254 SMO Tube Fittings (UNS S31254 / W.Nr. 1.4547, 6-Moly Super-Austenitic Instrumentation Fittings)
254 SMO tube fittings are mechanical-grip compression fittings machined from UNS S31254, a 6% molybdenum super-austenitic stainless steel that also carries a deliberate nitrogen addition and a touch of copper. One or two ferrules swage onto the tube's outside diameter and pull up to a leak-tight, gas-tight seal with a firm mechanical grip. The reason 254 SMO gets specified for a fitting comes down to one thing: chlorides. With a PREN around 43, it resists chloride pitting, crevice corrosion, and chloride stress-corrosion cracking far past where 316 and even 904L give out, it is a seawater-grade alloy, and it stays fully austenitic and non-magnetic doing it. Crestflo carries these fittings from 1/8-inch through 2-inch (50 mm) tube OD, single and double (twin) ferrule, imperial and metric. They are qualified to ASTM F1387 and offered to NACE MR0175 / ISO 15156 where sour service applies. Standard items ship from ready stock; specials are built to order, with delivery running 6-8 weeks. Request a quote to price out your configuration.
254 SMO® is a registered trademark of Outokumpu OYJ. Swagelok®, Parker®, and Hy-Lok® are trademarks of their respective owners. Crestflo supplies UNS S31254 to standard chemistry, is not affiliated with or endorsed by any brand or mill referenced here, and all interchange is strictly dimensional.
What 254 SMO Compression Tube Fittings Are
A 254 SMO compression tube fitting is built for fluid-system instrumentation: a body, a nut, and one or two ferrules that bite onto the tube's outer diameter as the nut is tightened. The joint comes apart and goes back together without losing integrity. No flaring, no welding, no elastomer seal sitting in the leak path. The seal is metal-to-metal, so the service window comes from the alloy, not from an O-ring failing first.
Scope matters here. These are instrumentation compression tube fittings in UNS S31254 super-austenitic stainless, not mill bar, not sheet or plate, not forgings or line pipe, and not electrical wire ferrules. ("6-moly" refers to the molybdenum content, not a plumbing part.) Crestflo's 254 SMO fittings land on small-bore instrument tubing for transmitters, analyzers, sample lines, chemical-injection systems, and process taps across seawater, offshore, brine, and chloride-bearing service, the high-chloride duty where 316 pits and cracks and 904L is reaching its limit.
Also Known As: UNS S31254, 1.4547, F44 & 6-Moly Stainless
Buyers and specifiers write this grade several ways, and they all point to the same 6% molybdenum super-austenitic. On drawings and material call-outs it shows up as 254 SMO, 254SMO, SMO 254, Alloy 254, 6Mo, 6-moly, or just "6% molybdenum stainless." Its formal designations are UNS S31254 (the US number) and EN / Werkstoff 1.4547 (the European one); its ASTM forging/flange grade is F44 (as in A182 F44). Ask for "254 SMO ferrule fittings," "UNS S31254 tube fittings," "S31254 compression fittings," or "6Mo tube fittings" and you get exactly what's described on this page, the terms are interchangeable.
| Designation type | Value |
|---|---|
| Trade / common names | 254 SMO, 254SMO, SMO 254, Alloy 254, 6Mo, 6-moly, 6% molybdenum stainless |
| UNS number | S31254 |
| EN / Werkstoff (W.Nr.) | 1.4547 (DIN 1.4547) |
| ASTM forging/flange designation | F44 (e.g. A182 F44) |
| Alloy family | Super-austenitic (6-moly) stainless steel |
UNS S31254 and Werkstoff 1.4547 describe one and the same alloy, so whichever designation shows up on a spec sheet, you are looking at the same 254 SMO super-austenitic. One caution: 254 SMO is not 253 MA, that is a heat-resistant austenitic grade for high-temperature service, a different alloy for a different job.
Why Choose 254 SMO for Instrumentation Tube Fittings
254 SMO is a 6% molybdenum, nitrogen-alloyed super-austenitic, nominally 20% chromium, 18% nickel, 6% molybdenum, with about 0.2% nitrogen and a small copper addition, and that chemistry is what makes it the right pick for a fitting in seawater and high-chloride service. The molybdenum and nitrogen together drive pitting and crevice resistance; the result is a PREN near 43, roughly double the ~24 of 316L and a clear step above the ~34 of 904L. In practice that means the alloy resists localized chloride attack at temperatures where those lower-moly grades pit and crack. Its high nickel and austenitic structure also give it strong resistance to chloride stress-corrosion cracking, the failure mode that most often retires 304 and 316 in warm chloride service, and the copper addition adds useful resistance in reducing acids such as sulfuric.
