1.4898 / Alloy 901 / UNS N09901 Forgings Nickel-iron-chromium precipitation-hardening superalloy for gas turbine discs, rotor shafts & seamless rolled rings

01What is 1.4898 / Alloy 901 / UNS N09901?

1.4898 (Alloy 901 / UNS N09901 / AMS 5660) is a precipitation-hardening nickel-iron-chromium superalloy developed for gas turbine discs and shafts. Its nominal chemistry is 40–45% nickel, 11–14% chromium, 5–7% molybdenum and 2.35–3.10% titanium, with iron making up the balance at roughly 34%.

Three deliberate compositional choices explain everything about how the alloy behaves. Titanium and aluminium precipitate the ordered γ′ phase, Ni₃(Ti,Al), during ageing. This is the primary strengthening mechanism and it is what gives Alloy 901 its high yield strength at temperature. Molybdenum at 5–7% is unusually high for this alloy class and provides solid-solution strengthening of the austenitic matrix, which is what sustains creep resistance rather than short-term strength. Iron at roughly a third of the alloy is the commercial compromise: it dilutes the nickel content, lowers cost, and, critically for a forging shop, widens the hot-working window so that large discs and shafts can be forged on conventional presses without the cracking sensitivity of a fully nickel-base alloy such as Waspaloy.

The practical consequence is a design limit of roughly 600 °C (1110 °F) for creep-limited and stress-limited service. Below that, Alloy 901 retains high yield strength and useful creep-rupture life. Above roughly 650 °C the γ′ coarsens and strength falls away quickly, which is where Inconel 718 (~650 °C) or Waspaloy (~760 °C) take over. Alloy 901 is not chosen for maximum temperature capability; it is chosen where that temperature capability is sufficient and where forgeability, section size and cost matter.

The alloy is non-magnetic in service because the matrix is austenitic (face-centred cubic) in all conditions, a useful distinguishing check against martensitic precipitation-hardening steels such as 17-4PH, which are strongly ferromagnetic. It also work-hardens rapidly, which drives the machining practice described in the fabrication section below.

02What are the equivalents of 1.4898? (N09901, AMS 5660, Alloy 901)

Engineers meet this alloy under at least ten different names depending on which standards body, engine OEM or supplier wrote the document in front of them. Every designation in the table below refers to the same chemistry, and Jiangyin Jiangnan Metal Co., Ltd. accepts purchase orders against any of them, supplying UNS N09901 / DIN 1.4898 material with a multi-designation mill test certificate.

On 1.4898 versus 2.4662: published datasheets cite both Werkstoff numbers for Alloy 901. The 1.4xxx series is normally reserved for steels and the 2.4xxx series for nickel alloys, so 2.4662 is arguably the more logical number for a 40–45% Ni alloy, but 1.4898 is in widespread commercial use and is what most buyers search for. If your drawing carries either number, quote it verbatim on the purchase order and we will cross-reference both on the certificate.

03What is the chemical composition of 1.4898 / Alloy 901?

The limits below are the nominal DIN 1.4898 / UNS N09901 chemistry, which is common to both AMS 5660 and AMS 5661. Every element is there for a reason, given in the last column, worth reading before you decide which limits actually matter for your part.

^† Phosphorus and cobalt limits are the values commonly specified in AMS 5660 rather than in the base DIN listing; confirm against the revision of the specification named on your order. All values are wt %. The mill test certificate for the actual heat governs in every case.

04What forged products are available in 1.4898 / UNS N09901?

Jiangyin Jiangnan Metal Co., Ltd. produces 1.4898 / Alloy 901 forgings by three process routes, chosen by part geometry rather than by preference. Open-die forging handles rotor and pump shafts, blocks, and large discs, the route that suits the alloy's relatively narrow hot-working window because it allows reheating between passes. Seamless ring rolling on 3 m and 6 m radial-axial mills produces rings, spacers and flange blanks, and is the most common route for this grade. Upset forging is used for short, large-cross-section discs and hubs where an axial upset gives better radial grain flow than drawing down would.

