What Is AMS 5628? A Complete Guide to 431 Stainless Steel Forgings
One specification, one alloy, and a set of process controls that decide whether a forged part survives a marine, subsea, or rotating-service life. Here is what AMS 5628 actually requires — and why buyers of critical forgings care.
AMS 5628 is an SAE aerospace material specification for 431 martensitic stainless steel — a heat-treatable 16% chromium, 2.5% nickel alloy (UNS S43100) supplied as bars, wire, forgings and tubing. It is the most corrosion-resistant of the common hardenable stainless grades, and is used for high-strength, corrosion-resistant forged parts such as aerospace fasteners, shafts, subsea valve bodies and marine fittings.
Key takeaways
- AMS 5628 governs the alloy 431 — the grade is the metal, the specification is the rule set it must satisfy.
- Nominal chemistry is 16% Cr, 2.5% Ni, ≈0.14% C, balance iron; it is the most corrosion-resistant hardenable stainless.
- It is heat-treatable: austenitize ≈1010 °C → oil quench → double temper, commonly to 262–302 HB.
- For critical parts, melt practice matters more than average chemistry — double or triple melt (VIM+ESR+VAR) controls inclusions.
- Typical uses of the material: aerospace fasteners and shafts, subsea wellheads, marine fittings, high-integrity boundary parts.
01 — Definition
The specification, in plain terms
AMS 5628 is an SAE aerospace material specification. It does not invent a new metal — it sets the rules a specific stainless steel must obey to be sold against that callout. The steel it governs is the martensitic grade 431 (UNS S43100), and the specification covers it as bars, wire, mechanical tubing, and — the form this guide is about — AMS 5628 forging parts.
Reading an AMS document is really reading a contract. It fixes the chemistry window, the melting and processing route the mill may use, the required mechanical properties, and the inspection every lot must pass before it ships. When a drawing calls out AMS 5628, the buyer is purchasing that whole envelope of controls — not just a bar of 431 that happens to be sitting in a warehouse.
A grade tells you what the metal is. A specification tells you what it had to survive to earn its stamp.
That distinction matters most where a hidden defect is expensive or dangerous. A general fabricator buying 431 for a bracket rarely needs the paper trail. A programme building rotating aerospace hardware, a subsea valve body, or a high-integrity boundary part needs every clause — because the specification is what lets them defend the part to a regulator or an auditor years later.
02 — The Alloy
431: the most corrosion-resistant martensitic stainless
Stainless steels fall into families defined by their microstructure. Austenitic grades (the familiar 304 and 316) are tough and highly corrosion resistant but cannot be hardened by heat treatment. Ferritic grades are magnetic and modestly corrosion resistant. Martensitic grades — where 431 sits — trade some corrosion resistance for something the others cannot offer: they harden dramatically when quenched and tempered, reaching strength closer to alloy steel while keeping useful stainless behaviour.
Within that martensitic family, 431 is prized for a specific reason. Its higher chromium content, paired with a deliberate nickel addition, makes it the most corrosion-resistant of the common hardenable stainless grades. The nickel also buys toughness, so a 431 part can run at genuinely high hardness without turning brittle — a combination most martensitic grades force you to choose between.
That is why designers reach for it when a part must be strong, hard-wearing, and exposed to a salt-laden or humid environment at the same time: shafts, fasteners, fittings, and valve components that would corrode in ordinary alloy steel and deflect or gall in a soft austenitic grade.
