Nickel alloy / Low-expansion superalloy / Open-die forging
Incoloy Alloy 908 Forgings
UNS N09908 / Alloy 908 / Chemistry to ASTM B872
Incoloy alloy 908 (UNS N09908) is an age-hardenable, cobalt-free nickel-iron superalloy with a low coefficient of thermal expansion, developed to jacket Nb3Sn cable-in-conduit superconductors operating near 4 K. Jiangyin Jiangnan Metal Co., Ltd., an open-die forging factory in Jiangyin, Jiangsu, China, forges alloy 908 to drawing as seamless rolled rings, discs, bars, shafts, flanges and sleeves, and supplies each piece with EN 10204 3.1 or 3.2 certification.
- UNS
- N09908
- Density
- 8.17g/cm³ (0.295 lb/in³)
- CTE at 93 °C
- 8.59µm/m·°C
- Yield at 4.2 K
- 1200MPa, annealed + aged
- Cobalt
- Free0.50 % max
About Incoloy alloy 908
Alloy 908 was developed by the Massachusetts Institute of Technology and the International Nickel Company as a conduit material for Nb3Sn superconducting magnets.
A cable-in-conduit conductor is wound at room temperature, held for 100 hours or more near 650 °C to react the Nb3Sn, then cooled to liquid helium temperature. If the conduit contracts more than the strand inside it, the brittle superconductor is strained and its current-carrying capacity falls. The conduit therefore needs a coefficient of thermal expansion close to that of Nb3Sn.
The iron-nickel matrix of alloy 908 lies in the Invar composition range, where magnetostriction offsets normal lattice expansion. Chromium is limited to about 4 % and cobalt is excluded so that the low-expansion behaviour is retained. Strength comes from γ′ Ni3(Al, Ti) precipitation, which forms during the same thermal cycle used to react the superconductor. The alloy resists overaging: γ′ does not transform to η, Ni3Ti, for times up to 100 hours at temperatures to 750 °C.
Published characterisation gives a 0.2 % offset yield strength near 1200 MPa at 4.2 K, fracture toughness above 200 MPa·√m at the same temperature, and fatigue crack growth rates about three times lower at 4.2 K than at room temperature.
Alloy 908 and Incoloy 27-7Mo are different materials. Incoloy alloy 908 (UNS N09908) is a low-expansion Fe-Ni superalloy. Incoloy 27-7Mo (UNS S31277) is a super-austenitic stainless steel containing about 27 % Ni, 21 % Cr and 7 % Mo. For high-molybdenum corrosion service, see Incoloy 926 or Incoloy 825.
Chemical composition of Incoloy alloy 908 (UNS N09908)
Composition limits to ASTM B872 and Special Metals published data, weight percent. Iron is the balance. Each heat is supplied with a ladle analysis on the material certificate. Product analysis is available to order.
| Element | Min | Max | Function |
|---|---|---|---|
| Nickel (Ni) | 47.0 | 51.0 | Sets the Invar matrix that gives the low expansion |
| Iron (Fe) | Balance | Matrix | |
| Chromium (Cr) | 3.75 | 4.50 | Oxidation and SAGBO resistance, limited to protect the CTE |
| Niobium (Nb, Cb) | 2.70 | 3.30 | Strengthening and grain-boundary control |
| Titanium (Ti) | 1.20 | 1.80 | γ′ Ni3(Al,Ti) former |
| Aluminium (Al) | 0.75 | 1.25 | γ′ Ni3(Al,Ti) former |
| Manganese (Mn) | — | 1.00 | Residual |
| Cobalt (Co) | — | 0.50 | Restricted. Alloy 908 is a cobalt-free grade |
| Copper (Cu) | — | 0.50 | Residual |
| Silicon (Si) | — | 0.50 | Residual |
| Carbon (C) | — | 0.030 | Low for toughness and weldability |
| Phosphorus (P) | — | 0.015 | Residual |
| Boron (B) | — | 0.012 | Grain-boundary strengthening |
| Sulfur (S) | — | 0.0050 | Low for hot workability |
| Melting route: VIM + ESR or VIM + VAR. State the required route on the enquiry. Projects using alloy 908 for magnet conduit normally specify double melting. | |||
Mechanical properties of Incoloy 908
Alloy 908 is supplied in the annealed condition and precipitation hardened afterwards, in most cases by the same reaction cycle that forms the Nb3Sn. Properties in the two conditions differ substantially. State on the order which condition the acceptance values apply to.
