254 SMO / F44 – UNS S31254
6Mo super-austenitic stainless steel with nitrogen
Super-austenitic stainless with 6% Mo and 0.2% N, with pitting and crevice resistance in seawater comparable to that of costlier nickel alloys. Mechanical strength superior to 316L thanks to nitrogen.
Designations and equivalents
| System | Designation |
|---|---|
| UNS | S31254 |
| W.Nr./EN | 1.4547 (X1CrNiMoCuN20-18-7) |
| DIN | X1CrNiMoCuN20-18-7 |
| JIS | SUS312L |
| Trade names | 254 SMO (Outokumpu), 6Mo |
| Others | SS 2378 |
Standards by product form
| Form | Standard |
|---|---|
| Bar | ASTM A276 / A479 |
| Forging/flange | ASTM A182 F44 |
| Plate | ASTM A240 |
| Tube | ASTM A312 / A269 / A249 |
| Fittings | ASTM A403 WPS31254 |
Tolerances and HS codes
- Forged bars (round and square): machining allowance from 2 mm (Ø 6–16 mm) to 24 mm (Ø 400–500 mm) on the diameter. E.g. Ø 100–125 mm finished: +7 mm, tolerance +6 / −0 (length up to 3,500 mm).
- Forged discs: allowance of 4 to 9 mm on the diameter and 3 to 11 mm on the height, plus-only tolerance.
- Cutting and machining: band saw −0 / +5 to +15 mm on length; waterjet +10 % of thickness on the diameter (min. 2 mm); machined parts −0 / +2 to +5 mm on the diameter. See the full tables (INF 002 and INF 003, the same for every alloy).
| Form | NCM (Brazil, 8 digits) | HS (international, 6 digits) | HTS (USA, 10 digits)* | CN (European Union, 8 digits)* |
|---|---|---|---|---|
| Forged bar | 7222.30.00 | 7222.30 | 7222.30.0024 | 7222 30 51 |
| Hot-rolled bar | 7222.11.00 | 7222.11 | 7222.11.0059 | 7222 11 11 |
| Plate (> 10 mm, width ≥ 600 mm) | 7219.21.00 | 7219.21 | 7219.21.0020 | 7219 21 10 |
| Seamless pipe | 7304.41.90 | 7304.41 | 7304.41.6045 | 7304 41 00 |
| Welded pipe | 7306.40.00 | 7306.40 | 7306.40.5085 | 7306 40 80 |
| Flange and fitting | 7307.21.00 | 7307.21 | 7307.21.5000 | 7307 21 00 |
Tariff class: stainless steel (by chemical composition, under the Chapter 72 Notes and Section XV Note 5).
*: US and EU codes for reference. NCM per current legislation (General Rules of Interpretation, Notes to Chapters 72, 74 and 75 and Section XV Note 5). Confirm the final code with your customs broker according to the item size and finish. HS = first 6 digits of the NCM.
Chemical composition (%)
ASTM A182 F44 / A240
| Element | Range (%) |
|---|---|
| C | ≤ 0.020 |
| Mn | ≤ 1.00 |
| P | ≤ 0.030 |
| S | ≤ 0.010 |
| Si | ≤ 0.80 |
| Cr | 19.5 – 20.5 |
| Ni | 17.5 – 18.5 |
| Mo | 6.0 – 6.5 |
| Cu | 0.50 – 1.00 |
| N | 0.18 – 0.25 |
| Fe | balance |
Mechanical properties
Solution annealed (ASTM A182 F44)
| Property | Value |
|---|---|
| Yield strength (0.2%) | ≥ 300 MPa |
| Tensile strength | ≥ 650 MPa |
| Elongation | ≥ 35% |
| Reduction of area | ≥ 50% |
| Hardness | ≤ 223 HBW (A240) |
Typical physical properties
| Property | Value |
|---|---|
| Density | 8.0 g/cm³ |
| Melting range | ≈ 1,320 – 1,390 °C |
| Modulus of elasticity | 195 GPa |
| Thermal conductivity (20 °C) | 14 W/m·K |
| Thermal expansion (20–100 °C) | 16.5 µm/m·K |
Heat treatment, welding and machining
| Item | Usual practice |
|---|---|
| Service temperature | Cryogenic up to ≈ 400 °C (risk of intermetallic phases above). |
| Heat treatment | Solution annealing 1,150 – 1,200 °C with rapid water quench. |
| Welding | Good, with a Mo over-alloyed consumable: AWS ERNiCrMo-3 or ERNiCrMo-13 (electrodes ENiCrMo-3/-12); avoid matching-composition consumables because of Mo segregation. |
| Machining | Machinability lower than carbon steel: it work hardens quickly; use sharp tools, positive rake, constant feed without 'rubbing' and abundant coolant. Harder than 316L because of the N and Mo. |
Filler metal for welding
The wire composition is usually not identical to the material: each page explains why.