Be precise about where 254 SMO sits. In chloride pitting resistance it is in the same tier as super-duplex 2507 (PREN ~43) and just below AL-6XN (UNS N08367, PREN ~45-47), the other common 6-moly super-austenitic that runs slightly higher molybdenum and nitrogen. The difference against duplex is structural: 254 SMO is fully austenitic, so it is non-magnetic in the annealed condition, it welds more forgivingly, and it keeps its toughness down to cryogenic temperatures with no ductile-to-brittle transition. Super-duplex 2507, by contrast, is ferromagnetic and carries roughly double the yield strength. So the honest trade is this: for chloride resistance with non-magnetic austenitic behavior and easy weldability, 254 SMO; for the same chloride resistance with far higher strength-per-wall, super-duplex 2507. Where the duty is milder, reducing acids or lighter chloride that 316 can't handle but seawater-grade is overkill, step down to 904L. Match the alloy to the media and don't over-specify.
For the full metallurgical picture, composition, mechanical and physical properties, PREN and CPT data, and corrosion-by-media detail, see the 254 SMO grade page. The reference composition is summarized below for convenience when writing specs. Values follow the applicable ASTM specification and can be customized to requirement; actual per-heat chemistry is reported on the material test certificate that ships with each order.
| Element | wt % (per applicable ASTM specification) |
|---|---|
| Carbon (C) | 0.020 max |
| Chromium (Cr) | 19.5–20.5 |
| Nickel (Ni) | 17.5–18.5 |
| Molybdenum (Mo) | 6.0–6.5 |
| Nitrogen (N) | 0.18–0.22 |
| Copper (Cu) | 0.50–1.00 |
| Manganese (Mn) | 1.00 max |
| Silicon (Si) | 0.80 max |
| Phosphorus (P) | 0.030 max |
| Sulfur (S) | 0.010 max |
| Iron (Fe) | Balance |
Molybdenum and nitrogen are the elements that separate 254 SMO from lower-alloy stainless in chlorides, while the high nickel underwrites chloride-SCC resistance and the copper helps in reducing acids. The very low carbon limit protects corrosion resistance in and around welds. The key strength and corrosion figures that make 254 SMO a fitting-grade super-austenitic are summarized here.
| Property | 254 SMO (UNS S31254) |
|---|---|
| Tensile strength (min) | 650 MPa (94 ksi) |
| Yield strength (0.2%, min) | 300 MPa (44 ksi) |
| Elongation (min) | 35% |
| Hardness | ≤ ~96 HRB |
| Density | ~8.0 g/cm³ (0.29 lb/in³) |
| Magnetic response | Austenitic; essentially non-magnetic in the annealed condition |
| PREN (Cr + 3.3×Mo + 16×N) | ≈ 43–45 |
Those figures are standard published minimums and nominal physical data for the solution-annealed condition; actual certified properties for a given heat are stated on the material test certificate. Note the yield strength, at 300 MPa it is nearly double that of 316, a direct result of the nitrogen content, though still well below a super-duplex. For the head-to-head against the neighboring grades, see 254 SMO vs Super Duplex vs AL-6XN; for the step down in alloy, compare 904L.
254 SMO Tube Fitting Sizes (1/8-inch to 2-inch OD): Imperial & Metric
Crestflo machines UNS S31254 tube fittings in both imperial fractional and metric tube-OD series, with end connections up to 2-inch (50 mm) OD. The chart below covers the standard range. Working pressure for any given fitting depends on size, wall thickness, and temperature; it comes from standard tube-fitting pressure data for the tube OD and wall, following ASTM/ASME practice, and the exact rated pressure for your configuration is confirmed on the quotation. Values follow the applicable standard and can be customized to requirement.
| Tube OD, imperial | Tube OD, metric | Availability |
|---|---|---|
| 1/8", 1/4", 3/8", 1/2" | 6 mm, 8 mm, 10 mm, 12 mm | Standard range |
| 5/8", 3/4", 1" | 14 mm, 16 mm, 18 mm, 20 mm, 22 mm, 25 mm | Standard range |
| up to 2" (50 mm) | up to 50 mm | Made-to-order |
254 SMO Tube Fitting End & Thread Connections: NPT, BSPP, JIC & Weld
Fittings come with tube-OD compression ends, tapered or parallel threads, or weld ends. All are available in national, international, and custom thread types.
| End type | Thread / connection standards |
|---|---|
| Tube-OD (compression) | Fractional inch and metric OD |
| Male / female tapered thread | NPT, NPTF, BSPT |
| Male / female parallel thread | BSPP, ISO parallel, SAE ORB |
| Flare / port | JIC 37°, SAE ORB |
| Weld end | Socket-weld, butt-weld, orbital |
Single vs Double Ferrule & Hardened vs Non-Hardened Options
UNS S31254 fittings come in single-ferrule and double (twin) ferrule designs, with hardened or non-hardened ferrules supplied depending on the job. This choice shapes gripping action, vibration resistance, and how many times a fitting can be reassembled. Like other high-nickel austenitics, 254 SMO is a tough, work-hardening alloy that can gall under repeated make-up, so ferrule selection and surface treatment are worth getting right, Crestflo applies anti-galling measures such as silver plating on request where high torque or repeated assembly is expected.
| Configuration | Description | When it is used |
|---|---|---|
| Double (twin) ferrule | Front ferrule seals, back ferrule grips | General instrumentation, vibration, gas-tight service |
| Single ferrule | One ferrule seals and grips | Simpler assemblies, certain hydraulic and OEM designs |
| Hardened ferrule | Case-hardened grip surface | Harder tube materials, maximum hold |
| Non-hardened ferrule | Softer grip surface | Thin-wall or softer tubing |
Replacement nuts and ferrules are stocked separately, and so are single-ferrule fittings and double-ferrule fittings.