| Condition | Tensile strength | Yield strength (0.2 % offset) | Elongation |
|---|---|---|---|
| Annealed | 827 MPa (120 ksi) min | 414 MPa (60 ksi) min | 30 % typ. |
| Annealed and precipitation hardened | 1172 MPa (170 ksi) | 827 MPa (120 ksi) | 12 % |
| Indicative values. Acceptance limits are set by the purchase order or the project material specification. | |||
Cryogenic performance at 4.2 K
Published characterisation of solution-annealed and aged alloy 908 reports an average 0.2 % offset yield strength of about 1200 MPa at 4.2 K, fracture toughness KIC above 200 MPa·√m, and fatigue crack growth rates about three times lower at 4.2 K than at 298 K. Material cold worked 20 % and then aged shows roughly 20 % higher yield strength and 10 % higher tensile strength at 4.2 K.
Weld metal is weaker in the properties that govern a pressure boundary. GTAW deposits aged 200 hours at 650 °C reach comparable yield strength but about 10 % lower tensile strength, with fracture toughness, elongation and Charpy absorbed energy at roughly 40 % of base metal. Joint design should account for this.
Physical and thermal properties
| 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 | 76.69 BTU·in/hr·ft²·°F |
| Design service temperature | down to −269 °C (4 K) | down to −452 °F |
Incoloy 908 compared with related grades
Alloy 908 is usually compared with alloy 909, alloy 903 and modified 316LN. Alloy 908 is the cobalt-free, chromium-bearing grade intended for service at 4 K.
| Grade | UNS | Nominal chemistry | Cobalt | Typical use |
|---|---|---|---|---|
| Incoloy 908 | N09908 | 49 Ni, 4 Cr, 3 Nb, 1.5 Ti, 1 Al, bal Fe | None | Nb3Sn magnet conduit, cryogenic structures |
| Incoloy 909 | N19909 | 38 Ni, 13 Co, 4.7 Nb, 1.5 Ti, 0.4 Si, bal Fe | 13 % | Gas-turbine rings, seals and shafts |
| Incoloy 903 | N19903 | 38 Ni, 15 Co, 3 Nb, 1.4 Ti, 0.9 Al, bal Fe | 15 % | Low-expansion aerospace hardware |
| Modified 316LN | S31653 | Austenitic Cr-Ni-Mo-N stainless steel | None | Alternative magnet jacket. Higher CTE, lower strength, lower cost |
| Cobalt content matters in nuclear and fusion service because of activation. Alloy 908 and 316LN are the cobalt-free options in this group. | ||||
Available forged shapes
Open-die forging requires no die tooling, so single pieces and small batches are economical. Alloy 908 is normally ordered in small quantities for one magnet or one test programme.
Seamless rolled rings and forged rings
Rolled to rectangular or contoured section. Used for magnet support structures and coil cases.
Round bars, shafts, spindles
Forged and turned bar for machining into fittings, terminations and joint hardware.
Discs, blanks, tube sheets
Upset-forged discs and pierced blanks, supplied rough machined or to finish dimensions.
Sleeves, bushings, hollow bars
Trepanned or mandrel-forged hollows for conduit and jacket sections.
Forged flanges and valve parts
Weld-neck, blind and custom flange geometries. Valve stems, seats and bodies.
Near-net shapes to drawing
Stepped shafts, nozzles, crankshafts and non-standard profiles forged close to the finished envelope.
