Summary
UNS S31254, better known as 254 SMO, is a super austenitic stainless steel with nominally 20 percent chromium, 18 percent nickel and 6 percent molybdenum, alloyed with nitrogen and copper. The molybdenum and nitrogen together lift its pitting resistance equivalent number above 43, which puts it in the same corrosion bracket as a super duplex grade while keeping a fully austenitic structure. Strength is roughly twice that of the 300 series, and the grade holds impact toughness alongside resistance to chloride stress corrosion cracking, pitting and crevice attack.
Two practical limits shape how it is made. It does not respond to heat treatment, so strength comes only from cold reduction. And hot work must stay inside 982 to 1149 °C, because above that the surface scales and workability drops away.
Jiangyin Jiangnan Metal melts, forges, heat treats, machines and tests this grade on one site in Jiangyin, Jiangsu. Rings roll to 6,000 mm outside diameter, single pieces reach 30 tonnes, and the minimum order is 10 kg with no minimum piece count.
Designation and equivalents
The same grade travels under a trade name, a UNS number, an ASTM forging symbol and a European number, which is why enquiries for it arrive under four different labels. All of the following describe one material.
| System | Designation |
|---|---|
| UNS | S31254 |
| Trade name | 254 SMO |
| EN number | 1.4547 |
| ASTM forgings | A182 Grade F44 |
| ASTM plate and sheet | A240 |
| ASTM seamless pipe | A312 |
| ASTM welded tube | A249 |
| ASTM seamless tube | A269 |
| ASTM welded pipe | A813, A814 |
Check which specification governs. ASTM A182 F44 is a pressure-part forging specification and carries its own composition limits and mechanical minimums. A general forging to ASTM A473 is a different acceptance basis. Tell us which applies, because the two are certified differently and the heat treatment record differs.
Chemical composition
The left column gives the ASTM A182 Grade F44 specified limits. The right column is a typical analysis for 254 SMO, useful for calculation work such as a PREN estimate.
| Element | A182 F44 limits | Typical analysis |
|---|---|---|
| Chromium, Cr | 19.5 – 20.5 | 20 |
| Nickel, Ni | 17.5 – 18.5 | 18 |
| Molybdenum, Mo | 6.00 – 6.50 | 6.1 |
| Nitrogen, N | 0.18 – 0.25 | 0.20 |
| Copper, Cu | 0.50 – 1.00 | — |
| Carbon, C | 0.020 max | 0.010 |
| Manganese, Mn | 1.00 max | — |
| Silicon, Si | 0.80 max | — |
| Phosphorus, P | 0.030 max | — |
| Iron, Fe | Balance | 55.69 |
Sulfur and other residual limits apply as stated in the governing edition of ASTM A182 and are reported on the certificate for each melt. Composition is verified on the liquid steel during refining and again on the solid product.
Pitting resistance
Pitting resistance equivalent number for this grade is above 43. It is calculated from chemistry as percentage chromium plus 3.3 times percentage molybdenum plus 16 times percentage nitrogen; one mill datasheet for seamless tube quotes a value of at least 42.5 on that basis. Treat PREN as a screening index for comparing grades, not as a prediction of service life. Chloride concentration, temperature, crevice geometry and surface finish all move the real outcome, so corrosion testing in the actual medium is the only sound basis for a critical selection.
Mechanical and physical properties
ASTM A182 F44 sets the minimums that a forging must meet on test. Typical values sit well above them and are given for design comparison only.
