What UNS S31277 is
S31277 was developed as a cheaper option than the nickel-base corrosion-resistant alloys for plant that has to handle several aggressive media.
It sits between the 6% molybdenum super-austenitic stainless steels and the nickel-chromium-molybdenum alloys. Molybdenum is taken up to roughly 7.2% and nitrogen to roughly 0.34%. Nitrogen is a strong austenite stabiliser, so it does the work of part of the nickel that would otherwise be needed to hold an austenitic structure, and it raises strength at the same time. Chromium handles oxidising media, nickel and molybdenum handle reducing media, and the three together handle pitting and crevice attack.
The S prefix in the UNS number matters when the purchase order is written. S31277 is a stainless steel, because iron is the balance of the analysis at roughly 37 to 40%. Forgings are ordered against ASTM A-series specifications, usually ASTM A182 for flanges and forged fittings or ASTM A479 for bar and forging stock. Many supplier sites file it with the nickel alloys, this one included, because that is where buyers look for it.
Naming. Enquiries come in as UNS S31277, S31277, alloy 27-7MO, 27-7Mo and INCOLOY® alloy 27-7MO. INCOLOY is a trademark of the Special Metals Corporation group of companies. The grade itself is fixed by the UNS number and the ASTM composition limits, and can be melted by any qualified mill.
Chemical composition
Composition limits in weight percent. Every heat we forge is supplied with the ladle analysis, and with a product analysis taken from the forging itself if the order asks for one.
| Element | Specified limit | Nominal | Role in the alloy |
|---|---|---|---|
| Carbon (C) | 0.020 max | < 0.02 | Held low to avoid chromium carbide precipitation at grain boundaries |
| Manganese (Mn) | 3.00 max | 2.5 | Raises nitrogen solubility in the melt |
| Phosphorus (P) | 0.030 max | – | Residual, held down for hot workability |
| Sulfur (S) | 0.010 max | – | Residual, held down for hot workability and toughness |
| Silicon (Si) | 0.50 max | 0.3 | Deoxidiser, limited to restrain sigma phase |
| Chromium (Cr) | 20.5 – 23.0 | 22.0 | Passive film former; resistance to oxidising media |
| Nickel (Ni) | 26.0 – 28.0 | 27.0 | Austenite stability and chloride SCC resistance |
| Molybdenum (Mo) | 6.5 – 8.0 | 7.2 | Pitting and crevice corrosion resistance; reducing acids |
| Nitrogen (N) | 0.30 – 0.40 | 0.34 | Austenite stabiliser, strength, pitting resistance |
| Copper (Cu) | 0.50 – 1.50 | 1.0 | Resistance to sulfuric and other reducing acids |
| Iron (Fe) | Balance | ≈ 38 | Base element, which is why the grade counts as a stainless steel |
PREN from the typical analysis is 22.0 + 3.3 × 7.2 + 16 × 0.34 = 51.2. Worked out on the specified minima of Cr 20.5, Mo 6.5 and N 0.30 it comes to 46.8, so even a heat at the bottom of the range still beats a typical 6% Mo grade.
Mechanical properties
Typical room-temperature values in the solution annealed condition. Acceptance values for an order come from the product standard and the purchase specification. The tested values for the heat supplied appear on the mill certificate.
| Property | Metric | Imperial |
|---|---|---|
| Tensile strength, Rm | ≈ 827 MPa | ≈ 120 ksi |
| Yield strength, Rp0.2 | ≈ 414 MPa | ≈ 60 ksi |
| Elongation, A5 | ≈ 50% | ≈ 50% |
| Hardness | ≈ 95 HRB | ≈ 95 HRB |
| Density | ≈ 8.0 g/cm³ | ≈ 0.289 lb/in³ |
| Solution anneal | 1121 – 1175 °C, rapid quench | 2050 – 2150 °F, rapid quench |
Yield strength is roughly double that of a standard 300-series austenitic grade, which comes from the 0.34% nitrogen. Wall thickness and part weight can often be cut to suit, and that recovers some of the alloy premium.
On heat treatment. Super-austenitic grades at this alloy content precipitate sigma and chi phases if they cool slowly through roughly 600 to 1000 °C, and both toughness and corrosion resistance go with them. Quench capacity has to suit the section thickness of the forging. For heavy sections, use a shop that runs this grade regularly.
Corrosion behaviour
Alloy 27-7MO was developed for process plant where one material has to cope with several different aggressive media. Published work on the grade reports resistance to the common process acids, hydrochloric, sulfuric and phosphoric, together with good resistance to pitting and crevice corrosion in chlorides.
