The 60-Second Background
AMS 5660 and AMS 5661 are SAE Aerospace Material Specifications that both cover Incoloy 901 (UNS N09901) — a nickel-iron-chromium age-hardenable superalloy developed in the 1960s. By substituting iron for roughly half the nickel content (approximately 37–42% Fe balance), Incoloy 901 achieves dramatically better hot forgeability compared to Inconel® 718 (a registered trademark of Special Metals Corporation), enabling turbine discs over 1,500 mm diameter with uniform mechanical properties, while maintaining continuous operating capability up to 600 °C (1,110 °F).
If both specifications cover the same alloy, why do two AMS numbers exist? Because in aerospace and power engineering, the application is as critical as the chemistry. A component rotating at 3,000 rpm under centrifugal stress and a stationary casing under internal pressure demand entirely different quality standards, even when made from the same metal. AMS 5660 and AMS 5661 encode that difference through inspection class, grain size control, and grain flow requirements.
The chemical composition, heat treatment procedure, and room-temperature mechanical property minimums of AMS 5660 and AMS 5661 are identical. The specifications differ only in how rigorously the forged part must be inspected and what internal microstructure it must achieve.
For complete product data — full composition tables, properties at temperature, heat treatment procedures — see our AMS 5660 & AMS 5661 Forging Parts product page for full composition tables, mechanical properties, and heat treatment data.
What AMS 5660 and AMS 5661 Both Share
- Alloy chemistry: Identical composition limits for all elements (Ni, Fe, Cr, Ti, Al, Mo, Co, Cu, C, S, P, B). A heat qualifying to one spec automatically meets compositional requirements of the other.
- Melting route: Both specifications require vacuum induction melting (VIM) with consumable electrode remelting (VAR or ESR) for premium inclusion control. Air-melt material is not permitted under either specification.
- Room-temperature mechanical properties: UTS, 0.2% proof strength, elongation, and reduction of area minimums are identical after standard two-stage aging.
- Heat treatment procedure: Identical two-stage aging cycle — solution anneal, intermediate age, final age — with the same temperature windows and hold times.
- Post-age hardness target: Both specify 302–388 HBW as shop-floor process verification.
- Elevated-temperature properties: Stress rupture life at 538 °C, creep rate, and elevated tensile property minimums are the same in both specifications.
- Supporting standards referenced: AMS 2269, AMS 2750, ASTM A388, EN 10204 3.1/3.2 — identical for both specs.
The Four Real Differences Between AMS 5660 and AMS 5661
Difference 1 — Ultrasonic Inspection Class (Most Critical)
AMS 5660 requires AMS 2630 Class A ultrasonic testing, using flat-bottom hole (FBH) reference #5 (5/64-inch diameter) as the maximum acceptable indication. AMS 5661 requires AMS 2630 Class AA — the premium tier — using FBH #3 (3/64-inch diameter). This represents a 64% reduction in maximum permissible reflector cross-sectional area. Class AA also requires tighter scan increments, immersion technique, and higher evaluator qualification than Class A.
This difference is consequential because fatigue crack growth in Ni-Fe superalloys at temperature is a power function of initial defect size. A void passing Class A but failing Class AA can reduce estimated disc fatigue life by 30–50% under representative LCF loading conditions.
Difference 2 — Grain Size Requirement
AMS 5660 requires grain size of ASTM No. 5 or finer (average diameter ≤ 63 µm). AMS 5661 requires ASTM No. 6 or finer (≤ 45 µm) in critical rotating sections. Finer grain reduces slip-band size to improve fatigue crack initiation resistance, and also improves ultrasonic wave propagation — a prerequisite for reliable Class AA inspection results.
Difference 3 — Product Form and Primary Application
AMS 5660 covers wrought bars, billets, and general semi-finished open-die forgings for static and semi-static structural parts such as valve bodies, flanges, casings, and bearing housings, where creep and overload govern failure.
AMS 5661 covers forged rings, turbine disc blanks, blade retention fasteners, and other high-speed rotating components. The seamless rolled ring is central to its purpose: turbine disc blanks, compressor drum rings, and rotor retention rings operating under centrifugal and vibration loads.
Difference 4 — Grain Flow Orientation
AMS 5661 mandates that grain flow shall align with the principal stress direction throughout the forging cross-section. For a ring: circumferential flow; for a disc: radial flow from bore to rim. Proof is required by macro-etching a sectioned coupon. AMS 5660 does not mandate a specific orientation or require proof by macro-etch.
Application Guide by Industry
The correct specification follows from the dominant failure mode in service. Here is a part-family breakdown across major industries that use Incoloy 901 forgings:
Gas Turbine Casings
Outer compressor casings, diffuser housings, exhaust casings. Static under internal pressure; creep governs life. Class A UT is sufficient.
Turbine Disc Blanks
Rotating compressor and turbine stage discs. LCF governs life. Class AA UT is mandatory; most OEMs require it regardless of rotation speed.
Valve Bodies & Flanges
High-temperature valve internals and pipeline flanges in nuclear and oil & gas. Pressure-loaded; fatigue is secondary to creep.
Seamless Compressor Rings
Drum rings and spacer rings in axial compressors. Centrifugal loading and vibratory stress require premium inspection and fine-grain structure.
Nuclear Reactor Internals
Structural internals in nuclear systems where superalloy forging properties are required. Always verify applicable nuclear code requirements with your design authority before ordering.
Blade Retention Fasteners
Tie bolts and locking rings at blade-disc interfaces. Fretting fatigue makes premium grain quality and Class AA inspection non-negotiable.