| Property | Minimum |
|---|---|
| Tensile strength | 650 MPa / 94 ksi |
| Yield strength, 0.2 per cent offset | 300 MPa / 44 ksi |
| Elongation in 50 mm or 4D | 35 per cent |
| Reduction of area | 50 per cent |
| Property | Metric | Imperial |
|---|---|---|
| Tensile strength | 680 MPa | 98,600 psi |
| Yield strength | 300 MPa | 43,500 psi |
| Elongation at break | 50 per cent | 50 per cent |
| Modulus of elasticity | 195 GPa | 28,300 ksi |
| Shear modulus | 75 GPa | 10,900 ksi |
| Poisson's ratio | 0.30 | 0.30 |
| Hardness, Brinell | 210 | 210 |
| Density | 8 g/cm³ | 0.289 lb/in³ |
| Thermal conductivity | 14 W/m·K | 97.2 BTU in/hr·ft²·°F |
| Thermal expansion, 20–100 °C | 16.5 µm/m·°C | 9.17 µin/in·°F |
Melting, forging, heat treatment, machining
Melting
S31254 runs the electric arc furnace route with ladle refining and vacuum oxygen decarburisation. VOD earns its place on this grade: carbon has to come down to 0.020 per cent maximum while 6 per cent molybdenum and 0.18 to 0.25 per cent nitrogen stay in the bath. Maximum heat size is 60 tonnes. Electroslag remelting is available where cleanliness or segregation control justifies it.
Forging
Hot work is carried out between 982 and 1149 °C. That is a narrow window by forging standards, and the upper bound is not a soft limit: above it the surface scales and workability falls off. The lower bound matters as much, because a cooling piece worked too cold in a grade with this work hardening rate will tear. Heavy sections therefore come back to the furnace more often than a carbon steel equivalent would. Presses available are 6,300 t, 4,000 t and 2,000 t, with radial-axial ring mills at 6 m, 3 m and 1 m, plus hammers from 750 kg to 5 t for smaller work.
Heat treatment
Solution annealing is at 1149 to 1204 °C followed by a water quench. The purpose is to put molybdenum and nitrogen back into solution and clear intermetallic precipitation picked up during hot work, which restores corrosion resistance; it does not raise strength. Post-forging annealing is advisable for that reason on any part going into a chloride-bearing service. The top of that range calls for real high-temperature furnace capacity, which three RTX-12-350KW car-bottom furnaces provide, alongside eleven other car-bottom and pit furnaces including two 15 m deep well furnaces for vertical quenching of long shafts. The grade does not respond to hardening heat treatment; strength is raised only by cold reduction.
Machining
This is a difficult grade to machine, from a very high work hardening rate combined with the absence of sulfur that would otherwise help chip breaking. Workable practice is sharp tools, machine tools with power in reserve, positive feeds, generous lubrication and slow speeds. Light rubbing passes are the main thing to avoid, since they glaze the surface and leave the next cut fighting a harder skin. Vertical lathes run to 5,000 mm swing, horizontal lathes to 14,000 mm between centres, and deep-hole drilling to 1,600 mm diameter by 10,000 mm.
Welding
All common processes are usable, but filler metal is required; welding without it produces poor strength properties. Recommended fillers are AWS A5.14 ERNiCrMo-3 and alloy 625, with covered electrodes matching AWS A5.11 ENiCrMo-12. The nickel-base filler is deliberate: a matching-composition filler would leave the weld metal short of the molybdenum needed to match parent-metal pitting resistance.
Product forms and size limits
Each limit below is set by a specific machine, so a part can be checked against the list before a drawing is sent.
| Form | Limit | Set by |
|---|---|---|
| Seamless rolled rings | OD 50 to 6,000 mm | 6 m radial-axial ring mill; upset and punch route below 500 mm |
| Forged rings and sleeves | To 6,000 mm OD | Press route for sections outside the mill envelope |
| Forged flanges | To drawing, ASME B16.5 and B16.47 | Press and vertical lathe |
| Shafts and spindles | To 14,000 mm turned length | Horizontal lathe; 15 m well furnace for vertical quench |
| Discs, tube sheets, blocks | To 5,000 mm machined diameter | 5,000 mm vertical turning lathe |
| Bored cylinders, hollow bar | To 1,600 mm dia × 10,000 mm | CNC deep-hole drilling machine |
| Round, flat and square bar | To drawing | Press and hammer route |
| Valve bodies, nozzles, pump parts | To drawing | Press and machine shop |
| Single piece weight | To 30,000 kg | 60 t heat size, 6,300 t press |
| Minimum order | 10 kg | No minimum piece count, prototypes accepted |
Where the grade is used
S31254 earns its cost where chlorides and halides attack conventional austenitic stainless. For some duties it has been reported as a cost-effective substitute for high nickel and titanium alloys.
