Your drawing says UNS S35000. That is not maraging steel.

Search for Maraging 350 and you will be told, on page after page, that it is UNS S35000. Distributor datasheets say it. Marketplace listings say it. Until recently, our own product page said it. It is repeated so consistently that most engineers reasonably assume it is settled.

It is not settled. It is a decades-old copy-and-paste error, and unlike most industry folklore it has a real failure mode attached: there is a live ASTM- and AMS-covered stainless steel that legitimately owns that UNS number, it is available as bar and forgings, and it is about half the strength of the alloy you probably designed for.

This article is written from the position of a shop that forges both alloys. We are not correcting the internet for sport — it is a recurring query on our technical desk, and the cases we do not catch in time become returned material.

1. Is UNS S35000 the same as Maraging 350?

If your drawing, your ERP material master or your supplier's quotation says “Maraging 350, UNS S35000”, nothing has necessarily gone wrong yet — the intent is usually obvious from context. The risk begins the moment that string is read by somebody who does not share your context: a trading company, an overseas mill, or an automated sourcing platform.

2. What is UNS S35000, actually?

AM 350 is a semi-austenitic precipitation-hardening stainless steel developed for aerospace sheet and bar applications. In the annealed condition it is largely austenitic; it is conditioned and sub-zero treated to transform to martensite, then tempered. Its identity is thoroughly documented under several parallel designations:

• AISI Type 633 and SUS 633 (JIS)
• ASTM A693 / ASME SA-693 — precipitation-hardening stainless plate, sheet and strip
• AMS 5548 — corrosion and heat-resistant sheet and strip, 16.5Cr–4.5Ni–2.9Mo–0.10N
• AMS 5745 — corrosion and heat-resistant bars and forgings, same chemistry
• MIL-S-8840, SAE J467 (AM-350), ASTM A579 Grade 61

Note what is absent from that chemistry: cobalt, and the 18% nickel that defines the maraging family. And note what is present: 16.5% chromium, which is why AM 350 is genuinely corrosion resistant while Maraging 350 rusts in a humid warehouse without protective coating.

These are not variants of one another. They are different alloy systems that happen to share the number 350 for unrelated historical reasons — one from a strength target in ksi, the other from a producer's internal alloy series.

3. Why the mix-up is expensive: the AMS 5745 trap

Most designation errors are harmless because no supplier can actually fulfil the wrong reading. This one is different, and the reason is AMS 5745.

AMS 5745 covers AM 350 in the form of bars and forgings. So when a purchase order reads “forged bar, UNS S35000, 200 mm diameter”, that is not a nonsensical request that will bounce back with a query. It is a completely ordinary, quotable, shippable line item — for a stainless steel forging. A stainless mill reading that PO has no reason to suspect you meant something else, will produce to AMS 5745, and will issue a mill certificate that is entirely correct against the material you specified.

What arrives is a corrosion-resistant forging with an ultimate tensile strength in the region of 1,250–1,400 MPa depending on condition. What your calculations assumed was 2,415 MPa. Everything about the paperwork will pass incoming inspection unless someone thinks to check the chemistry against the intended alloy rather than against the ordered designation.

4. AMS 6515 or AMS 6521? Product form decides

The second half of the problem is specification rather than designation, and it is the part that most supplier pages — including, until this revision, our own Maraging 350 product page — get wrong.

AMS specifications for maraging steel are organised primarily by product form. A single grade will have one specification for wrought long products and a different one for flat products, because the sampling frequency, test coupon orientation, thickness-dependent property tables and acceptance criteria genuinely differ between a 600 mm forged disc and a 3 mm strip.

For the 12% cobalt grade, the wrought long-product specification is AMS 6515: Steel, Maraging, Bars, Forgings, Tubing, and Rings, 18.5Ni – 12.0Co – 4.9Mo – 1.40Ti – 0.10Al, Double Vacuum Melted, Annealed. SAE created it to cover UNS K93540. If you are buying a forging, a ring or a heavy bar, that is the number that belongs on the order.

For full technical details on composition, heat treatment, melt routes, mechanical properties and available sizes, see our Maraging 350 forged parts guide.