Size and weight range
| Parameter | Range | Notes |
|---|---|---|
| Ring outside diameter | 200 to 3000 mm | Seamless rolled. Larger sizes by enquiry |
| Bar diameter | 60 to 800 mm | Forged and rough turned |
| Single-piece weight | 5 to 8000 kg | Per finished forging |
| Minimum order | 1 piece | No die cost on open-die work |
| Delivery condition | As-forged, solution annealed, or annealed and aged | State on the order |
| Machining | Rough turned, proof machined, or finish machined | To drawing tolerances |
Forging route for alloy 908
The route is set by two constraints. The finishing temperature must stay high enough that γ′ does not precipitate during deformation, and grain boundaries must not be exposed to oxygen under stress at temperature.
| Stage | Description |
|---|---|
| Melting | VIM + ESR or VIM + VAR ingot, chemistry to ASTM B872. Mill certificate traced to the heat number. |
| Homogenisation | Extended soak to dissolve segregation before deformation. |
| Open-die forging | Cogging and upsetting with controlled reduction ratio and finishing temperature, to break down cast structure and refine grain size. |
| Ring rolling | For ring geometries, pierced and rolled to a seamless ring with circumferential grain flow. |
| Solution anneal | Around 980 °C for a mill anneal, or about 1050 °C for a full solution anneal that reverses prior cold work. Cycle agreed with the customer. |
| Aging | Where the customer does not run a Nb3Sn reaction cycle, aged near 650 to 720 °C in a controlled atmosphere. |
| Machining | Rough, proof or finish machined to drawing. |
| NDT and testing | Ultrasonic testing, dimensional inspection, mechanical and chemical testing. |
| Certification | EN 10204 3.1, or 3.2 with third-party witness. |
State the reaction heat treatment on the enquiry. Where the Nb3Sn cycle is 650 °C for 200 hours, that cycle also serves as the aging treatment, and the delivery condition should be solution annealed only. Material supplied already aged and then given a second long soak will be overaged. This is the most frequent specification error on alloy 908 enquiries.
Testing, inspection and certification
Ultrasonic testing
To EN 10228-3, SEP 1921 or ASTM A388, at the acceptance class stated on the order.
Dye penetrant and magnetic particle
PT to ASTM E165 on request. Alloy 908 is normally examined by PT rather than MT.
Tensile, hardness, grain size
Room-temperature tensile and hardness as standard. Cryogenic tensile and KIC testing subcontracted to an accredited laboratory when specified.
EN 10204 3.1 and 3.2
3.1 as standard. 3.2 with SGS, BV, TUV or a customer inspector witnessing, on request.
Applications
Alloy 908 is a specialist material rather than a general-purpose corrosion alloy. Its main applications are:
- Fusion and high-field magnets. Conduit and jacket sections for Nb3Sn cable-in-conduit conductors, coil case components, magnet support structures.
- Cable fabrication tooling. Mandrels, forming dies and fixtures that must track the thermal contraction of the conductor through the reaction cycle.
- Cryogenic structural hardware. Flanges, joint boxes, terminations and supports operating between 4 K and room temperature.
- Low-expansion tooling and metrology fixtures. Where dimensional stability across a temperature range matters more than corrosion resistance.
- Research and test rigs. One-off forged parts for magnet test facilities and cryostats, where open-die forging avoids die tooling cost.
For chemical processing, oil and gas, or general pressure-vessel service, alloy 908 is not suitable. Inconel 625, Incoloy 825 and Hastelloy C-276 are the usual grades for that service, and are also available as open-die forgings.
Supplier
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory located at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. The company produces open-die forgings and seamless rolled rings in carbon steel, alloy steel, tool steel, stainless steel and nickel-based superalloys, including Incoloy alloy 908 (UNS N09908).
For alloy 908 the company supplies forged rings, seamless rolled rings, discs, blanks, round bars, shafts, flanges, sleeves, bushings, hollow bars and tube sheets, forged to customer drawing with no minimum order quantity. Material chemistry follows ASTM B872. Mechanical properties, heat treatment condition and ultrasonic acceptance class are agreed on the order. Ultrasonic testing is carried out to EN 10228-3, SEP 1921 or ASTM A388, and forgings are certified to EN 10204 3.1, or 3.2 with third-party witness.
Enquiries: sales@steelforgepieces.com, +86-189-2135-9659, www.steelforgepieces.com. Quotations are returned within 24 hours of receiving a drawing or dimensions.
Frequently asked questions about Incoloy alloy 908
What is Incoloy alloy 908?
Incoloy alloy 908 (UNS N09908) is an age-hardenable, cobalt-free nickel-iron superalloy with a low coefficient of thermal expansion. It contains 47 to 51 % nickel, about 4 % chromium, 3 % niobium, 1.5 % titanium and 1 % aluminium, with the balance iron, and is strengthened by γ′ Ni3(Al,Ti) precipitation. The alloy was developed by MIT and the International Nickel Company as a conduit material for Nb3Sn cable-in-conduit superconductors operating near 4 K.
