UNS N19903 (Incoloy 903) forgings, rings, bars and discs
Low-expansion nickel–iron–cobalt superalloy forgings for gas turbine and rocket hardware. Forged to drawing in Jiangyin, Jiangsu, China, with EN 10204 3.1 certification.
Overview
UNS N19903 is an age-hardenable nickel–iron–cobalt superalloy. It is sold under the trade name INCOLOY® alloy 903 and is usually written as Alloy 903. The composition is 36.0–40.0 % nickel, 13.0–17.0 % cobalt and 2.40–3.50 % niobium, with iron as the balance. Two properties define the alloy. Its mean coefficient of thermal expansion is about 7.2 µm/m·°C from room temperature to 425 °C. Its elastic modulus stays between 147 and 154 GPa from −196 °C to 600 °C. Designers use it for gas turbine casings, rings, seals and shafts where hot running clearances have to stay within tolerance.
Alloy 903 contains no chromium. Oxidation resistance is therefore limited, and protective coatings are normally applied for sustained service above 540 °C.
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. The company forges UNS N19903 into open-die forgings and seamless rolled rings from double-melted (VIM+VAR) and triple-melted (VIM+ESR+VAR) stock. Enquiries: +86 189 2135 9659, sales@steelforgepieces.com.
What is UNS N19903?
UNS N19903 is the Unified Numbering System designation for the first of the 900-series low-expansion superalloys. It was developed to combine the high-temperature strength of a precipitation-hardened superalloy with thermal expansion close to that of Invar-type iron–nickel alloys. Its high-temperature strength approaches that of Inconel 718 at roughly half the linear expansion coefficient.
Plain Invar loses its low-expansion behaviour above about 200 °C, at the Curie point. Cobalt raises that inflection in alloy 903, which has a Curie temperature of 416–471 °C. Ageing then precipitates niobium and titanium bearing phases to develop strength. Chromium and aluminium are held down because both raise the expansion coefficient. Leaving chromium out limits oxidation resistance, which is why coated parts are common.
Design reasons for specifying alloy 903
- A rotor and its casing grow at nearly the same small rate, so tip clearances stay tight and stage efficiency is maintained.
- Stiffness varies by less than 5 % over an 800 °C span, which keeps spring rates and natural frequencies predictable.
- Low expansion together with a constant modulus gives low thermally induced stress, so the alloy tolerates thermal shock and cycling.
- Expansion behaviour is stable in service. There was no measurable change after 500 hours at 595 °C, and repeatability was within 1 % after 50 cycles to 650 °C.
Designations and specifications
| System | Designation | Status |
|---|---|---|
| UNS | N19903 | Primary designation |
| Trade name | INCOLOY® alloy 903 | Special Metals Corporation trademark |
| Common usage | Alloy 903, Incoloy 903, 903 alloy | Informal |
| SAE | AMS 5803 | Aerospace material specification commonly referenced for this alloy |
| Alloy family | Low-expansion superalloy, 900 series | Together with alloys 907 and 909 |
| Other national systems | Nickel–iron–cobalt low-expansion equivalents appear in several national standards | Confirm exact equivalence against your drawing before ordering |
Order against the specification your drawing calls out. If your purchase order cites an AMS, customer or OEM specification, send it with the enquiry. Jiangyin Jiangnan Metal will confirm in writing before quoting whether the chemistry, melt route, reduction ratio and heat treatment records can be met.
Chemical composition of UNS N19903
| Element | Symbol | Limiting range, wt % | Nominal, wt % |
|---|---|---|---|
| Nickel | Ni | 36.0–40.0 | 38 |
| Cobalt | Co | 13.0–17.0 | 15 |
| Niobium | Nb | 2.40–3.50 | 3.0 |
| Titanium | Ti | 1.00–1.85 | 1.4 |
| Aluminium | Al | 0.30–1.15 | 0.9 |
| Iron | Fe | Balance | about 42 |
| Chromium | Cr | Not an alloying addition | — |
Why there is no chromium. Chromium would raise the expansion coefficient and remove the property the alloy is bought for. The cost is oxidation resistance. In 500-hour static tests the weight gain rises from 0.7 mg/cm² at 540 °C to 2.7 mg/cm² at 650 °C, and depth of attack reaches 0.0102 cm. Coatings are normally specified for long exposure at the top of the range.