Fatigue vs. Creep — The Root Reason for the Split
The AMS 5660 / AMS 5661 division reflects a fundamental difference in how materials fail under different loading regimes. The two specifications address this by applying different inspection rigor to catch the defects most relevant to each failure mode.
Why AMS 5660 is appropriate for creep-governed static parts
A stationary casing at 550 °C accumulates damage by slow plastic deformation under sustained load. Small internal voids passing Class A do not propagate rapidly under static stress; they may remain subcritical through a 100,000-hour design life. What matters is bulk compositional control and correct heat treatment — both rigorously required by AMS 5660. The additional cost of Class AA inspection adds no risk-reduction value for this failure mode.
Why AMS 5661 is mandatory for fatigue-governed rotating parts
A turbine disc undergoes a full strain cycle with every start, load change, and shutdown. Fatigue crack growth rate in Ni-Fe superalloys at temperature is a power function of initial defect size. A sub-surface void passing Class A (FBH #5) but failing Class AA (FBH #3) can reduce predicted disc fatigue life by a factor of 2–3 under representative LCF conditions. The cost of Class AA UT is typically 3–7% of total forging cost — a fraction of the consequence of an undetected defect in a rotating disc.
Several leading gas turbine OEMs mandate AMS 5661 for all rotating components regardless of speed, because the cost of Class AA UT is far smaller than the engineering liability of a disc failure in service. If your customer’s drawing lists AMS 5661, treat it as a hard requirement — no substitution without written engineering disposition from the design authority.
Full AMS 5660 vs AMS 5661 Specification Comparison
| Parameter | AMS 5660 | AMS 5661 |
|---|---|---|
| Alloy Common Name | Incoloy 901 | Incoloy 901 |
| UNS Designation | UNS N09901 | UNS N09901 |
| Chemical Composition | Identical to AMS 5661 | Identical to AMS 5660 |
| Melting Route | VIM + VAR or ESR per AMS 5660 / 5661 requirements | VIM + VAR or ESR per AMS 5660 / 5661 requirements |
| Product Forms | Bars, billets, open-die forgings | Forged rings, discs, rotating fasteners |
| UT Inspection Class | AMS 2630 Class A | AMS 2630 Class AA |
| FBH Reference Size | #5 (5/64” diameter) | #3 (3/64” diameter) — 64% smaller area |
| Grain Size Requirement | ASTM No. 5 or finer (≤ 63 µm) | ASTM No. 6 or finer (≤ 45 µm) |
| Grain Flow Proof | Not required | Macro-etch proof; principal stress direction |
| Surface Finish | Standard | Premium for fatigue-sensitive surfaces |
| Room-Temp Tensile (min) | Identical to AMS 5661 | Identical to AMS 5660 |
| Heat Treatment | Two-stage aging (identical) | Two-stage aging (identical) |
| Hardness Target | 302–388 HBW (identical) | 302–388 HBW (identical) |
| Stress Rupture at 538 °C | Same minimum as AMS 5661 | Same minimum as AMS 5660 |
| Governing Failure Mode | Creep / sustained overload | Fatigue — LCF and HCF |
| EN 10204 Certificate | 3.1 standard; 3.2 on request | 3.2 mandatory for rotating parts |
| Typical Cost Premium | Baseline | +8–15% above AMS 5660 |
| AMS 5661 satisfies AMS 5660? | Yes — AMS 5661 is a superset. AMS 5660 does not satisfy AMS 5661. | |
How to Write a Correct Purchase Order
Specifying the wrong AMS number is one of the most common procurement errors in superalloy forging. Use these checklists:
✓ AMS 5660 Purchase Order Checklist
- AMS 5660 with revision letter (e.g., “AMS 5660E”)
- Product form: open-die forging, bar, or billet
- Weight or dimensions — or reference to a forging drawing
- Heat treatment condition: solution-annealed and aged
- UT inspection: AMS 2630 Class A (specify Class AA if required despite AMS 5660 designation)
- EN 10204 3.1 or 3.2 material test report — specify which
- Quantity, delivery date, and port of destination
✓ AMS 5661 Purchase Order Checklist
- AMS 5661 with revision letter (e.g., “AMS 5661C”)
- Product form: seamless rolled ring or precision forging
- Ring dimensions: OD × ID × height (as-forged envelope), or ring drawing reference
- Heat treatment condition: solution-annealed and aged
- UT: AMS 2630 Class AA — explicitly call out FBH #3 acceptance
- Grain size: ASTM No. 6 or finer — state explicitly on PO
- Grain flow verification: macro-etch on sectioned coupon — state explicitly
- EN 10204 3.2 material test report (specify if third-party witness inspection is required)
- Any OEM material supplements required by your customer (customer to advise)
- Quantity, delivery date, and port of destination
If your drawing specifies AMS 5661 but the application is a static component, do not substitute AMS 5660 without formal engineering disposition signed by the design authority. The specification on a drawing is a design decision. Unauthorized substitution is a non-conformance under most OEM quality plans.
Expertise and Experience Signals — Why Trust This Content
- Jiangsu Liangyi has manufactured AMS 5660 and AMS 5661 forgings since 1997 — over 25 years of continuous production experience
- ISO 9001:2015 certified QMS; in-house heat treatment pyrometry to AMS 2750
- In-house AMS 2630 Class A and Class AA UT equipment and qualified inspectors
- Single-part forging range: 30 kg to 30,000 kg per piece
- EN 10204 3.1 and 3.2 mill certificates issued on every shipment; EN 10204 3.2 third-party witness available on request
- Customers in 50+ countries across aerospace, gas turbine, nuclear power, and oil & gas
Frequently Asked Questions — AMS 5660 vs AMS 5661
Need AMS 5660 or AMS 5661 Forgings?
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