Because Alloy 901 is expensive per kilogram, near-net-shape forging pays for itself faster here than on carbon or stainless grades. Removing 30–50% of the machining stock on a turbine disc profile is frequently the difference between a viable quotation and an uncompetitive one, and on hollow shafts above roughly Ø100 mm bore, starting from a trepanned billet rather than a solid bar cuts raw-material input substantially.

• Gas turbine discs
• Turbine & compressor rotor shafts
• Seamless rolled rings
• Compressor spacers
• Forged flanges
• Tube sheets
• Sleeves & bushings
• Valve stems & spindles
• Valve seat rings & bodies
• Pump shafts
• Forged blocks & blanks
• Round & flat bar
• High-temperature bolting stock
• Custom near-net-shape forgings

Maximum envelopes are for this grade specifically and are smaller than our carbon-steel limits. Superalloy forging is constrained by press load, reheat cycles and the narrow hot-working window, not by press opening. Single-piece weight range across all forms: 10 kg to 15 000 kg. The dimensional maxima are individual limits and are not all achievable at once. A ring at 6000 mm OD and a shaft at 10 000 mm length are each possible, but not at maximum section as well, because the 15 000 kg single-piece weight governs. Use the weight calculator to check your geometry, and confirm the envelope at enquiry stage.

05What are the mechanical properties of 1.4898 / Alloy 901?

Alloy 901 is supplied either solution treated (soft, for onward machining, hot work or welding) or solution treated, stabilised and aged, usually written STA, which is the fully hardened condition most finished parts are delivered in. The difference between the two is roughly a doubling of yield strength.

These are indicative typical values, not design allowables. Guaranteed minima are set by the specification named on your order (AMS 5660, AMS 5661 or the customer specification) and the actual certified values are those on the mill test certificate for the heat supplied. Section size matters: heavy discs and rings above roughly 200 mm thickness commonly test 5–10 % below thin-section values because the quench rate at the core is lower. For design work, use the allowables in the governing specification or in MMPDS, not a supplier web page.

How properties fall off with temperature

The reason Alloy 901 is a turbine-disc alloy rather than a room-temperature structural alloy is that it holds most of its strength to about 600 °C. The table below shows the general shape of that retention.

Indicative values illustrating the shape of the retention curve. Above roughly 550 °C, time-dependent deformation matters more than the short-term tensile numbers above; see creep and stress rupture.

06What are the physical properties of 1.4898? (density, thermal, magnetic)

Typical published values for the alloy class. Thermal conductivity around 10 W/m·K is roughly a quarter that of carbon steel. This is the single most important number for anyone machining the alloy, because almost none of the cutting heat leaves through the chip.

07What heat treatment is used for 1.4898 / Alloy 901 forgings?

Alloy 901 uses a three-stage cycle, and the middle stage is the one people leave out. Solution treatment dissolves the γ′ and puts everything back into solution. The stabilisation hold then precipitates grain-boundary carbides. This step exists specifically to restore notch-rupture ductility, and skipping it produces a part that passes a smooth tensile test and then fails notch-sensitively in service. Only after that does the long ageing hold precipitate the fine intragranular γ′ that carries the strength.

Hold times in the table are for thin sections. Scale the solution and stabilisation holds by section thickness at roughly 30 minutes per 25 mm above a 25 mm section, keeping the 24-hour ageing hold fixed. The quench from solution temperature must be fast: a slow cool through 900–750 °C lets coarse γ′ and grain-boundary films form, and neither can be recovered by subsequent ageing. For heavy discs, water quenching introduces its own residual-stress problem; see the failure modes section on quench cracking.

08Creep and stress rupture: the property that actually sizes the part

For any part running above roughly 550 °C, the tensile table stopped being the governing data some time ago. What sizes a turbine disc is creep: the slow, time-dependent deformation under a stress well below the yield strength. Alloy 901 was designed around this. The 5–7% molybdenum in solid solution raises the matrix creep resistance, and the boron and grain-boundary carbides suppress grain-boundary sliding, which is the mechanism that would otherwise dominate at these temperatures.

