UNS N09908 is an age-hardenable nickel-iron superalloy sold commercially as INCOLOY alloy 908. It contains 47 to 51 percent nickel, 3.75 to 4.5 percent chromium, 2.7 to 3.3 percent niobium, 1.2 to 1.8 percent titanium and 0.75 to 1.25 percent aluminium, with iron making up the balance. The alloy has a low coefficient of thermal expansion, high tensile strength and good fracture toughness down to 4 K. Cobalt is limited to 0.50 percent maximum.
The alloy was developed by the Massachusetts Institute of Technology together with the International Nickel Company as a conduit material for Nb₃Sn cable-in-conduit superconducting magnets. The sheath has to expand and contract at the same rate as the brittle superconductor inside it. Alloy 908 was specified for ITER model coils and for the Tokamak Physics Experiment, and it is now used for other high-strength cryogenic structures as well.
Jiangyin Jiangnan Metal Co., Ltd. forges UNS N09908 to customer drawings as rings, seamless rolled rings, shafts, bars, discs, sleeves, bushings, flanges and tube sheets, and ships worldwide from Jiangyin, Jiangsu Province, China.
Grade summary
- UNS designation
- N09908
- Common names
- Incoloy 908, alloy 908, INCOLOY alloy 908
- Type
- Precipitation-hardening nickel-iron superalloy, γ′ Ni₃(Al,Ti)
- Defining property
- Coefficient of thermal expansion matched to Nb₃Sn
- ASTM specification
- ASTM B872, plate, sheet and strip, annealed temper
- Density
- 8.17 g/cm³ (0.295 lb/in³)
- Melting range
- 1361 to 1410 °C (2482 to 2570 °F)
- Primary use
- Nb₃Sn superconducting magnet conduit, cryogenic structures
- Forged by
- Jiangyin Jiangnan Metal Co., Ltd., Jiangyin, Jiangsu, China
Chemical composition of UNS N09908
The composition below follows the limits used for UNS N09908 in ASTM B872. Cobalt is capped at 0.50 percent, which makes the alloy suitable for neutron flux environments where activation of cobalt is a concern.
| Element | Content (%) | Role in the alloy |
|---|---|---|
| Nickel, Ni | 47.0 to 51.0 | Matrix former, sets the low expansion behaviour |
| Iron, Fe | Balance, about 35.6 to 44.6 | Matrix, the Ni-Fe pairing gives Invar-type expansion |
| Chromium, Cr | 3.75 to 4.50 | Oxidation resistance without spoiling expansion |
| Niobium, Nb (Cb) | 2.70 to 3.30 | Strengthening, forms γ″ and Laves phases |
| Titanium, Ti | 1.20 to 1.80 | γ′ Ni₃(Al,Ti) precipitation hardening |
| Aluminium, Al | 0.75 to 1.25 | γ′ precipitation hardening |
| Manganese, Mn | 1.00 max | Residual, deoxidiser |
| Cobalt, Co | 0.50 max | Restricted, the alloy is specified as cobalt free |
| Copper, Cu | 0.50 max | Residual |
| Silicon, Si | 0.50 max | Residual, deoxidiser |
| Carbon, C | 0.030 max | Held low to limit carbide networks |
| Phosphorus, P | 0.015 max | Impurity |
| Boron, B | 0.012 max | Grain boundary strengthening |
| Sulfur, S | 0.005 max | Impurity, kept very low for hot workability |
Composition limits are given for guidance. The governing chemistry for any order is the one written on the purchase specification and reported on the mill certificate.
Mechanical and physical properties
The values below are typical room temperature figures for the alloy in the aged condition. UNS N09908 is used mainly at cryogenic temperature, where both strength and toughness rise. Data at 4 K and 77 K should be requested against the specific heat treatment on your order.
| Property | Metric | Imperial |
|---|---|---|
| Tensile strength | 1172 MPa | 170,000 psi |
| Yield strength, 0.2% offset | 827 MPa | 120,000 psi |
| Elongation at break | 12% | 12% |
| Property | Metric | Imperial |
|---|---|---|
| Density | 8.17 g/cm³ | 0.295 lb/in³ |
| Melting range | 1361 to 1410 °C | 2482 to 2570 °F |
| Coefficient of thermal expansion at 93 °C | 8.59 µm/m·°C | 4.77 µin/in·°F |
| Thermal conductivity | 11.05 W/m·K | 76.7 BTU·in/hr·ft²·°F |
Heat treatment and SAGBO control
UNS N09908 is strengthened by γ′ Ni₃(Al,Ti) precipitation. A typical route is solution annealing near 980 to 1010 °C followed by precipitation ageing between 650 and 730 °C. In Nb₃Sn magnet work the ageing is normally combined with the superconductor reaction heat treatment, which holds the assembly near 650 °C for 100 to 200 hours, so the conduit ages during the same cycle.