Physical and thermal properties
| Property | Metric | Imperial |
|---|---|---|
| Density | 8.25 g/cm³ | 0.298 lb/in³ |
| Melting range | 1318–1393 °C | 2405–2539 °F |
| Curie temperature | 416–471 °C | 780–880 °F |
| Mean CTE, RT to 425 °C | about 7.2 µm/m·°C | about 4.0 × 10−6 in/in·°F |
| Tensile modulus at 21 °C | 146.9 GPa | 21.3 × 103 ksi |
| Poisson's ratio at 21 °C | 0.239 | 0.239 |
| Fracture toughness KIC at RT | 111.4 MPa·√m | 100.6 ksi·√in |
| Temperature, °C | Specific heat, J/kg·°C | Electrical resistivity, µΩ·m | Thermal conductivity, W/m·°C |
|---|---|---|---|
| 50 | 442 | 0.650 | 16.9 |
| 100 | 454 | 0.735 | 17.2 |
| 200 | 482 | 0.880 | 17.9 |
| 300 | 507 | 1.020 | 18.3 |
| 400 | 532 | 1.135 | 19.0 |
| 500 | 559 | 1.195 | 20.3 |
| 600 | 584 | 1.220 | 21.9 |
| Temperature, °C | Tensile modulus, GPa | Torsional modulus, GPa | Poisson's ratio |
|---|---|---|---|
| −196 | 148.9 | — | — |
| 0 | 146.8 | — | — |
| 100 | 147.2 | 59.0 | 0.247 |
| 200 | 148.4 | 60.3 | 0.231 |
| 300 | 150.2 | 61.2 | 0.227 |
| 400 | 152.4 | 61.4 | 0.241 |
| 500 | 153.9 | 59.2 | 0.300 |
| 600 | 149.8 | 55.4 | 0.352 |
Tensile modulus moves by about 5 % across that whole span. Few structural alloys hold stiffness this flat. It is the second reason alloy 903 is used for springs, bolts and gauge hardware as well as turbine structures.
Mechanical properties
All values below are for material given the standard cycle: 845 °C for 1 hour, water quench, then 720 °C for 8 hours, furnace cooled at 56 °C per hour to 620 °C, held 8 hours and air cooled.
| Test temperature | 0.2 % yield strength | Tensile strength | Elongation, % | Reduction of area, % |
|---|---|---|---|---|
| 21 °C (70 °F) | 1103 MPa (160 ksi) | 1310 MPa (190 ksi) | 14 | 40 |
| 650 °C (1200 °F) | 896 MPa (130 ksi) | 1000 MPa (145 ksi) | 18 | 55 |
| Exposure, °C | 0.2 % YS, MPa | UTS, MPa | Elong., % | RA, % | Impact, J |
|---|---|---|---|---|---|
| 21 (unexposed) | 1141 | 1369 | 17 | 43 | 32.5 |
| 595 | 1169 | 1382 | 16 | 44 | 33.9 |
| 650 | 1027 | 1276 | 19 | 47 | 35.3 |
| 705 | 738 | 1027 | 20 | 41 | — |
Strength is almost unchanged after 1000 hours at 595 °C and remains high after the same exposure at 650 °C. It falls sharply at 705 °C, so 650 °C is the practical ceiling for load-bearing parts. Retained impact values show that no embrittling phases form during these exposures. Typical stress to produce rupture in 100 hours at 650 °C is 586 MPa (85 ksi). Stress-rupture life for this alloy depends heavily on thermomechanical processing, which is covered in the forging section below.