03 — Chemistry
What is actually in AMS 5628
The composition window is narrow on the elements that decide performance and generous only on the residuals. Chromium builds the passive film that resists corrosion; nickel stabilises the structure and adds toughness; carbon is what lets the steel harden at all. Sulphur and phosphorus are held low because they form the inclusions that seed fatigue cracks and hurt toughness.
| Element | Min | Max | Role |
|---|---|---|---|
| Carbon (C) | 0.12 | 0.20 | Hardenability |
| Chromium (Cr) | 15.00 | 17.00 | Corrosion film |
| Nickel (Ni) | 1.25 | 2.50 | Toughness |
| Manganese (Mn) | — | 1.00 | Deoxidiser |
| Silicon (Si) | — | 1.00 | Deoxidiser |
| Phosphorus (P) | — | 0.040 | Residual, low |
| Sulphur (S) | — | 0.030 | Residual, low |
| Iron (Fe) | — | Bal. | Base metal |
Values are typical of the 431 window; always work to the current released revision of the specification for your programme.
A well-run mill aims for the centre of each window rather than skating along a limit. Aiming a heat at roughly 16% chromium and the full 2.5% nickel, for example, leaves process margin — small melt-to-melt variation still lands comfortably inside the box instead of failing a certificate on a rounding error.
04 — Properties
Mechanical properties you can design to
Because 431 is heat-treatable, its properties are not a single number — they are a function of the temper chosen. Higher tempering temperatures trade strength for toughness; lower tempers do the reverse. The table below shows a representative hardened-and-tempered condition, which is where most forged parts are supplied.
| Property | Typical value | Note |
|---|---|---|
| Tensile strength | ~860–1050 MPa | 125–150 ksi, temper-dependent |
| 0.2% yield strength | ~650–860 MPa | Rises with lower temper |
| Elongation | 15–20% | In 4D |
| Hardness (H&T) | 262–302 HB | Common delivery band |
| Hardness (annealed max) | ≤302 HB | For machining |
| Impact (Charpy) | retained cold | Nickel gives low-temp toughness |
Illustrative ranges for orientation — actual acceptance values follow the drawing and the released specification revision.
The property worth dwelling on is toughness at high hardness. A valve stem or a fastener needs to resist wear and hold thread strength, which pushes hardness up. In most hardenable steels that same hardness makes the part fracture-prone. The nickel in 431 keeps meaningful impact energy even toward sub-zero service temperatures — which is exactly why it appears in fracture-mechanics-driven designs where a crack must not run.
05 — Why Forge
Why a forging beats bar stock and casting
You can buy 431 as bar and simply machine a part from it. For critical service, forging is preferred, and the reason is grain flow. Forging works the metal so its internal fibre follows the shape of the part, instead of being cut straight through by a lathe. That aligned structure carries load and resists fatigue far better along its length — the difference between a part whose strength follows its geometry and one whose strength is sliced across it.
- Reduction ratio. How much the ingot cross-section is reduced governs how thoroughly the cast structure is broken down. A ratio of 4:1 is a common floor; pushing toward 6:1 refines grain further and elongates inclusions so they are easier to detect on ultrasonic inspection.
- Soundness. Forging closes the porosity a casting can carry, which matters for pressure-boundary and rotating parts where an internal void is a crack waiting to start.
- Inspectability. A properly worked forging presents a clean, directional structure that ultrasonic testing can read reliably — an advantage that only exists if the reduction was real.
This is where forged AMS 5628 components earn their premium over off-the-shelf machined bar: the geometry, the grain flow, and the inspection record are engineered together rather than left to chance.
06 — Cleanliness
Melt practice decides how clean the steel really is
Two lots of 431 can meet the same chemistry and still behave very differently in fatigue, because what limits a critical part is usually not the average composition — it is the worst inclusion in it. Melt practice is how a mill controls that. There are three broad routes, and the specification and application together decide which is acceptable.
EAF + AOD
Lowest cost, highest inclusion content. Fine for non-structural work, but not the route for a fracture-critical AMS 5628 part.
VIM + VAR / ESR
A remelt step strips inclusions and gas, sharply improving cleanliness and consistency. The practical choice for many demanding forgings.
VIM + ESR + VAR
Two remelts. ESR chemically removes sulphur into the slag; VAR then delivers the tightest inclusion control for the most exacting parts.