Creep data is normally correlated using the Larson–Miller parameter, P = T·(C + log₁₀ t) where T is absolute temperature in kelvin, t is rupture time in hours and C is a material constant, conventionally taken as 20 for this alloy class. The value of the parameter is that a single master curve of stress against P lets you trade temperature against time: a 1000-hour test at 700 °C tells you something usable about a 100 000-hour life at 600 °C.

09How corrosion and oxidation resistant is Alloy 901?

Alloy 901 has good corrosion resistance in the atmospheres normally found in jet engine operation, which is the environment it was qualified for. It is worth being precise about what that does and does not mean.

At 11–14%, the chromium content is deliberately lower than in alloys chosen primarily for oxidation resistance. Inconel 601 carries 21–25%, Incoloy 800 runs 19–23%. That chromium was traded away to make room for the molybdenum and titanium that deliver strength and creep resistance. The consequence is that Alloy 901 has slightly lower scaling resistance than Type 309 or Type 310 stainless steel, and it is not the alloy to choose if oxidation resistance is your first requirement rather than your second.

10Alloy 901 vs Inconel 718, Waspaloy, A286 and Inconel 706

These five alloys compete for the same job: the rotating discs, shafts and spacers of gas and steam turbines. They are not interchangeable, and the differences that matter are the strengthening mechanism, the temperature ceiling, and how easily the alloy can be forged and welded.

Composition ranges are nominal; cost indices are indicative raw-material ratios with A286 as the baseline and move with LME nickel, ferro-chrome and ferro-molybdenum. Service temperatures are creep-limited practical design limits, not oxidation limits.

11How is 1.4898 melted, and why does the melt route matter?

For a turbine disc alloy, the melt route is not a detail. It is the single decision that most affects both price and whether the part will pass its ultrasonic inspection. Titanium-bearing superalloys are reactive; residual oxide and nitride inclusions become crack-initiation sites under the cyclic loading a disc sees, and they will not machine out because they are distributed through the section.

Jiangyin Jiangnan Metal Co., Ltd. sources Alloy 901 input stock by any of these routes and states the route explicitly on the mill test certificate. If your drawing does not specify one, we quote VIM + ESR as the default. If the part is a rotating aerospace component, specify the route. Do not leave it to the supplier to choose, because the price difference between EAF+VOD+ESR and triple melt is large enough that an unspecified order will be quoted on the cheaper route by someone.

12How do you forge, weld and machine 1.4898 / Alloy 901?

Forging

Alloy 901 is forged between 1120 °C and 1010 °C (2050–1850 °F). Light finishing reductions can be taken down to about 870 °C (1600 °F) but not below. Below that the alloy work-hardens faster than it recrystallises and cracking risk rises sharply. For rapid forging the metal temperature should not exceed 1120 °C, because incipient melting at grain boundaries is the failure mode at the top of the range and it destroys the billet irrecoverably.

That is a working window of roughly 110 °C, which is wide by superalloy standards and narrow by carbon-steel standards. In practice it means multiple reheats on any substantial part, and it is the reason Alloy 901 forgings carry a forging cost per kilogram several times that of alloy steel. Aim for a total forging ratio of at least 4:1 to break down the as-cast ingot structure, and specify the grain-flow direction on the drawing. For discs, radial or tangential flow rather than whatever the die happens to produce.

Welding

Alloy 901 can be welded by inert-gas arc processes, principally GTAW, but it is regarded as difficult to weld. Like every γ′-hardened superalloy with a high titanium plus aluminium content, it is susceptible to strain-age cracking: during heating for post-weld heat treatment, γ′ precipitates in the heat-affected zone at the same time as residual welding stresses are relaxing, and the now-hardened HAZ cannot accommodate the strain. The result is cracking that appears after welding rather than during it.

Machining

Two properties dominate: the alloy work-hardens rapidly, and its thermal conductivity of about 10 W/m·K means almost none of the cutting heat leaves through the chip. Everything else follows from those two facts.