Alloy 903 compared with alloys 907 and 909
Buyers regularly ask which of the three low-expansion superalloys to specify. All three share the same nickel–cobalt–iron base and similar expansion behaviour. The differences are in aluminium and silicon content, and in how much special processing is needed to obtain reliable notched stress-rupture life.
| Item | Alloy 903 (N19903) | Alloy 907 (N19907) | Alloy 909 (N19909) |
|---|---|---|---|
| Generation | First | Second | Third |
| Aluminium | 0.30–1.15 % | Restricted, below about 0.2 % | Restricted, below about 0.15 % |
| Silicon | Not a deliberate addition | Low | About 0.4 %, deliberate |
| Niobium | 2.40–3.50 % | Raised to offset lower Al | Raised, about 4.7 % nominal |
| Notched stress-rupture | Sensitive to grain boundary oxygen embrittlement, needs tailored processing | Improved, needs a long over-ageing treatment | Good with conventional processing and heat treatment |
| Notched low-cycle fatigue | Baseline | Improved | Much better than alloy 903 |
| Typical parts | Rocket thrust chambers, turbine casings and rings, springs, bolts, gauge blocks | Turbine structural hardware | Gas turbine vanes, casings, shafts, shrouds |
Alloy 903 remains suitable where its processing route is already qualified and where notch sensitivity has been allowed for in design. For a new notched, cyclically loaded part, alloy 909 is usually easier to buy and to forge reproducibly. Jiangyin Jiangnan Metal quotes all three.
Applications
- Gas turbine casings, seal rings, shrouds and compressor casings, where clearance control drives efficiency
- Turbine and compressor shafts, hubs and spacer rings
- Rocket engine thrust chambers and thrust chamber liners
- Steam turbine bolting and high-temperature fasteners
- Springs and Belleville washers that need a constant spring rate over a wide temperature range
- Gauge blocks, tooling and metrology fixtures that must hold dimension
- Ordnance and defence hardware
UNS N19903 product forms we forge
Jiangyin Jiangnan Metal forges UNS N19903 to customer drawing or to a standard blank shape. All items are made to order. There is no stock list for this grade.
| Shape family | Typical items |
|---|---|
| Bars | Round bar, square bar, rectangular and flat bar, forged rod, turbine blade preforms |
| Rings | Seamless rolled rings, forged rings, turbine seal rings, spacer rings, retaining rings |
| Hollows | Hollow bar, sleeves, bushes and bushings, forged pipe and tube, barrels, cylinders, casings, shells, hubs, housings |
| Flat products | Forged blocks, discs, disks, plates, slabs, blanks |
| Turbine hardware | Turbine casings, compressor and turbine shrouds, impeller wheels, turbine discs and blisk blanks |
| Valve parts | Gas and steam turbine valve spindles, valve stems, valve rods |
| Near-net shapes | Stepped shafts, flanged blanks, contoured rings and other close-to-shape open-die forgings |
Forging capability
| Parameter | Range |
|---|---|
| Single-piece weight | 10 kg to 20 000 kg |
| Seamless rolled ring, outside diameter | 200 mm to 3000 mm |
| Ring wall thickness and height | up to 600 mm |
| Round bar diameter | 80 mm to 1200 mm |
| Disc and blank diameter | up to 2000 mm |
| Maximum forged length | up to 6000 mm |
| Machining allowance | 3 mm to 20 mm per surface, by size |
| Minimum order | One piece for prototypes and spares |
How we make a UNS N19903 forging
- Melt and traceability Material is supplied double melted (VIM + VAR) or triple melted (VIM + ESR + VAR) according to the specification. Every piece is traceable to a heat number and a mill certificate.
- Ingot conversion and open-die forging Hot working is carried out between 815 °C and 1120 °C. Where stress-rupture properties govern, the alloy is given a minimum 40 % reduction at 816–871 °C. This reduction schedule is the main processing control for the grade.