Oxidising media
The 22% chromium holds a stable passive film in nitric acid, mixed acids and oxidising salt solutions. Nickel alloys with more molybdenum but less chromium are weaker here.
Reducing media
Nickel at 27%, molybdenum at 7.2% and around 1% copper give useful resistance in dilute sulfuric and phosphoric acid service, including phosphoric acid carrying chloride and fluoride contamination.
Chlorides
A PREN of about 51 puts the critical pitting temperature well above that of 6% Mo grades. The high nickel content also improves resistance to chloride stress-corrosion cracking, which is what limits 316L and 904L in hot chloride service.
Testing. On request we run ASTM G48 Method A or C ferric chloride pitting and crevice tests at a temperature the customer nominates, and ASTM A262 Practice E intergranular corrosion tests. Samples are taken from the forging after final heat treatment and the results go on the certificate.
Comparison with other corrosion-resistant grades
The question is usually whether to move up from a 6% Mo stainless or down from a nickel alloy. Typical figures:
| Grade | UNS | Ni % | Cr % | Mo % | N % | PREN |
|---|---|---|---|---|---|---|
| 316L | S31603 | 12 | 17 | 2.1 | 0.05 | 25 |
| 904L | N08904 | 25 | 20 | 4.3 | 0.05 | 35 |
| 254 SMO | S31254 | 18 | 20 | 6.1 | 0.20 | 43 |
| AL-6XN | N08367 | 24 | 21 | 6.3 | 0.22 | 45 |
| Alloy 27-7MO | S31277 | 27 | 22 | 7.2 | 0.34 | 51 |
| Alloy 625 | N06625 | 61 | 21.5 | 9.0 | – | 51 |
| Alloy C-276 | N10276 | 57 | 16 | 16 | – | 75 |
Figures are typical and are given for orientation only. Alloy 625 also carries about 3.6% niobium, which the standard PREN expression takes no account of. Material selection has to be made by the specifying engineer against the actual medium, temperature, chloride concentration and pH.
Against 254 SMO and AL-6XN
S31277 carries roughly one percent more molybdenum and half again as much nitrogen, which takes PREN from the low-to-mid forties up to about 51. Nickel is also much higher than in 254 SMO, 27% against 18%. That matters where the governing risk is chloride stress-corrosion cracking rather than pitting.
Against alloy 625 and C-276
In many environments S31277 gets close to alloy 625 and alloy 22 on well under half the nickel, which is why the grade exists. It does not replace C-276, which has 16% molybdenum and a PREN of about 75 and belongs a class above for hot reducing acids and severe chloride service. Where an existing specification calls for C-276, treat a proposed change to S31277 as a substitution that needs corrosion data for that exact service.
We forge Inconel 625, Hastelloy C-276, Hastelloy C-22, Incoloy 926 and Incoloy 825 in the same shop, so we can price alternatives against the same drawing.
Forged product forms in UNS S31277
All parts are made to customer drawing, or to finished dimensions plus an agreed machining allowance. Rings are rolled on a radial-axial ring mill. Solid and stepped parts are open-die forged under hydraulic press.
- Seamless rolled ringsRolled, with grain flow round the circumference
- Forged flangesWeld neck, blind, slip-on, lap joint, orifice
- Forged shaftsStraight, stepped, eccentric, hollow-bored
- Forged discs and blanksUpset discs, pancakes, blanks for machining
- Forged sleeves and bushingsBored, trepanned, mandrel-forged
- Tube sheetsHeat exchanger and condenser tube sheets
- Round bars and billetsForged and peeled or turned
- Valve componentsBodies, bonnets, seat rings, stems, balls
- Forged tubes and pipesHollow forged, bored, heavy wall
- Blocks and rectangular forgingsManifolds, nozzles, custom sections
Forging size range for UNS S31277
| Form | Size range | Notes |
|---|---|---|
| Seamless rolled rings | OD 200 – 3000 mm, height up to 800 mm | Wall from 30 mm; rectangular or profiled section |
| Forged flanges | DN 15 – DN 2000 | ASME B16.5, B16.47, EN 1092-1 or to drawing |
| Shafts and stepped bars | Ø 60 – 800 mm, up to 6000 mm long | Straightness and run-out to agreed tolerance |
| Discs and blanks | Ø up to 1500 mm | Upset forged for radial grain flow |
| Sleeves and bushings | OD up to 1200 mm | Bored or trepanned from solid |
| Single piece weight | 5 kg – 8000 kg | Heavier parts quoted on enquiry |
| Minimum order | 1 piece | Prototypes and spare parts accepted |
| Typical lead time | 25 – 45 days | From drawing approval, material in stock |
If a part falls outside this range, send the drawing anyway. Oversized geometries can often be split into forged segments, or quoted as a machined-from-solid alternative.