Atmosphere control during ageing
Alloy 908 is susceptible to stress-accelerated grain-boundary oxidation, usually abbreviated SAGBO. Oxygen penetrates the grain boundaries when the alloy is held at ageing temperature under stress in an oxygen-bearing atmosphere, and intergranular cracking follows. Ageing has to be carried out in vacuum, in argon or in a controlled low-oxygen atmosphere. Most purchase specifications for this alloy state the requirement explicitly.
Forging is carried out hot, in the temperature range where the alloy retains good ductility. Its resistance to oxygen embrittlement is high enough to allow hot fabrication without cracking. Die and tooling cleanliness matters, because surface contamination picked up during forging can initiate cracks during later ageing.
Heat treatment temperatures above are indicative. Work to the cycle stated in your own specification or drawing, and state the intended cycle at enquiry stage so the forging condition and stock allowance can be set correctly.
UNS N09908 forged products
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory, so UNS N09908 parts are made to your drawing rather than picked from a size catalogue. The usual forms are listed below.
- Forged ringsOpen-die and seamless rolled rings, plain or profiled
- Seamless rolled ringsRing rolled for continuous circumferential grain flow
- Shafts and spindlesStraight, stepped and eccentric shafts
- Round bars and billetsForged bar, cut to length or to weight
- Discs and blanksSolid discs, pierced discs, gear and turbine blanks
- Sleeves and bushingsHollow forgings, bored and trepanned
- FlangesForged flange blanks and finished flanges to drawing
- Tube sheets and platesForged tube sheets, blocks and rectangular blanks
- Valve componentsValve bodies, bonnets, seat rings and forged valve stems
Size and weight range
| Form | Range |
|---|---|
| Ring outside diameter | 100 to 3,000 mm |
| Bar and shaft diameter | 80 to 1,200 mm |
| Maximum length | up to 6,000 mm |
| Single piece weight | 5 kg to 8,000 kg |
| Delivery condition | As forged, rough machined, solution annealed, or solution annealed and aged |
| Minimum order | Single piece, prototypes and one-off parts accepted |
Send a drawing, or the outside diameter, inside diameter, height and quantity, and a quotation follows within 24 hours.
Melting route, testing and certification
Melting
UNS N09908 is a high cleanliness superalloy. Material is supplied double melted, VIM plus ESR as standard, with VIM plus VAR available where the specification calls for it, so that segregation, inclusion content and gas levels stay inside superalloy limits. Ingot is upset and drawn on the open-die press to break down the cast structure and develop grain flow along the loaded direction of the part.
Non-destructive testing
Forgings are ultrasonically tested to EN 10228-3, SEP 1921 or ASTM A388, to the acceptance class stated on the order. Liquid penetrant inspection to ASTM E165 and dimensional inspection reports are available on request.
Certification
Every consignment is supplied with a mill certificate to EN 10204 3.1, giving heat number, chemical analysis, heat treatment condition, mechanical test results and NDT results. EN 10204 3.2 certificates with third-party witnessing by TÜV, BV, SGS, Lloyd's or your own inspector are issued when the order requires them.
Applications for UNS N09908 forgings
UNS N09908 was designed for superconducting magnet construction, and that remains its main application. Its strength and toughness at 4 K also suit it to other cryogenic work.
- Fusion and superconducting magnets. Conduit and sheathing for Nb₃Sn cable-in-conduit conductors, magnet structural rings, support hardware and clamps, including ITER and Tokamak Physics Experiment coil work.
- Superconductor manufacturing tooling. Forming and compaction tooling for Nb₃Sn cable, where the tooling has to hold dimensions through the reaction heat treatment.
- Cryogenic structures. High-strength components for liquid helium and liquid nitrogen service, cryostat internals, and load-bearing parts down to 4 K.
- Precision dimensional control. Rings, sleeves and shafts in instruments and assemblies where clearance has to be held over a wide temperature swing.
- Research and accelerator hardware. Magnet components in neutron flux environments where cobalt-bearing alloys are excluded because of activation.
UNS N09908 compared with related low-expansion alloys
Four controlled-expansion superalloys come up repeatedly in enquiries. The difference that matters most is cobalt content and the presence of chromium.
| Alloy | UNS | Ni | Co | Cr | Nb | Typical use |
|---|---|---|---|---|---|---|
| Alloy 908 | N09908 | 49 | 0.5 max | 4 | 3 | Cryogenic Nb₃Sn magnet conduit |
| Incoloy 903 | N19903 | 38 | 15 | none | 3 | Aero-engine rings, casings, shafts |
| Incoloy 907 | N19907 | 38 | 13 | none | 4.7 | Higher strength aero rings and seals |
| Incoloy 909 | N19909 | 38 | 13 | none | 4.7 | Aero rings and seals, silicon modified |
Values are nominal and indicative only, for orientation between grades. Use the governing specification for design work.