• Rough machine in the solution-treated condition, finish after ageing. This is the single biggest lever on machining cost for this grade.
• Never dwell and never rub. If the tool stops cutting and starts burnishing, the surface glazes and work-hardens, and the next pass has to cut through a harder layer than the one before. Maintain positive feed contact at all times.
• Carbide tooling for uninterrupted cuts, which gives the highest removal rates. Cobalt HSS for interrupted cuts, close-tolerance finishing, and any operation where minimising cold work matters more than speed.
• Rigid setup, sharp positive-rake geometry, flood coolant. Deflection produces exactly the rubbing that causes glazing.
• Expect roughly 10–15% of the machinability of free-machining steel. Quote the machining hours accordingly.

13Alloy 901 failure modes and how to prevent them

These are the failure mechanisms that actually appear in Alloy 901 parts, in rough order of how often we see them in incoming material assessments and customer investigations. Each is preventable at the specification stage, which costs nothing, and expensive to catch at receipt.

14Production capability: 1.4898 / Alloy 901 forging manufacturer

Jiangyin Jiangnan Metal Co., Ltd. operates an open-die forging plant at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, employing approximately 460 people including 9 senior engineers and 32 intermediate engineers. The plant has produced open-die forgings and seamless rolled rings since 2008 and exports to more than 40 countries.

Lead times run from order confirmation to ex-works dispatch and assume input stock is available. Alloy 901 is not a stock grade at most mills. Melt-stock lead time dominates the schedule, so early enquiry materially improves the delivery date.

15Standards, non-destructive examination and quality

Specifications we produce to

• AMS 5660
• AMS 5661
• AMS 5830 (filler)
• DIN 1.4898
• W.-Nr. 2.4662
• UNS N09901
• ISO 9723 / 9725
• AFNOR Z8NCDT42
• AECMA prEN 2176–2178
• GE B50A305B
• PWA 1002 / 1003
• Customer specifications

Non-destructive examination

Every 1.4898 forging is ultrasonically inspected. Note that Alloy 901 is non-magnetic, so magnetic-particle inspection does not apply. Surface examination is by liquid penetrant. This trips up buyers who copy an NDE clause across from a martensitic steel drawing.

Certification

EN 10204 3.1 mill test certificates are issued as standard, covering heat chemistry, mechanical results, heat-treatment records and NDE results, with full heat-number traceability. EN 10204 3.2 certificates with third-party witness through Lloyd's Register, DNV, Bureau Veritas, ABS or TÜV are available on request; nominate the inspection body at order stage so hold points can be scheduled. Quality management is certified to ISO 9001:2015.

16How to specify a 1.4898 / Alloy 901 forging order

Alloy 901 carries two decisions that most grades do not: the melt route and whether the stabilisation hold is included. Leave either unstated and you will be quoted against whichever interpretation is cheapest. These seven steps close that gap.

1. Confirm the designation. Write UNS N09901 / DIN 1.4898, or AMS 5660 for aerospace. Avoid ordering against "Incoloy 901" or "Nimonic 901" alone. Those are Special Metals Corporation trademarks, not specifications.
2. Specify the melt route. VIM + ESR for general industrial, VIM + VAR for aerospace rotating parts, VIM + ESR + VAR triple melt for the most critical discs and shafts. Use the melt-route selector if you are unsure.
3. State the heat-treatment condition explicitly. Either "solution treated" or "solution treated + stabilised + aged (STA)". If STA, require all three stages by temperature and time on the certificate, and ask for the furnace charts.
4. Provide the drawing and the grain-flow requirement. Dimensions, machining stock, tolerances, surface finish, and for discs the required grain-flow direction with the macroetch acceptance standard.
5. Define NDE. Ultrasonic acceptance class to ASTM A388, EN 10228-3 or SEP 1921, plus liquid penetrant per ASTM E165, not magnetic particle, which does not work on this alloy.
6. Specify certification. EN 10204 3.1, or 3.2 with the inspection body named. Add any notched stress-rupture or grain-size acceptance requirement here.
7. Give quantity, delivery target and destination. Standard forgings ship in 10–14 weeks; AMS 5660 material with source approval takes 14–18 weeks.