Manufacturing route
Super-austenitic grades have a narrow hot working window and work-harden quickly. The route below is followed on every heat.
- Melting. Electric arc furnace with AOD or VOD refining, then electroslag remelting. ESR is what brings the sulfur, phosphorus and inclusion level down far enough for the grade to work hot, and it gives the sound centre a heavy forging needs.
- Ingot inspection. Ingots are conditioned and checked before the first heat, so that surface defects do not get folded into the part.
- Open-die forging. Worked from a controlled start temperature down to a set finishing temperature. Reduction ratio is chosen to break down the as-cast structure and to run grain flow along the main stress direction of the part.
- Ring rolling. Rings are pierced, then rolled radially and axially so grain flow follows the circumference. This is why a rolled ring performs better than a ring cut from plate.
- Solution annealing. Held at 1121 to 1175 °C, then quenched fast enough to get through the sigma and chi precipitation range clean. Furnace charts are recorded and issued with the certificate.
- Rough machining. Scale and the affected surface layer are taken off and the part is brought to the size needed for volumetric inspection.
- Non-destructive testing. Ultrasonic inspection to the ordered class, with dye penetrant on machined surfaces where the order calls for it.
- Certification and dispatch. Mill certificate issued, part marked with heat number and order reference, then packed for sea or air freight.
Testing and certification
| Test | Standard | Supplied |
|---|---|---|
| Chemical analysis | Ladle and, on request, product analysis | Every heat |
| Tensile and hardness | ASTM A370 / ISO 6892-1 | Every heat lot |
| Ultrasonic testing | EN 10228-3, SEP 1921, ASTM A388 | Class to order |
| Dye penetrant | ASTM E165 / EN ISO 3452 | On request |
| Intergranular corrosion | ASTM A262 Practice E | On request |
| Pitting and crevice corrosion | ASTM G48 Method A or C | On request, temperature nominated |
| Positive material identification | Portable XRF or OES | On request |
| Grain size and microstructure | ASTM E112 | On request |
| Material certificate | EN 10204 3.1 | Every order |
| Third-party certificate | EN 10204 3.2, by TUV, BV, SGS, DNV or Lloyd's Register | On request |
Third-party witness inspection at the works is welcome. It is arranged when the order is placed, so hold points are agreed before forging starts.
Applications
Published application areas for alloy 27-7MO include pollution control, power generation, marine, chemical processing, pulp and paper, and oil and gas. The parts we are asked for most often in this grade:
Chemical and process plant
Pressure vessel flanges, nozzles, tube sheets and shell rings for shell-and-tube heat exchangers. Columns, towers, reactors and preheaters. Forged shafts and impeller hubs for chemical and plunger pumps on acid duty.
Flue gas desulfurisation and pollution control
Scrubber components, absorber internals, damper shafts and duct flanges, where hot chloride-bearing condensate attacks lower-alloy stainless steels.
Oil and gas
Wellhead and Christmas tree components, valve bodies, bonnets, seat rings and stems, subsea and deepwater production hardware, and downhole sand-control components that need both the strength and the corrosion resistance.
Pulp, paper and marine
Bleach plant equipment, digester and washer components, seawater handling and desalination hardware, and forged parts for shipbuilding and offshore structures.
Power and general machinery
Turbine and compressor rings, gearbox and power-transmission components, rolls, wheels, manifolds and crankshafts for mills and mixers.
Information needed for a quotation
Six items let us price the job without a round of clarification emails.
- Finished geometry. A drawing, or finished dimensions plus the machining allowance wanted on each surface. For rings, give OD, ID and height.
- Product standard. ASTM A182 for flanges and forged fittings, ASTM A479 for bar and forging stock, plus any ASME Section II or client specification that applies.
- Delivery condition. Solution annealed and quenched is standard. Say so if a different condition or a simulated post-weld heat treatment applies.
- Tests. Ultrasonic class, corrosion tests and their test temperature, PMI, impact requirements.
- Certification level. EN 10204 3.1 from the works, or 3.2 countersigned by a nominated third party.
- Quantity and destination. Piece count, required date and destination port, so lead time and freight can be priced with the material.
Send it to sales@steelforgepieces.com. PDF, STEP and DWG are all fine, and so is a dimensioned photograph of the old part.
Frequently asked questions
What is UNS S31277?
UNS S31277, sold commercially as alloy 27-7MO, is a 7% molybdenum super-austenitic stainless steel. A typical analysis is 27% nickel, 22% chromium, 7.2% molybdenum, 0.34% nitrogen and about 1% copper, with iron as the balance. Nitrogen stabilises the austenite and raises strength, which lets the alloy reach a PREN of roughly 51 on much less nickel than a nickel-base corrosion-resistant alloy needs.