Specify UNS N09908 where cobalt has to be excluded, for example in neutron flux environments or where a customer specification bans it, and where service temperature is cryogenic. The 903, 907 and 909 grades are intended for elevated temperature aero-engine hardware, where their cobalt content is an advantage.
Frequently asked questions about UNS N09908
What is UNS N09908?
UNS N09908 is an age-hardenable nickel-iron superalloy sold commercially as INCOLOY alloy 908. It contains 47 to 51 percent nickel, 3.75 to 4.5 percent chromium, 2.7 to 3.3 percent niobium, 1.2 to 1.8 percent titanium and 0.75 to 1.25 percent aluminium, with iron making up the balance. The alloy has a low coefficient of thermal expansion, high tensile strength and good toughness at cryogenic temperature, and cobalt is limited to 0.50 percent maximum.
Is UNS N09908 the same as Incoloy 908?
Yes. UNS N09908 is the Unified Numbering System designation for the alloy sold commercially as INCOLOY alloy 908. Both names refer to the same material and either may be used on a purchase order.
Which standard covers UNS N09908?
ASTM B872 covers UNS N09908 plate, sheet and strip in the annealed condition. There is no dedicated ASTM product specification for N09908 forgings, so forgings are supplied against the customer purchase specification, with chemistry to the B872 limits and with mechanical testing, NDT and certification agreed on the order.
What is UNS N09908 used for?
UNS N09908 was developed as conduit and sheathing material for Nb₃Sn cable-in-conduit superconducting magnets, including ITER and TPX fusion magnets, because its thermal expansion is matched to Nb₃Sn. It is also used for high-strength cryogenic structures, magnet support hardware, forming tooling for superconductor cables, and precision components that must hold dimensions across a wide temperature range.
Why must UNS N09908 be heat treated in vacuum or inert atmosphere?
UNS N09908 is susceptible to stress-accelerated grain-boundary oxidation, abbreviated SAGBO. Oxygen penetrates the grain boundaries when the alloy is held at ageing temperature under stress in an oxygen-bearing atmosphere, and intergranular cracking follows. Ageing in vacuum, in argon or in a controlled low-oxygen atmosphere prevents this, and most purchase specifications state the requirement.
What forged shapes are available in UNS N09908?
Jiangyin Jiangnan Metal Co., Ltd. supplies UNS N09908 as open-die forged rings, seamless rolled rings, shafts, spindles, round bars, discs, blanks, hollow bars, sleeves, bushings, flanges, tube sheets, valve bodies and valve stems, all produced to the customer drawing rather than to a fixed catalogue of sizes.
Who supplies UNS N09908 open-die forgings?
Jiangyin Jiangnan Metal Co., Ltd., an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, produces UNS N09908 (Incoloy alloy 908) forgings to customer drawings and ships worldwide. Enquiries go to sales@steelforgepieces.com or +86-189-2135-9659.
What testing and certification is supplied with UNS N09908 forgings?
UNS N09908 forgings are ultrasonically tested to EN 10228-3, SEP 1921 or ASTM A388 as specified on the order. Chemical analysis, mechanical testing and dimensional inspection are reported on a certificate to EN 10204 3.1, or to EN 10204 3.2 when third-party witnessing is required.
How does UNS N09908 differ from Incoloy 903, 907 and 909?
Incoloy 903, 907 and 909 are cobalt-bearing low-expansion superalloys developed mainly for aero-engine rings, casings and shafts at elevated temperature. UNS N09908 is essentially cobalt-free and carries about 4 percent chromium, which gives it oxidation resistance and suits it to cryogenic service and to neutron flux environments where cobalt is undesirable.
Request a quotation for UNS N09908 forgings
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory in Jiangyin, Jiangsu Province, China. The company forges UNS N09908 (Incoloy alloy 908) rings, seamless rolled rings, shafts, bars, discs, sleeves, bushings, flanges and tube sheets to customer drawings, supplies certificates to EN 10204 3.1 and 3.2, and exports worldwide.
Send a drawing, or the outside diameter, inside diameter, height, quantity and required delivery condition. A quotation follows within 24 hours.
Factory No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, ChinaPhone +86-189-2135-9659
Email sales@steelforgepieces.com
Web www.steelforgepieces.com Email an enquiry
Document notes
UNS N09908 (Incoloy alloy 908) forgings datasheet. Published by Jiangyin Jiangnan Metal Co., Ltd., No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. Last updated 31 August 2026.
Composition and property data are compiled from published specifications for UNS N09908 and are given for guidance. The governing documents for any order are the purchase specification and the mill certificate issued with the material.