17Top 10 mistakes when ordering 1.4898 / Alloy 901 forgings

18Drawing callout template for 1.4898 / Alloy 901

Copying the block below into your CAD material callout removes most of the ambiguity that causes requotes and rejections on this grade. Adjust the melt route, condition and NDE class to suit the part.

MATERIAL:      UNS N09901 / DIN 1.4898
               (also satisfies AMS 5660, AMS 5661, AISI 681,
                ISO 9723, AFNOR Z8NCDT42, generic equivalent
                of Incoloy 901 / Nimonic 901)

MELT ROUTE:    VIM + ESR                 // VIM + VAR or VIM + ESR + VAR
                                         // for aerospace rotating parts

CONDITION:     Solution treated + stabilised + aged (STA)
               Stage 1  1080-1107 deg C, 2 h min, water quench
               Stage 2   760- 802 deg C, 2-4 h, air cool
               Stage 3   704- 746 deg C, 24 h, air cool
               ALL THREE STAGES MANDATORY. Furnace charts required.

FORGING:       Forge 1120-1010 deg C. Finish above 870 deg C.
               Total forging ratio 4:1 minimum.
GRAIN FLOW:    Radial, verified by macroetch per ASTM E381
GRAIN SIZE:    ASTM E112 No. 4-7, no duplex structure

NDE:           UT per ASTM A388 (or EN 10228-3 quality class 3)
               PT per ASTM E165 Type I, Method C
               MAGNETIC PARTICLE NOT APPLICABLE - alloy is non-magnetic

MECHANICAL:    Tensile per ASTM E8 at RT, longitudinal + transverse
               Notched stress-rupture test required on rotating parts
HARDNESS:      302-363 HB in the aged condition

CERTIFICATION: EN 10204 3.1 mill certificate
               // or 3.2 with third-party witness, body to be named
MARKING:       Heat number, condition, drawing number and melt route,
               vibro-etched on a non-functional surface

19Where is 1.4898 / Alloy 901 used?

Alloy 901 exists because of one application, and that application still dominates its use. Everything else is a secondary market that inherited the alloy because the properties happened to suit.

20Glossary

1.4898

German Werkstoff number for the Alloy 901 chemistry. Also cited as 2.4662 in some references. Generic equivalents: UNS N09901, AMS 5660, AISI 681.

UNS N09901

Generic Unified Numbering System designation for the 40–45 % Ni, 11–14 % Cr, 5–7 % Mo, 2.35–3.10 % Ti precipitation-hardening superalloy. The safest neutral name for a purchase order.

Incoloy® 901 / Nimonic® 901

Registered trademarks of Special Metals Corporation for their own material of this composition. Not specifications, and not names an independent producer can supply under.

AMS 5660 / AMS 5661

SAE Aerospace Material Specifications covering Alloy 901 bar, forgings and rings, the dominant aerospace specifications for the grade.

γ′ (gamma prime)

The ordered Ni₃(Ti,Al) precipitate that provides the principal strengthening. Its volume fraction and coarsening rate set the creep limit of the alloy.

Stabilisation treatment

The intermediate 760–802 °C hold between solution treatment and ageing. Precipitates grain-boundary carbides and restores notch-rupture ductility. Frequently and wrongly omitted.

STA

Solution treated, stabilised and aged; the fully hardened supply condition for finished parts.

Strain-age cracking

Cracking in the weld heat-affected zone of γ′-hardened superalloys, caused by precipitation during post-weld heat treatment while residual welding stresses are relaxing.

Larson–Miller parameter

P = T(C + log₁₀ t), with C conventionally 20 for this alloy class. Correlates creep-rupture data so that short high-temperature tests can be extrapolated to long service lives.

Notch-rupture ductility

The ability to sustain stress-rupture loading at a stress concentration without premature failure. The property the stabilisation hold exists to protect.