Seawater and desalination
Saltwater handling systems, desalination plant, heat exchangers and associated forged flanges, tube sheets and shafts.
Flue gas cleaning
Flue gas desulfurisation scrubbers and flue-gas cleaning equipment, where chloride condensate attacks lesser grades.
Pulp and paper
Bleaching equipment exposed to chloride and chlorine dioxide bleach liquors.
Chemical process plant
Process equipment in the chemical industry, including forged valve bodies, nozzles and pump components.
Oil and gas
Components used in petroleum production, and tall oil distillation columns.
Food processing
Food processing equipment where brine and acidic media rule out standard 316.
Testing and certification
Every forging leaves with chemical analysis and mechanical test results traceable to the melt number. Non-destructive examination is to EN 10228-3, SEP 1921 or ASTM A388 as the order specifies, with the class and acceptance level set by the purchase order rather than assumed. Certification is issued to EN 10204 3.1, or 3.2 with third-party witness. The works holds CCS, BV, DNV, LR and NK approvals, and inspection can be arranged through one of those societies or through your own nominated inspector. Positive material identification, corrosion testing to ASTM A262 or ASTM G48, ferrite measurement and impact testing are available as supplementary requirements when called for.
Common questions
What is UNS S31254?
A super austenitic stainless steel with nominally 20 per cent chromium, 18 per cent nickel and 6 per cent molybdenum, alloyed with nitrogen and copper. It is widely known as 254 SMO, appears in ASTM A182 as forging grade F44, and carries the European number 1.4547. Pitting resistance equivalent number is above 43, which is comparable to a super duplex stainless steel.
What is the PREN of UNS S31254?
Above 43, reached by raising molybdenum to 6 per cent. The calculation is percentage chromium plus 3.3 times percentage molybdenum plus 16 times percentage nitrogen, and one mill datasheet quotes at least 42.5 for seamless tube on that formula. PREN compares grades from chemistry; it does not predict service life in a given medium.
Can the grade be hardened by heat treatment?
No. It does not respond to heat treatment, and hardening is possible only through cold reduction. Solution annealing at 1149 to 1204 °C with a water quench restores corrosion resistance rather than raising strength.
What is the forging temperature range?
982 to 1149 °C for forging, upsetting and related operations. Going above that range causes scaling and reduces workability, so the upper limit is worth holding to. Annealing after hot work is advisable to recover maximum corrosion resistance.
Which filler metal should be used for welding?
Filler is required, since welding without it gives poor strength properties. Use AWS A5.14 ERNiCrMo-3 or alloy 625, with covered electrodes matching AWS A5.11 ENiCrMo-12.
Is it difficult to machine?
Yes, because of a very high work hardening rate and the absence of sulfur. Sharp tools, machine tools with power in reserve, positive feeds, plenty of lubrication and slow speeds give good results.
What is the minimum order, and can you work from a drawing?
Minimum order is 10 kg with no minimum piece count, so single prototypes are accepted alongside production lots. Send a drawing, 3D model or sample with the specification, ultrasonic class and certificate type. Lead time runs 20 to 60 days depending on section size and heat treatment route.
Request a price on UNS S31254
Send the drawing or a size list with quantity, governing specification, ultrasonic requirement and certificate type. Quotations normally go out within one working day, stating the forging route and heat treatment plan so you can see how the part will be made.
Melting works and open-die forge, since 1997
Sources
- AZoM, Stainless Steel Grade 254 SMO (UNS S31254). Typical composition and physical, mechanical and thermal properties; annealing, hot working, machining, welding and application data.
- Ferrobend, ASTM A182 / A182M F44. Specified composition limits and mechanical minimums for forging grade F44.
- Langley Alloys, What is the PREN of S31254. Pitting resistance equivalent number above 43.
- Alleima, 254 SMO seamless tube and pipe datasheet. PREN formula and quoted value for seamless tube.
- ASTM A182 / A182M, Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service.
- ASTM A473, Standard Specification for Stainless Steel Forgings.
Typical property values on this page are published figures for grade identification and do not replace the governing standard. Contractual properties are those stated on the purchase order and on the material certificate issued with the goods. Page reviewed 5 October 2026.