Super-austenitic stainless · 7 % molybdenum · Open-die forging
Incoloy Alloy 27-7Mo Forgings
UNS S31277 · Alloy 27-7MO · ASTM A479 / A182 · ASME Code Case 2458
Incoloy alloy 27-7Mo (UNS S31277) is a 7 % molybdenum, nitrogen alloyed super-austenitic stainless steel. Iron is the balance, so it is a stainless steel and not a nickel-base alloy. Its resistance to pitting and crevice corrosion is higher than the 6 % molybdenum grades, and in mixed acid and chloride service it is close to alloys 625 and C-276. Jiangyin Jiangnan Metal Co., Ltd., an open-die forging factory in Jiangyin, Jiangsu, China, forges 27-7Mo to drawing as seamless rolled rings, flanges, shafts, discs, sleeves and bars, with EN 10204 3.1 or 3.2 certification.
- UNS
- S31277
- Molybdenum
- 7.2% typical (6.5–8.0)
- Nitrogen
- 0.34% typical (0.30–0.40)
- PREN
- ~51Cr + 3.3Mo + 16N, calculated
- Yield
- 414MPa, solution annealed
What Incoloy alloy 27-7Mo is
Alloy 27-7Mo sits between the 6 % molybdenum super-austenitic stainless steels and the nickel-base corrosion alloys. S31254 and N08367 run short of margin in warm chloride and mixed acid service. Alloys 625, 622 and C-276 cover that service but cost several times more per kilogramme. Raising molybdenum to about 7.2 %, nitrogen to 0.34 % and nickel to 27 % produces a stainless steel that covers much of the same ground at a lower alloy price.
Each addition has a defined job. Nickel and nitrogen hold the structure fully austenitic, with no ferrite or martensite. Molybdenum and nitrogen carry the pitting and crevice resistance. Chromium covers oxidising media. The copper addition of 0.5 to 1.5 % improves behaviour in reducing acids such as sulfuric. Because both mechanisms are covered, the grade is used in mixed acid duty where alloys tuned to one mechanism fail.
Nitrogen also raises strength. Annealed yield strength is roughly double that of type 316L at around 50 % elongation, so in many designs wall thickness can be reduced rather than the material simply substituted.
Specification note. Most Incoloy grades are nickel-base, so 27-7Mo is often enquired against ASTM B564, the nickel alloy forging specification. Iron is the balance of S31277 and the correct route is the stainless family: ASTM A479 for bar and forging stock, ASTM A182 for forged flanges, fittings and valve parts. We raise this at quotation stage, because a certificate written to the wrong specification is rejected at inspection.
Not the same grade. Incoloy 27-7Mo (UNS S31277) is a corrosion resistant super-austenitic stainless steel. Incoloy alloy 908 (UNS N09908) is a low expansion nickel-iron superalloy used for cryogenic magnet conduit.
Chemical composition of Incoloy 27-7Mo (UNS S31277)
Limiting composition in weight percent, iron balance. Each heat is supplied with a ladle analysis on the material certificate. Product analysis and PMI on the finished forging are available to the order.
| Element | Min | Max | Typical | Function |
|---|---|---|---|---|
| Nickel (Ni) | 26.0 | 28.0 | 27.0 | Austenite stability, resistance to reducing media and chloride stress corrosion cracking |
| Chromium (Cr) | 20.5 | 23.0 | 22.0 | Passive film, resistance to oxidising media |
| Molybdenum (Mo) | 6.5 | 8.0 | 7.2 | Pitting and crevice corrosion resistance |
| Nitrogen (N) | 0.30 | 0.40 | 0.34 | Austenite stability, strength, localised corrosion resistance |
| Copper (Cu) | 0.5 | 1.5 | 0.8 | Resistance to sulfuric and other reducing acids |
| Manganese (Mn) | — | 3.00 | 2.5 | Raises nitrogen solubility during melting |
| Silicon (Si) | — | 0.50 | 0.3 | Deoxidation residual |
| Carbon (C) | — | 0.020 | 0.010 | Held low to avoid chromium carbide precipitation |
| Iron (Fe) | Balance | ≈ 42 | Base element, and the reason S31277 is classified as stainless steel | |
| Phosphorus and sulfur limits follow the governing product specification. Melting route: EAF + AOD/VOD, with ESR forging stock where the specification or section size requires it. State the required route on the enquiry. | ||||
Pitting resistance equivalent number. PREN = %Cr + 3.3 × %Mo + 16 × %N. On the typical analysis above this gives about 51, against 43 to 45 for the 6 % molybdenum grades. PREN ranks alloys, it is not a design value. For critical duty, add an ASTM G48 critical pitting or crevice temperature test to the test plan.