Typical applications
- Seawater systems (piping, pumps, exchangers)
- Offshore: injection water and firefighting
- Pulp and paper bleaching
- Desalination
- Gas scrubbers (FGD)
- Chemical industry with halides
Alternatives: 904L in less aggressive media; super duplex (F55) with higher mechanical strength; Alloy 625 or C-276 above the pitting/crevice limit of 6Mo.
254 SMO / F44: complete guide
What 254 SMO steel is
254 SMO belongs to the family of special stainless steels, in the "6Mo" group. It is an iron-chromium-nickel-molybdenum alloy with nitrogen, not a copper-nickel alloy: copper enters at only 0.5 – 1.0%. The designation F44 is that of the ASTM A182 forging standard, and 254 SMO is Outokumpu's trade name.
Chemical composition and role of the elements
Molybdenum (6.0 – 6.5%) and nitrogen (0.18 – 0.25%) are the elements that make the difference: the PREN, calculated as Cr + 3.3 Mo + 16 N, is between 42 and 45, versus 23 – 28 for 316L. Nitrogen also raises the yield strength to twice that of 316L and stabilizes the austenite. The high nickel (17.5 – 18.5%) is needed to keep the austenitic structure with so much chromium and molybdenum.
Heat treatment and welding
Supplied solution annealed at 1,150 – 1,200 °C and water quenched, a higher temperature than for common austenitics in order to dissolve the sigma and chi phases that molybdenum promotes. Welding is done without preheating and without post-weld heat treatment, but necessarily with a nickel-alloy consumable such as ERNiCrMo-3 (625), because a weld with a matching consumable segregates molybdenum and loses pitting resistance. Dustre performs solution annealing in its own furnaces as part of its heat treatment service.
Corrosion and limits of use
It resists natural and chlorinated seawater, brines, acidic solutions with chlorides and bromides, and gas desulfurization media. It is an alloy for aqueous corrosion: the ASME code allows it up to about 400 °C, and prolonged exposure above 600 °C precipitates intermetallic phases that embrittle the material.
Applications
Seawater heat exchangers and piping, desalination plants, FGD absorbers, pulp bleaching equipment, offshore components and pump and valve parts for severe chlorides. Alternatives: 904L for less severe media and super duplex F53 when more mechanical strength is needed.
Supply forms and Dustre services
Round bars and blocks forged in open die, cut to size on a band saw, with rough or finish machining, solution annealing in our own furnaces and hardness, tensile and impact testing. All material is supplied with a 3.1 inspection certificate and heat traceability.
Frequently asked questions
What is the chemical composition of 254 SMO / F44?
Per ASTM A182 F44 / A240: C ≤ 0.020; Mn ≤ 1.00; P ≤ 0.030; S ≤ 0.010; Si ≤ 0.80; Cr 19.5 – 20.5.
Which standards cover 254 SMO / F44?
Bar: ASTM A276 / A479. Forging/flange: ASTM A182 F44. Plate: ASTM A240. Tube: ASTM A312 / A269 / A249. Fittings: ASTM A403 WPS31254.
Up to what temperature can 254 SMO / F44 be used?
Cryogenic up to ≈ 400 °C (risk of intermetallic phases above).
Which materials can replace 254 SMO / F44?
904L in less aggressive media; super duplex (F55) with higher mechanical strength; Alloy 625 or C-276 above the pitting/crevice limit of 6Mo.