The mechanism behind ESR is worth understanding, because it is what makes very low sulphur possible in large heats. As a consumable electrode is remelted through a molten slag layer, the slag reacts with sulphur in the steel and traps it, so it does not return to the solidifying ingot. That reaction is why electroslag remelting can reach sulphur levels a vacuum route alone cannot, and why triple-melt stock underpins the cleanest forgings.
The average chemistry passes the certificate. The largest inclusion decides the fatigue life.
07 — Heat Treatment
How a 431 forging is hardened
Heat treatment is where a soft, machinable forging becomes the load-bearing part on the drawing. For 431 the cycle follows a clear sequence, and each stage exists for a reason.
Austenitize
The forging is brought to roughly 1010 °C and soaked so the carbon and alloying elements dissolve into a uniform high-temperature structure ready to transform.
Oil quench
Rapid cooling — typically in oil — locks the structure into hard, strong but brittle martensite. At this point the part is at maximum hardness and minimum toughness.
Temper to target
Reheating below the transformation range relieves stress and converts the raw martensite toward the required strength-toughness balance. Temper temperature is the main dial the metallurgist turns.
Double temper
A second tempering pass — for instance near 620 °C — transforms retained austenite and stabilises properties, landing a repeatable hardness such as the 262–278 HB band seen on delivered parts.
A note that trips up specifiers: 431 should generally be avoided in the intermediate temper range that induces embrittlement. A competent supplier tempers either low, for strength, or high, for toughness — and does not park the part in the brittle window in between.
08 — Verification
How the part is proven before it ships
A specification is only as good as the inspection that backs it. For AMS 5628 forgings destined for critical service, a typical acceptance package layers several independent checks so no single test carries the whole risk. The list below describes standard industry practice for the material.
- Ultrasonic testing (UT). Often specified across the full volume, to find internal discontinuities the eye and the surface never reveal. This is where the earlier forging reduction pays off, because a well-worked structure reads cleanly.
- Inclusion rating (ASTM E45). A microscopic assessment of the type and severity of non-metallic inclusions — the same populations melt practice was chosen to control. Clean stock returns low severity ratings such as B1 / C1.
- Impact testing. Charpy specimens confirm toughness, and for cold-service parts they are pulled at low temperature to prove the part will not fracture where it lives.
- Hardness and tensile. Confirms the heat treatment actually delivered the specified strength band across the lot.
- First Article Inspection (FAI). For aerospace programmes, a full dimensional and requirements verification, documented on AS9102 forms, before series production is released.
- Certification & traceability. Mill certificates tie every result back to the heat, so a part can be defended to an auditor years later.
Where a programme calls for tests beyond a supplier's in-house scope, they are commonly performed by accredited third-party laboratories and reported on the certificate — a normal and accepted part of the supply chain.
09 — Applications
Where AMS 5628 forgings are used
Across the industry, the pattern for this material is the same: a part that must stay strong and hard in an environment that would corrode ordinary steel. Typical applications of AMS 5628 / 431 material include:
High-strength bolts, pins and fittings that resist marine atmosphere without losing thread strength.
Shafts and rotating hardware where fatigue life and inclusion cleanliness are the governing drivers.
Corrosion resistance and toughness under pressure for oil-and-gas hardware on the seabed.
Load-bearing parts exposed to salt spray where a passive film and real strength both matter.
High-integrity boundary parts demanding tight cleanliness and full traceability.
Wear- and corrosion-resistant stems and trim that hold tolerance in aggressive media.
10 — Buyer's Checklist
How to specify AMS 5628 forgings well
If you are sourcing these parts, the drawing callout is the start, not the finish. The points below separate a supplier who can hold the specification from one who merely quotes to it.
- State the melt practice. For fracture-critical work, name double- or triple-melt explicitly rather than assuming; it is the single biggest lever on cleanliness and cost.
- Fix the reduction ratio. Ask for the forging reduction and require it on the certificate, so grain flow and UT integrity are contractual, not hoped-for.