Mechanical properties of Incoloy 27-7Mo
27-7Mo is supplied solution annealed. It is not an age hardening alloy, so no heat treatment raises its strength above the values below.
| Property | Metric | Imperial |
|---|---|---|
| Tensile strength | 827 MPa | 120 ksi |
| Yield strength (0.2 % offset) | 414 MPa | 60 ksi |
| Elongation in 50 mm / 2 in. | 50 % | 50 % |
| Hardness | ≈ 95 HRB | ≈ 95 HRB |
| Yield strength at 538 °C | > 172 MPa | > 25 ksi |
| Structure | Fully austenitic, non-magnetic | |
| Typical values. Acceptance limits, test location and specimen orientation are set by the governing specification (ASTM A479, ASTM A182) and by the purchase order. | ||
Testing on forgings
Heavy sections cool slowly, so the anneal and quench are set from section thickness rather than from a fixed cycle. A test result only represents the forging if the specimen does, so specimen location is agreed before production: prolongation, sacrificial ring, or separately forged test block.
Physical and fabrication data
| Property | Value | Note |
|---|---|---|
| Density | ≈ 8.0 g/cm³ | ≈ 0.29 lb/in³ |
| Magnetic response | Non-magnetic | Fully austenitic in the annealed condition |
| Hot working range | ≈ 1150 → 950 °C | Start and finish temperature, with controlled reheats |
| Solution anneal | ≈ 1120 – 1180 °C | Rapid water quench, cycle set from section thickness |
| Age hardening | Not applicable | Strength comes from nitrogen and work, not precipitation |
| Welding filler | Over-alloyed Ni-base | ERNiCrMo-3, -4, -10 or -13 type consumables. Matching filler under-alloys the weld |
| Machinability | Work hardening | Rigid setups, positive rake, heavy feeds, no dwelling in the cut |
| Values are indicative for the alloy class and for our forging route. Confirm with us if a figure is design critical. | ||
Corrosion performance
27-7Mo is specified for environments that exceed the limits of 6 % molybdenum stainless steels: warm chlorides, mixed oxidising and reducing acids, and the wet acidic condensates of air pollution control equipment.
- Pitting and crevice corrosion. Above the 6 % molybdenum super-austenitics. Reported critical pitting temperature above 85 °C by ASTM G48 Method C, critical crevice temperature around 45 °C by Method D.
- Chloride stress corrosion cracking. Resists SCC in sodium chloride solutions to saturation at the boiling point, which is where the 27 % nickel content pays off against lean austenitics.
- Mixed acids. Performs where oxidising and reducing acids are present together.
- Flue gas desulfurization. Suited to the condensates of FGD systems on high sulfur coal fired plant.
- Seawater and marine service. Used in seawater handling equipment where high molybdenum austenitics are specified.
Furnace loading. Above about 925 °C, avoid stagnant oxidising conditions. Parts stacked face to face, or laid against each other in a furnace, can suffer rapid local oxidation at the contact crevices. We space and support 27-7Mo forgings in the furnace, and the same applies to any heat treatment carried out downstream.