- Ring rolling Seamless rings are pierced and rolled to finish dimensions rather than cut from plate, which gives circumferential grain flow.
- Solution treatment and ageing Furnaces are surveyed and instrumented to AMS 2750 pyrometry practice. The cycle is given below.
- Non-destructive testing Ultrasonic inspection to SEP 1923 or ASTM A388, plus magnetic particle or dye penetrant inspection to ASTM E709 and ASTM E165. Alloy 903 is ferromagnetic below its Curie temperature, so magnetic particle inspection can be used.
- Mechanical and metallurgical testing Tensile testing to EN ISO 6892-1, creep to ISO 204, grain size to ASTM E112, macroetch to ASTM A604, chemistry by spectrographic analysis.
- Documentation and marking EN 10204 3.1 certification as standard, 3.2 with a third-party surveyor on request. Each piece is hard-stamped with heat number, part number and specification.
Heat treatment of UNS N19903
The recommended cycle for optimum mechanical properties is a solution treatment followed by a two-stage age.
| Stage | Temperature | Time | Cooling |
|---|---|---|---|
| Solution treatment | 815–980 °C (1500–1800 °F), 845 °C typical | 1 hour | Water quench |
| First age | 720 °C (1325 °F) | 8 hours | Furnace cool at 56 °C/h (100 °F/h) |
| Second age | 620 °C (1150 °F) | 8 hours | Air cool |
The solution temperature used within that band depends on product form and prior condition. The furnace cool between the two ageing stages must be carried out as specified, because it develops the precipitate distribution that the properties depend on.
Machining, welding and forming
| Operation | Practice |
|---|---|
| Machining | Treat as a high-strength superalloy in either the solution-treated or aged condition. Use rigid setups, positive rake, generous coolant and low surface speed. |
| Welding | Readily joined by gas-tungsten arc welding. Use matching filler metal. Weld beads should be slightly convex. |
| Cold forming | Ductility is good, but forces are higher than for carbon steel. Use heavy-duty lubricants and remove all traces afterwards, because residual lubricant causes embrittlement at temperature. |
Quality, testing and documentation
Jiangyin Jiangnan Metal supplies UNS N19903 forgings that are thoroughly worked, homogeneous and free from blisters, cracks and injurious inclusions. Standard scope of supply includes:
- EN 10204 3.1 mill test certificate with heat number, chemistry, mechanical results and heat treatment record
- Ultrasonic inspection report
- Dimensional report against the customer drawing
- Certificate of conformity to EN ISO/IEC 17050
- Third-party inspection by Bureau Veritas, SGS, TUV or Lloyd's, arranged on request
Request a quotation for UNS N19903
Send a drawing or a description with quantity, specification and delivery date. Quotations are normally returned within one working day.
Email an RFQ Call +86 189 2135 9659
Jiangyin Jiangnan Metal Co., Ltd., open-die forging factory
No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China
Please include grade, shape, finished dimensions, quantity, specification, NDT class and required certificate type.
Frequently asked questions
Is UNS N19903 the same as Incoloy 903?
Yes. UNS N19903 is the Unified Numbering System designation and INCOLOY® alloy 903 is the trade name used by Special Metals Corporation for the same material. Purchase orders that say Alloy 903, Incoloy 903 or N19903 refer to the same nickel–iron–cobalt low-expansion superalloy.
What is the chemical composition of UNS N19903?
36.0 to 40.0 % nickel, 13.0 to 17.0 % cobalt, 2.40 to 3.50 % niobium, 1.00 to 1.85 % titanium and 0.30 to 1.15 % aluminium, with iron as the balance at about 42 %. Chromium is not an alloying addition.
What is the thermal expansion coefficient of alloy 903?