Is UNS S31277 a stainless steel or a nickel alloy?
It is a stainless steel. The S prefix in the UNS number identifies it as stainless, and iron is the balance of the analysis at roughly 37 to 40%. Suppliers often list it with nickel alloys because of its 27% nickel content, but it is ordered against ASTM A-series specifications such as A182, A240 and A479, not against the nickel alloy standards.
What is the PREN of UNS S31277?
About 51. Using PREN = %Cr + 3.3(%Mo + 0.5%W) + 16%N and a typical analysis, 22 + 3.3 × 7.2 + 16 × 0.34 = 51.2. Compare 254 SMO at about 43 and AL-6XN at about 45. Nickel-base alloy 625 works out at about the same figure.
Which standards cover UNS S31277 forgings?
S31277 appears in ASTM A182 for forged flanges and fittings, ASTM A479 for bar and forging stock, ASTM A240 for plate, sheet and strip, and ASTM A312 and A249 for pipe and tube. Forgings are inspected to EN 10228-3, SEP 1921 or ASTM A388 for ultrasonic testing and certified to EN 10204 3.1 or 3.2. ASTM B564 covers nickel alloy forgings and does not apply to this grade, although some catalogues still quote it.
How does UNS S31277 compare with 254 SMO and AL-6XN?
S31277 runs about 7.2% molybdenum against roughly 6.1% in 254 SMO (S31254) and 6.3% in AL-6XN (N08367), and about 0.34% nitrogen against 0.20 to 0.22%. PREN goes from the low-to-mid forties up to roughly 51, with better resistance to pitting, crevice corrosion and reducing acids. Nickel is also higher at 27% against 18% and 24%, which helps in chloride stress-corrosion cracking service.
Can UNS S31277 replace Inconel 625 or Hastelloy C-276?
In many process environments it can, at lower cost, because its corrosion resistance comes close to alloy 625 and alloy 22. It does not replace C-276, which has 16% molybdenum and a PREN of about 75 and sits a class above for hot reducing acids and severe chloride service. Any substitution has to be approved by the specifying engineer against the actual medium, temperature, chloride level and pH.
What is the solution annealing temperature for UNS S31277?
Solution anneal between 1121 and 1175 °C (2050 to 2150 °F), then quench fast. Super-austenitic grades at this alloy content will precipitate sigma and chi phases if they cool slowly through roughly 600 to 1000 °C, so quench capacity has to suit the section thickness of the forging.
What sizes of UNS S31277 forgings can be produced?
Jiangyin Jiangnan Metal Co., Ltd. produces UNS S31277 seamless rolled rings up to about 3000 mm outside diameter, forged discs and blocks up to about 1500 mm, shafts up to about 6000 mm long, and single pieces up to about 8 tonnes. Check the exact size range for a given part with the works before ordering.
Is UNS S31277 suitable for sour service?
It is used in oil and gas service including downhole sand-control products, and slow strain rate and U-bend test data on the grade has been published. Acceptance for a given sour application has to be checked against the NACE MR0175 / ISO 15156 Annex A limits for hydrogen sulfide partial pressure, chloride concentration, temperature and pH, and confirmed by the project specification.
What certification is supplied with the forgings?
Every heat is supplied with an EN 10204 3.1 mill certificate issued by the works, covering heat number, chemical analysis, heat treatment record, mechanical test results and NDT results. EN 10204 3.2 certificates countersigned by TUV, BV, SGS, DNV or Lloyd's Register are available where the order calls for third-party witness.
What is the minimum order quantity and the lead time?
Single-piece orders are accepted, including prototypes and spares. Lead time is typically 25 to 45 days from drawing approval when raw material is in stock, and longer when a dedicated ESR heat has to be melted for the order.
Who manufactures UNS S31277 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 forges UNS S31277 rings, flanges, shafts, discs, sleeves and tube sheets to customer drawings. Enquiries go to sales@steelforgepieces.com or +86-189-2135-9659.
Cite this page
This datasheet is published by the works that forges the grade. It may be quoted or cited with attribution.
Jiangyin Jiangnan Metal Co., Ltd. “UNS S31277 forgings (alloy 27-7MO): composition, properties and ordering data.” steelforgepieces.com, updated 1 September 2026. Open-die forging factory, No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. Contact: sales@steelforgepieces.com, +86-189-2135-9659.
Figures on this page come from published data on alloy 27-7MO and from our own works records. Typical values are marked as such. The values that apply to a delivered part are the ones on its mill certificate. Corrections and additional data are welcome at sales@steelforgepieces.com.