Incoloy 27-7Mo compared with neighbouring grades
The selection question is usually whether 27-7Mo can replace a nickel-base alloy, or whether it is an upgrade from a 6 % molybdenum austenitic. The alloy content answers it.
| Grade | UNS | Ni | Cr | Mo | N | PREN* | Position |
|---|---|---|---|---|---|---|---|
| Type 316L | S31603 | 12 | 17 | 2.5 | — | ≈ 24 | Baseline austenitic, pits in warm chlorides |
| 254 SMO | S31254 | 18 | 20 | 6.1 | 0.20 | ≈ 43 | 6 % Mo super-austenitic |
| AL-6XN | N08367 | 24 | 21 | 6.3 | 0.22 | ≈ 45 | 6 % Mo super-austenitic, higher nickel |
| Incoloy 27-7Mo | S31277 | 27 | 22 | 7.2 | 0.34 | ≈ 51 | 7 % Mo super-austenitic |
| Alloy 625 | N06625 | Bal. | 21 | 9 | — | ≈ 51 | Nickel-base, markedly higher cost |
| Alloy C-276 | N10276 | Bal. | 16 | 16 | — | ≈ 69 | Nickel-base benchmark for severe service |
| *PREN calculated as %Cr + 3.3 × %Mo + 16 × %N from nominal composition, for ranking only. Comparisons across alloy families are indicative, since nickel-base grades resist mechanisms that PREN does not describe. | |||||||
27-7Mo is a step up from S31254 and N08367, and a lower cost alternative to alloy 625 in many chloride and mixed acid duties. For hot concentrated hydrochloric acid, wet chlorine or tight sour service requirements, C-276 or 625 remain the correct choice, and we forge those grades as well. Send the service conditions with the enquiry if you want a comment on grade selection.
Incoloy 27-7Mo forged shapes we produce
Open-die forging needs no die tooling, so a single piece costs the same per kilo as a batch. 27-7Mo is a made to order grade rather than a stock item.
Seamless rolled rings and forged rings
Ring rolled for continuous circumferential grain flow. Closures, retainers, pump casings and shell courses.
Forged flanges
Weld neck, blind, slip-on, long weld neck and ring joint geometries to ASTM A182 dimensions or to drawing.
Shafts, spindles, round bars
Straight, stepped and eccentric shafts for pumps, agitators and compressors. Forged and peeled bar for machine shops.
Discs, blanks, tube sheets
Upset forged discs and heavy blanks for heat exchangers and pressure vessels, rough or finish machined.
Sleeves, bushings, hollow bars
Trepanned or mandrel forged hollows, supplied as forged or bored and honed.
Valve parts and near-net shapes
Bodies, bonnets, seat rings, wedges, plugs and valve stems. Nozzles and non-standard profiles forged close to the finished envelope.
Size and weight range
| Parameter | Range | Notes |
|---|---|---|
| Ring outside diameter | 200 – 3000 mm | Seamless rolled, larger by enquiry |
| Bar diameter | 40 – 800 mm | Forged and rough turned |
| Disc and blank diameter | up to 2000 mm | Thickness to drawing |
| Single-piece weight | 5 – 8000 kg | Per finished forging |
| Flanges and fittings | DN 15 – DN 1200 | Class 150 – 2500 or to drawing |
| Minimum order | 1 piece | No die cost on open-die work |
| Delivery condition | As forged · Solution annealed · Machined | State on the order |
How we forge alloy 27-7Mo
A high molybdenum austenitic cracks if it is worked too cold, and loses corrosion resistance if it cools too slowly after annealing. The route below is set around both limits.