VIM / ESR / VAR

Vacuum induction melting, electroslag remelting and vacuum arc remelting; the melt and remelt processes whose combination defines cleanliness, price and ultrasonic quality.

EN 10204 3.1 / 3.2

Inspection document types. 3.1 is validated by the manufacturer's independent inspection representative; 3.2 is countersigned by the purchaser's or a third-party inspector.

Duplex grain structure

A mixture of very fine and very coarse grains from incomplete recrystallisation, usually from finishing forging too cold. Causes erratic ultrasonic response and unpredictable properties.

22Technical references

Chemistry, heat-treatment, mechanical and corrosion data on this page are drawn from the published standards and engineering references below. Values on any mill test certificate we issue are independent and traceable to calibrated equipment.

1. AMS 5660, "Alloy Bars, Forgings, and Rings, Corrosion and Heat Resistant, 42.5Ni – 12.5Cr – 6.0Mo – 2.9Ti – 0.02B, Consumable Electrode Melted, Solution and Precipitation Heat Treated", SAE International.
2. AMS 5661, "Alloy Bars, Forgings, and Rings, Corrosion and Heat Resistant, 42.5Ni – 12.5Cr – 6.0Mo – 2.9Ti – 0.02B", SAE International.
3. AMS 5830, "Alloy Welding Wire, Corrosion and Heat Resistant, 42.5Ni – 12.5Cr – 6.0Mo – 2.9Ti", SAE International.
4. ISO 9723, "Nickel and nickel alloy bars", and ISO 9725, "Nickel and nickel alloy forgings", International Organization for Standardization.
5. EN 10204:2004, "Metallic products — Types of inspection documents", CEN, Brussels.
6. EN 10228-3, "Non-destructive testing of steel forgings — Part 3: Ultrasonic testing of ferritic or martensitic steel forgings", CEN.
7. SEP 1921, "Ultrasonic testing of steel forgings", Stahl-Eisen-Prüfblatt, Verein Deutscher Eisenhüttenleute.
8. ASTM A388/A388M, "Standard Practice for Ultrasonic Examination of Steel Forgings", ASTM International.
9. ASTM E165/E165M, "Standard Practice for Liquid Penetrant Testing for General Industry", ASTM International.
10. ASTM E8/E8M, "Standard Test Methods for Tension Testing of Metallic Materials", ASTM International.
11. ASTM E112, "Standard Test Methods for Determining Average Grain Size", ASTM International.
12. ASTM E381, "Standard Method of Macroetch Testing Steel Bars, Billets, Blooms, and Forgings", ASTM International.
13. ASTM E139, "Standard Test Methods for Conducting Creep, Creep-Rupture, and Stress-Rupture Tests of Metallic Materials", ASTM International.
14. ASM Handbook, Volume 1: Properties and Selection: Irons, Steels, and High-Performance Alloys, ASM International, Materials Park, OH; sections on wrought heat-resistant alloys.
15. ASM Handbook, Volume 4E: Heat Treating of Nonferrous Alloys, ASM International; precipitation hardening of Fe-Ni-base superalloys.
16. ASM Handbook, Volume 14A: Metalworking: Bulk Forming, ASM International; forging of heat-resistant alloys.
17. Donachie, M.J. and Donachie, S.J., Superalloys: A Technical Guide, 2nd Edition, ASM International, 2002.
18. Reed, R.C., The Superalloys: Fundamentals and Applications, Cambridge University Press, 2006.
19. Larson, F.R. and Miller, J., "A Time-Temperature Relationship for Rupture and Creep Stresses", Transactions of the ASME, Vol. 74, 1952, pp. 765–775.
20. Special Metals Corporation, technical datasheets for INCOLOY® alloy 901 and NIMONIC® alloy 901, retrievable at specialmetals.com.
21. MMPDS, Metallic Materials Properties Development and Standardization, Battelle Memorial Institute; for design allowables where the application requires them.

Standards cited are the revisions known at the date of the last page review. For procurement, always reference the revision in force at the contract date. All trademarks are the property of their respective owners.