What is the UNS number for Incoloy 908?
N09908. It is also written as alloy 908 or nickel alloy 908. It is not the same material as Incoloy 27-7Mo (UNS S31277), a super-austenitic stainless steel that is sometimes listed alongside it in error.
Which standard covers Incoloy alloy 908?
ASTM B872 covers precipitation-hardening nickel alloy plate, sheet and strip including UNS N09908, and is the normal reference for chemistry.
There is no ASTM forging specification for N09908, as the grade is not listed in ASTM B564. Forgings are supplied to ASTM B872 chemistry, with mechanical properties, heat treatment and testing agreed on the purchase order or the project material specification. Send the project specification and we will quote against it directly.
What is Incoloy 908 used for?
Conduit and jacket material for Nb3Sn cable-in-conduit superconductors in fusion and high-field magnet programmes, tooling for fabricating those cables, cryogenic structural parts and magnet supports, and low-expansion tooling where the structure must track the thermal contraction of the superconductor.
Why does Incoloy 908 have a low coefficient of thermal expansion?
Its iron-nickel matrix lies in the Invar composition range, where magnetostriction offsets normal lattice expansion. Chromium is limited to about 4 % and cobalt is excluded to retain that behaviour, with strength supplied by γ′ precipitation instead. Because the effect is magnetic in origin, alloy 908 is not a low-permeability material. Confirm any magnetic requirement before ordering.
What is SAGBO, and why does it matter for alloy 908?
SAGBO is stress-accelerated grain boundary oxidation, in which oxygen attacks grain boundaries under stress at elevated temperature and embrittles the material. Alloy 908 was formulated with sufficient SAGBO resistance to permit hot fabrication without cracking, but the long reaction heat treatment near 650 °C still requires a low-oxygen or inert atmosphere with residual stress controlled. State the atmosphere and cycle on the enquiry so the forging and annealing route can be matched to it.
How strong is Incoloy 908 at 4.2 K?
In the solution-annealed and aged condition, published data give an average 0.2 % offset yield strength of about 1200 MPa at 4.2 K, fracture toughness KIC above 200 MPa·√m, and fatigue crack growth rates about three times lower at 4.2 K than at room temperature. At room temperature the aged alloy gives about 1172 MPa tensile strength and 827 MPa yield strength.
Which forged shapes can be made in alloy 908?
Seamless rolled rings, forged rings, discs and blanks, round bars, shafts and spindles, forged flanges, sleeves, bushings, hollow bars, tube sheets and near-net shapes forged to drawing. Open-die forging requires no die tooling, so single pieces and small quantities are economical, which is how alloy 908 is normally ordered.
What certification and testing comes with the forgings?
EN 10204 3.1 as standard, or 3.2 with third-party witness on request. Ultrasonic testing to EN 10228-3, SEP 1921 or ASTM A388 at the specified acceptance class. Chemical analysis, room-temperature tensile, hardness and grain size are reported as standard. Cryogenic tensile and fracture toughness testing can be subcontracted to an accredited laboratory when the project specification requires it.
What information is needed to quote?
A drawing or the finished dimensions, the quantity, the delivery condition (as-forged, solution annealed, or annealed and aged), the governing or project specification, the ultrasonic acceptance class, and the certification level. Send it to sales@steelforgepieces.com or WhatsApp +86-189-2135-9659. Quotations are returned within 24 hours.
References
Property data on this page is compiled from the sources below. Where a project specification differs, the specification governs.
- Special Metals Corporation, INCOLOY alloy technical bulletins.
- ASTM International, ASTM B872, Standard Specification for Precipitation-Hardening Nickel Alloys Plate, Sheet, and Strip.
- MIT Plasma Fusion Center, PFC/RR-93-2, Incoloy 908 Database Report.
- L. S. Toma et al., Mechanical Properties of Incoloy 908, An Update, Advances in Cryogenic Engineering Materials.
- AZoM, INCOLOY Alloy 908 (UNS N09908).
Last reviewed 18 August 2026, technical sales, Jiangyin Jiangnan Metal Co., Ltd.