| Stage | What happens |
|---|---|
| Forging stock | EAF + AOD/VOD melted bar or billet, ESR where the specification or section size requires it. Incoming mill certificates traced to the heat number and supplied with the forging. |
| Heating | Soaked to temperature with parts spaced and supported, not stacked, to avoid crevice oxidation above 925 °C. |
| Open-die forging | Cogged and upset under press or hammer with controlled reduction ratio, starting near 1150 °C and finishing above roughly 950 °C, with reheats as required. |
| Ring rolling | For ring geometries: pierced and rolled to a seamless ring with circumferential grain flow. |
| Solution anneal | Typically 1120 to 1180 °C, rapid water quench, cycle sized to section thickness. This is the step that restores corrosion resistance. |
| Machining | Rough turned, proof machined or finish machined to your drawing tolerances. |
| NDT and testing | Ultrasonic testing, dimensional inspection, mechanical and chemical testing, corrosion testing where specified. |
| Certification | EN 10204 3.1, or 3.2 with third party witness. Heat number and specification hard stamped or vibro etched as agreed. |
Cooling after annealing. Slow cooling through the intermediate temperature range precipitates intermetallic phases in a 7 % molybdenum austenitic. Corrosion resistance drops, and a tensile test will not show it. If your own shop re-anneals or stress relieves these forgings after machining, agree the cycle with us first.
Testing, inspection and certification
Ultrasonic testing
To EN 10228-3, SEP 1921 or ASTM A388, at the acceptance class stated on your order.
Dye penetrant examination
PT to ASTM E165 on request. 27-7Mo is non-magnetic, so it is examined by PT rather than MT.
Tensile, hardness, grain size
Room temperature tensile and hardness as standard. Impact testing and elevated temperature tests when specified.
ASTM A262 and ASTM G48
Intergranular corrosion testing, and G48 pitting or crevice temperature testing where the anneal is part of the acceptance criteria.
PMI and product analysis
Positive material identification on the finished forging, plus product analysis against the ladle certificate.
EN 10204 3.1 / 3.2
3.1 as standard. 3.2 with SGS, BV, TÜV, LR, DNV or your own inspector witnessing, on request.
Where Incoloy 27-7Mo forgings are used
Applications are in pollution control, power, marine, chemical processing, pulp and paper, and oil and gas, where chlorides and acids occur together.
- Air pollution control. Flue gas desulfurization scrubber internals, spray headers, nozzles, flanges and rings on high sulfur coal fired plant.
- Chemical processing. Reactor and column nozzles, flanges, agitator and pump shafts, pump casings for mixed acid duty.
- Heat exchangers and pressure vessels. Tube sheets, forged shell flanges, covers and heads for shell and tube exchangers, air receivers and columns.
- Oil and gas. Wellhead and Christmas tree components, subsea and deepwater production hardware, manifold blocks, valve bodies and stems.
- Marine and seawater. Seawater pump components, sleeves, bushings and rings for offshore and desalination service.
- Pulp, paper and pharmaceutical. Bleach plant fittings, rolls, wheels and process module components.
These are drawing specific parts, so we quote against the drawing and the specification rather than from a catalogue. Where a customer is still comparing materials, we can quote the same part in 27-7Mo, Incoloy 926, Incoloy 825, Inconel 625 and Hastelloy C-276, so the cost difference is visible before the material is fixed.
About the 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, super-austenitic stainless steel and nickel-based alloys, including Incoloy alloy 27-7Mo (UNS S31277).
For 27-7Mo the company supplies forged and seamless rolled rings, flanges, shafts, discs, blanks, sleeves, bushings, hollow bars, tube sheets, nozzles, valve parts and round bars, forged to customer drawing with no minimum order quantity. Material is ordered under the stainless specifications ASTM A479 and ASTM A182 rather than the nickel alloy specification ASTM B564, and is supplied solution annealed and rapidly quenched. 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 27-7Mo
What is Incoloy alloy 27-7Mo?
Incoloy alloy 27-7Mo (UNS S31277) is a 7 % molybdenum, nitrogen alloyed super-austenitic stainless steel containing about 27 % nickel, 22 % chromium, 7.2 % molybdenum and 0.34 % nitrogen, with iron as the balance. Nickel and nitrogen give a stable austenitic structure. Molybdenum, chromium and nitrogen give resistance to pitting and crevice corrosion above the 6 % molybdenum super-austenitics, and in many environments close to nickel-base alloys 625, 622 and C-276.
Is Incoloy 27-7Mo a stainless steel or a nickel alloy?