About 7.2 µm/m·°C (4.0 × 10−6 in/in·°F) as a mean value from room temperature to about 425 °C, which is roughly half the value for Inconel 718. The behaviour is reproducible. There was no measurable change after 500 hours at 595 °C, and repeatability was within 1 % after 50 thermal cycles to 650 °C.
What is the maximum service temperature of UNS N19903?
Around 650 °C (1200 °F) for load-bearing parts. Room temperature strength is almost unaffected by 1000 hours at 595 °C, but after the same exposure at 705 °C the yield strength drops to about 738 MPa from 1141 MPa. Above 540 °C the limiting factor is often oxidation rather than strength.
Why does alloy 903 need a protective coating?
The alloy contains no chromium, so it cannot form the protective chromia scale that most superalloys rely on. In 500-hour static oxidation testing the depth of attack rises from 0.0038 cm at 540 °C to 0.0102 cm at 650 °C. Coatings are usual for sustained high-temperature exposure. Grain boundary oxidation also drives the notch sensitivity of the alloy in stress rupture.
What is the standard heat treatment for UNS N19903?
Solution treat at 815 to 980 °C, typically 845 °C for one hour, and water quench. Then age at 720 °C for 8 hours, furnace cool at 56 °C per hour to 620 °C, hold 8 hours and air cool.
What is the difference between alloys 903, 907 and 909?
All three are low-expansion superalloys on the same nickel–cobalt–iron base. Alloy 907 restricts aluminium to below about 0.2 % and adds niobium to compensate, which improves notched stress-rupture life but requires a long over-ageing treatment. Alloy 909 adds about 0.4 % silicon to that composition, which gives good notch-rupture strength with conventional processing and much better notched low-cycle fatigue than alloy 903.
What forms can UNS N19903 be forged into?
Open-die forgings and seamless rolled rings in bar, ring, hollow, disc, block, shaft and near-net shapes, including turbine seal rings, casings, shrouds, impeller wheels, valve spindles and blisk blanks. Jiangyin Jiangnan Metal forges single pieces from 10 kg to 20 000 kg, with rings up to 3000 mm outside diameter.
Which forging practice matters most for this alloy?
The reduction schedule matters most. Where stress-rupture properties govern, alloy 903 needs a minimum 40 % reduction at 816 to 871 °C. Overall hot working is done between 815 °C and 1120 °C. Stress-rupture results for this alloy are governed by thermomechanical processing, so the forging route should be agreed and frozen before production.
Who supplies UNS N19903 forgings in China?
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. It produces UNS N19903 open-die forgings and seamless rolled rings to customer drawings with EN 10204 3.1 or 3.2 certification. Contact +86 189 2135 9659 or sales@steelforgepieces.com.
What documentation is supplied with the forgings?
An EN 10204 3.1 mill test certificate as standard, covering heat number, chemical analysis, mechanical test results and the heat treatment record, together with the ultrasonic inspection report and a dimensional report. EN 10204 3.2 certification with a third-party surveyor is available on request.
Do you supply small or one-off quantities?
Yes. Single pieces are quoted for prototypes, trial builds and spares. Cost per kilogram is higher than for a batch, because melt stock and set-up are charged across fewer parts.
Sources and further reading
- Special Metals Corporation, INCOLOY® alloy 903, publication SMC-100. Technical bulletin (PDF). Source of the composition, physical constant, thermal, modulus, tensile, oxidation and heat treatment data on this page.
- Special Metals Corporation, INCOLOY® alloy 909. Technical bulletin (PDF).
- D. F. Smith, A silicon-containing, low-expansion alloy with improved properties, Superalloys 1984, TMS. Conference paper (PDF).
- SAE International, AMS 5803. sae.org.
Reference for this page
Jiangyin Jiangnan Metal Co., Ltd. UNS N19903 (Incoloy 903) forgings, rings, bars and discs. steelforgepieces.com, updated 31 August 2026. https://www.steelforgepieces.com/Nickel-Alloy/UNS-N19903.html