- Define the temper and hardness band. Specify the delivered hardness range and, if service is cold, the temperature at which impact values must be met.
- Name the tests and standards. UT coverage, ASTM E45 acceptance, tensile and impact — spelled out, with acceptance criteria attached.
- Require full traceability. Heat-number-linked mill certificates and, for aerospace, the FAI package on the first article.
- Build in lead time. Dedicated triple-melt ingot production adds weeks; plan the schedule around melt availability, not just machining.
Get those six right and the specification does its job — the part that arrives is the part you designed, with the paperwork to prove it.
Need AMS 5628 forging parts to a real specification?
Jiangsu Liangyi Co., Limited is a China-based forging manufacturer operating under an ISO 9001 quality management system. We supply 431 / AMS 5628 forgings and can arrange heat treatment, ultrasonic testing and material certification to your specification — including third-party inspection where a programme requires it.
View AMS 5628 Forging Parts → Full chemistry, process options and quote requests are available on the product page linked above.Reference
Glossary of key terms
- AMS 5628
- An SAE aerospace material specification covering 431 corrosion-resistant steel as bars, wire, forgings and tubing.
- 431 / UNS S43100
- A martensitic stainless steel, nominally 16% chromium and 2.5% nickel, hardenable by heat treatment.
- Martensitic stainless steel
- A stainless family that can be hardened by quenching and tempering, unlike austenitic grades.
- ESR (Electroslag Remelting)
- A remelting process that passes the steel through a molten slag to remove sulphur and inclusions.
- VAR (Vacuum Arc Remelting)
- A remelting process under vacuum that further reduces gas and inclusion content for a cleaner ingot.
- Reduction ratio
- The ratio by which an ingot's cross-section is reduced during forging; higher ratios refine grain and improve inspectability.
- ASTM E45
- The standard test method for rating the type and severity of non-metallic inclusions in steel.
- FAI (First Article Inspection)
- A documented verification (AS9102 in aerospace) that a first production part meets every drawing requirement.
FAQ
Frequently asked questions
Is AMS 5628 the same as 431 stainless steel?
Not quite. 431 is the alloy; AMS 5628 is the SAE specification that governs 431 in bar, wire, forging and tubing form. The specification adds the chemistry limits, processing controls, properties and inspection a 431 product must meet to be supplied against that callout.
What is the chemical composition of AMS 5628?
A martensitic stainless of roughly 16% chromium and 2.5% nickel, with about 0.14% carbon and the balance iron, plus controlled manganese, silicon and low sulphur and phosphorus.
Can 431 / AMS 5628 be heat treated to high strength?
Yes. It is austenitized near 1010 °C, oil quenched to form martensite, then tempered — often twice — to reach the required strength and toughness. Higher tempers favour toughness, lower tempers favour strength.
What are the typical mechanical properties of AMS 5628?
In the quenched and tempered condition, tensile strength is typically around 860–1050 MPa, yield around 650–860 MPa, elongation 15–20%, and a common delivered hardness band of 262–302 HB, all varying with the temper chosen.
Why choose a forging over machined bar for these parts?
Forging aligns the grain flow with the part geometry and closes internal porosity, giving better fatigue life and inspectability than bar stock cut straight through — which is why critical parts are forged.
What melt practice should I request?
For non-critical work, standard air melt may suffice. For fatigue- or fracture-critical parts, specify double-melt or triple-melt (VIM + ESR + VAR) to control inclusions and sulphur. State it explicitly on the order.
Sources & standards
References
- SAE International — AMS 5628: Steel, Corrosion Resistant, Bars, Wire, Forgings, and Tubing (16.2Cr–2.5Ni). Published by SAE International.
- ASTM International — ASTM E45, Standard Test Methods for Determining the Inclusion Content of Steel. Published by ASTM International.
- SAE International — AS9102, Aerospace First Article Inspection Requirement.
This article is for general engineering guidance. Always design and procure to the current released revision of the applicable specification.