A stainless steel. Iron is the balance of the composition, which is why the grade carries the stainless UNS number S31277 rather than an N prefixed nickel alloy number. Forgings are ordered under ASTM A479 and ASTM A182, not under ASTM B564.
What is the UNS number for Incoloy 27-7Mo?
S31277. The grade is also written as alloy 27-7MO, 27-7Mo or Incoloy 27-7MO. It is a different material from Incoloy alloy 908 (UNS N09908), which is a low expansion nickel-iron superalloy.
Which standards cover Incoloy 27-7Mo forgings?
ASTM A479 / ASME SA479 for bar and forging stock, ASTM A182 / ASME SA182 for forged flanges, fittings and valve parts, ASTM A240 / SA240 for plate, sheet and strip, ASTM A312 for pipe and ASTM A213 or A249 for tube.
Design stresses for ASME Boiler and Pressure Vessel Code construction are given in ASME Code Case 2458, which lists the grade as austenitic Fe-27Ni-22Cr-7Mo-Mn-Cu-N alloy UNS S31277. Send us your project specification and we will quote against it.
What is Incoloy 27-7Mo used for?
Pollution control, power, marine, chemical processing, pulp and paper, and oil and gas. Typical forged parts are FGD scrubber hardware, nozzles and flanges, mixed acid reactor and column components, pump and agitator shafts, heat exchanger tube sheets and shell flanges, seawater pump parts, and wellhead, manifold and valve components.
How does 27-7Mo compare with 254 SMO, AL-6XN, alloy 625 and C-276?
At about 7.2 % molybdenum and 0.34 % nitrogen, 27-7Mo calculates to a PREN near 51, against 43 to 45 for the 6 % molybdenum grades S31254 and N08367. In many chloride and mixed acid services it performs close to alloy 625 at a lower alloy cost. For hot concentrated hydrochloric acid, wet chlorine or tight sour service requirements, C-276 remains the correct choice.
How is Incoloy 27-7Mo heat treated after forging?
Solution annealed and rapidly water quenched, typically from about 1120 to 1180 °C, with the cycle set from section thickness. Rapid cooling prevents the intermetallic phases that reduce corrosion resistance and toughness. Stagnant oxidising conditions above about 925 °C should be avoided during heating, because rapid local oxidation can occur at crevices between stacked parts.
Which forged shapes can you make in 27-7Mo?
Seamless rolled rings, forged rings and hollows, flanges, shafts and eccentric shafts, discs and blanks, sleeves, bushings, tube sheets, nozzles, valve bodies, seat rings and stems, gear blanks and round bars, as forged with allowance or finish machined to drawing. Open-die forging needs no die tooling, so single pieces and small batches are economical.
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 your acceptance class. Chemical analysis and PMI, room temperature tensile, hardness and grain size are reported as standard. ASTM A262 intergranular corrosion testing and ASTM G48 pitting or crevice testing are available when the specification calls for them.
What do you need in order to quote?
A drawing or the finished dimensions, the quantity, the governing material specification and edition, the delivery condition and machining allowance, the ultrasonic acceptance class, and the certification level. Send it to sales@steelforgepieces.com or WhatsApp +86-189-2135-9659. Quotations come back within 24 hours.
Data sources
Property data on this page is compiled from the sources below. Where your project specification differs, the specification governs.
- Special Metals Corporation — INCOLOY alloy technical bulletins, including alloy 27-7MO.
- ASTM International — ASTM A182/A182M, Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, and Valves and Parts.
- ASTM A479/A479M, Stainless Steel Bars and Shapes for Use in Boilers and Other Pressure Vessels (bar and forging stock, UNS S31277).
- ASTM A240/A240M, Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels.
- ASME Boiler and Pressure Vessel Code, Code Case 2458, Austenitic Fe-27Ni-22Cr-7Mo-Mn-Cu-N Alloy (UNS S31277).
- ASTM G48, Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution.
Last reviewed 19 August 2026 · Technical sales team, Jiangyin Jiangnan Metal Co., Ltd.