the part.

3. The six specifications

Nominal compositions and product-form scopes below follow the published SAE International specification titles and scope statements. These are nominal values, not the permitted ranges inside the purchased document.

4. Master comparison matrix

Read the two highlighted rows together. Identical grade, identical UNS number, identical chemistry — and two specification numbers that are not interchangeable, because the product form differs. That single pair explains most of the misrouted enquiries we receive on this family.

5. What each rung on the ladder costs you

Strength in this family is bought with titanium, and paid for in fracture toughness. The exchange rate is steep, and it is not linear.

Look at what toughness does between the 300 and the 350 grade. The strength gain is real, but the crack length a component tolerates before unstable fracture scales with the square of K_Ic. Halving toughness cuts the tolerable defect size to a quarter — and with it, the inspection interval. In damage-tolerant design, climbing the ladder often makes a part more expensive to own, not less.

6. Three different melt-route phrases in one family

This one costs people money, and it stays invisible unless you read the specification titles side by side.

• AMS 6512 and AMS 6514 are titled consumable electrode vacuum melted.
• AMS 6511 and AMS 6515 are titled double vacuum melted.
• AMS 6520 and AMS 6521 are titled consumable electrode melted — with no reference to vacuum in the title at all.

Three phrasings, one alloy family. If your quality plan contains a blanket statement such as "all maraging material shall be VIM + VAR", the specification titles alone will not enforce it consistently across all six. The fix takes one line: state the melt route explicitly on the purchase order rather than inferring it from the specification you invoked.

7. Flash welded ring ceilings are not the same across specifications

All four wrought specifications cover flash welded rings, but they do not cover them to the same size. AMS 6511 extends to flash welded rings through 6.000 inches (152.40 mm). AMS 6515 caps at 5.000 inches (127.00 mm) inclusive.

That is a small clause with a large consequence. A ring above the ceiling falls outside the flash-welded-ring scope of the specification, and the compliant route is a seamless rolled ring produced from forging stock under the same specification — not a flash welded ring with a concession attached. Discovering this at first article inspection rather than at design release is an expensive way to learn it.

8. None of these specifications delivers a hardened part

Every specification in this family is a soft-condition specification. The wrought specifications deliver annealed material; the flat-product specifications deliver solution heat treated material. In practice that means roughly 28–34 HRC arriving at your dock, machinable with coated carbide, and nowhere near the strength the grade is named after.

The 250 in "250 grade" is a capability — the tensile strength in ksi the material will reach after a correctly executed aging cycle, typically around 480 °C for three to six hours, air cooled. Until that cycle runs, the part is soft.

This is also why the machine-then-age sequence is standard practice with maraging steels. Dimensional change through aging is on the order of 0.05% linear, so parts can be finish machined soft and hardened afterwards without a grinding operation. It is one of the few genuine process advantages this family holds over quenched-and-tempered ultra-high-strength steels — and it only works if the delivery condition on the purchase order is correct.

9. Writing a callout that survives first article

A complete 18Ni maraging callout has five parts. Missing any one of them creates an ambiguity a supplier is entitled to resolve in the cheapest direction.

The five-step selection procedure

1. Fix the product form first. Wrought part, or flat product? That decision eliminates four of the six specifications immediately.
2. Choose the grade from a damage tolerance calculation, not from a strength table. The required critical crack length and inspection interval should drive the rung, not the peak tensile number.
3. State the melt route explicitly. The title language is not uniform across the family, so write the requirement rather than inferring it.
4. State the delivery condition and name who ages the part. This is the single most common dispute on maraging orders.
5. Pin the revision and add the UNS number. Specifications in this family are actively revised — AMS 6511 reached revision C in 2023, adding a stress corrosion cracking precaution, pyrometry requirements and clarified transverse testing. A supplier can misread 6514 for 6512; nobody misreads K93120 for K92890.

If you are ready to source, our AMS 6512 forging parts page covers the full range of available product forms — open die forgings, seamless rolled rings and machined blanks — with the exact callout we recommend for each form.

10. Glossary

18Ni maraging steel

A family of ultra-high-strength, low-carbon iron–nickel alloys that develop strength by precipitating intermetallic compounds during a low-temperature aging treatment, rather than by the martensite-plus-carbon mechanism used in quenched-and-tempered steels.

Aging

The low-temperature heat treatment, typically around 480 °C, that precipitates Ni₃Ti and Ni₃Mo and develops the grade's rated strength. Performed after machining, and followed by air cooling.

Annealed condition

The soft, as-delivered state of wrought maraging material, roughly 28–34 HRC. Sometimes called the solution annealed condition. Machinable, and not yet at rated strength.

Austenite reversion

The transformation of martensite back to austenite when aging temperature is too high, generally above roughly 510 °C. It causes a permanent strength loss that re-aging alone cannot recover.

CEVM

Consumable electrode vacuum melted. A remelting route in which a solid electrode is progressively melted under vacuum, refining cleanliness and solidification structure.

Double vacuum melted

Vacuum induction melting followed by vacuum arc remelting, commonly written VIM + VAR. The melt route called out in the titles of AMS 6511 and AMS 6515.

Flash welded ring

A ring formed by bending bar stock and joining the ends by flash welding. Covered by the wrought specifications only up to stated diameter limits, which differ between specifications.

Fracture toughness (K_Ic)

The resistance of a material to crack propagation, expressed in MPa·m^0.5. It governs the critical crack length a component can tolerate, and it falls sharply as maraging grade rises.

Stabilized specification

An SAE status indicating a document that is no longer actively developed but remains available for existing programmes. Generally unsuitable for new design work.

UNS number

Unified Numbering System designation. It identifies the alloy itself, independently of which specification, product form or country's standard invokes it.

11. Frequently asked questions