AMS 6521 and AMS 6522 cover sheet, strip and plate. Placing either on a forging order creates a specification that cannot be satisfied as written: the supplier must either query it, quietly substitute the correct specification, or certify to a document whose test requirements do not map onto the product being delivered. All three outcomes cost time, and the third one can survive undetected into a qualification audit.

5. Maraging steel designation cross-reference table

This is the table we keep pinned above our technical desk. It covers the four commercial maraging grades plus the two stainless alloys that cause the collision.

MIL-S-46850 covers maraging steel across grades and product forms and is frequently invoked alongside the AMS numbers on defence work. Chemistry figures are nominal values taken from specification titles and are given for identification, not for acceptance — always work from the current issue of the specification itself.

6. Where do published sources disagree?

We would rather tell you where the ground is soft than pretend it is all solid.

The designation UNS K93160 is used inconsistently across the industry. Some reputable distributors list it against Maraging 300 flat products; others list it against Maraging 350; several list it in both places on the same website. AMS 6521 is described by some sources as a C300 sheet specification and by others as a 350-series sheet specification. We are not going to resolve that here, because resolving it requires reading the current issue of each document rather than reading what other websites say about them.

What matters is the practical rule that falls out of the disagreement:

7. How to check a drawing that says UNS S35000

You have inherited a legacy drawing, or received one from a customer, and the material callout is UNS S35000. Work through these in order before you release the purchase order.

1. Does the drawing quote a strength anywhere?

   If it calls for 350 ksi / 2,415 MPa UTS, or 340 ksi yield, or a hardness of 52–56 HRC, the intent is unambiguously the maraging grade. AM 350 cannot reach those numbers in any condition. Proceed to Maraging 350 and correct the callout.

2. Does the drawing call for corrosion resistance, passivation or a stainless finish?

   If it does — and it does not specify plating or a protective coating — the intent may genuinely be AM 350. Maraging 350 has no meaningful corrosion resistance and is always supplied with a surface treatment for exposed service. A drawing that says “no plating required, passivate per QQ-P-35” is describing a stainless steel.

3. Does the drawing invoke an AMS number alongside the UNS number?

   This is usually decisive. AMS 6515, 6521 or 6522 means maraging. AMS 5745 or 5548 means AM 350. Where the AMS number and the UNS number contradict each other, the AMS number is the stronger evidence of intent, because it is the document that actually controls the material.

4. Still ambiguous?

   Send the drawing to us or to your mill before quoting. A five-minute technical review is cheaper than a rejected forging, and considerably cheaper than a forging that is not rejected.

8. How to write a Maraging 350 material callout

Paste this into your drawing note or purchase order line and the ambiguity disappears. Replace the bracketed values with your own requirements.

The single most valuable line there is the negative one. Explicitly excluding the alloy you do not want converts a silent substitution into a contractual non-conformance, which changes who carries the cost if it happens.

9. Five specification mistakes we see every month

1. UNS number with no specification number. The most common of all. It leaves grade, product form, melt route and condition entirely to the supplier's discretion.
2. A sheet specification on a forging order. AMS 6521 or 6522 quoted for a 400 mm forged disc. The test coupon requirements do not translate, so somebody has to improvise — and improvisation is not traceable.
3. Trade name only. “VASCOMAX 350” is a registered trademark of Carpenter Technology Corporation. Using it alone on a PO placed with a different producer specifies nothing enforceable; pair it with AMS 6515 and UNS K93540.
4. Werkstoff number without the chemistry. European drawings often carry 1.6356 or 1.6355 alone. Both map to the 350-series maraging composition, but a non-European supplier may not have the DIN cross-reference to hand.
5. Copying the melt-method clause from the wrong specification. If you cite “Method A / B / C per AMS 6521 Section 6.1” on a forging order, you have cited a section of a sheet specification. Cite the melt route in plain words instead: “vacuum induction melted and vacuum arc remelted (VIM + VAR)”.

10. How each market designates this alloy

The designation that arrives on an enquiry depends heavily on the market it was written in. If you are sourcing across regions, these are the equivalences your suppliers will expect to see.

11. Questions we get asked

12. Sources and further reading