Available 254 SMO Fitting Shapes & Connectors
UNS S31254 covers the full geometry range, so you can build a complete instrument run in a single material:
- Union, elbow, tee, cross, and reducer fittings
- Male connectors, female connectors, and adapters for joining mismatched threads
- Bulkhead unions, port connectors, caps, and plugs for sealing off unused lines
- Weld connectors (socket, butt, and orbital)
254 SMO comes in related fitting families too, including 37° JIC flare fittings and DIN 2353 bite-type fittings, all combinable within the same instrument tubing system. For the full geometry range and ferrule design, start at the tube-fittings product hub.
Corrosion & Service: Seawater, Chlorides, Chloride SCC, Bleaching & FGD
What a fitting actually sees in service is the whole reason to specify 254 SMO over 316 or 904L. This alloy was built for high-chloride and seawater duty, and that is exactly where it earns its place. The table below sums up how UNS S31254 behaves against the media that drive fitting selection. These are the grade's published, standard-referenced characteristics, not a per-fitting rating; the corrosion allowance and suitability for a specific stream stay the specifier's call.
| Media / service | 254 SMO behavior | Note |
|---|---|---|
| Chlorides / seawater / brackish water | Excellent | The defining application for the grade |
| Chloride stress-corrosion cracking (Cl-SCC) | Highly resistant | High nickel, the key advantage over 304/316 |
| Crevice corrosion | Excellent | High critical pitting and critical crevice temperature |
| Bleaching / chlorination (pulp & paper) | Excellent | Chlorine dioxide and bleach-plant streams |
| Flue-gas / FGD scrubber condensate | Excellent | Chloride-bearing SO₂ absorber environments |
| Sulfuric acid (H₂SO₄), dilute–moderate | Good | Copper addition aids reducing-acid resistance |
| Phosphoric acid (H₃PO₄) | Good | Including contaminated process grades |
| Sour service (H₂S) | Suitable | Within NACE MR0175 / ISO 15156 limits |
Where chloride pitting resistance has to come with higher strength, thin-wall, high-pressure runs where yield matters, super-duplex 2507 matches 254 SMO on PREN but roughly doubles the yield, so it is the better fit; see super-duplex 2507 tube fittings. Where the chloride load is even harsher, AL-6XN (UNS N08367), the sister 6-moly running slightly higher molybdenum and nitrogen, is the close alternative. And where the duty is lighter than seawater-grade, 904L is the economical step down.
Standards, Testing & NACE MR0175 Sour-Service Compliance
Crestflo qualifies its compression tube fittings to ASTM F1387, the performance standard for mechanically attached fittings. That is the basis for the dimensional interchange and mechanical-integrity claims below. For sour service, UNS S31254 fittings are offered to NACE MR0175 / ISO 15156 where the application qualifies; confirm the hardness and material-condition limits in the standard against your specific service before committing. The grade's product-form specifications sit in the ASTM A-series for high-alloy stainless, A213 and A269 (seamless tube), A249 (welded tube), A276 and A479 (bar), and A182 F44 (forgings and fittings), each with a matching ASME SA designation; we hold the standard approvals that apply to instrumentation tube fittings. Line-pipe specifications such as A312 sit outside this instrumentation scope.
| Standard / certification | Scope | Applies here |
|---|---|---|
| ASTM F1387 | Mechanically attached fitting performance | Yes |
| NACE MR0175 / ISO 15156 | Sour-service materials (where the application qualifies) | On request |
| EN 10204 MTC 3.1 / 3.2 | Material test certification | Yes |
| ASTM A213 / A269 / A479 | S31254 seamless tube & bar feedstock | As applicable |
| ISO 9001 / 14001 / 45001 & PED | Quality, environmental, safety, pressure | Certified |
These fittings go through a wide battery of tests. Hydraulic proof, burst, and impulse come first. Then pneumatic, flexure-fatigue, rotary-flex, vibration, and thermal-cycling work, plus pull-out and vacuum checks. Positive material identification (PMI) confirms the alloy on every piece, important on a 6-moly super-austenitic, where the molybdenum, nitrogen, and nickel content is exactly what buys the chloride performance and must be verified, not assumed. NDT is standard and done in-house. Destructive testing goes out to labs accredited under ISO/IEC 17025. Third-party inspection is available on request.
Material Traceability & Certification
Every UNS S31254 part is marked with its own heat/lot number. Each shipment comes standard with an EN 10204 MTC 3.1 certificate, and 3.2 is available on request. PMI confirms grade at the part level, every time. That documentation backs solid audit trails for seawater, offshore, chemical, and sour-service work, the jobs where a mis-specified or under-alloyed fitting is not a small problem. On a 6-moly, under-alloying is the specific risk: a fitting quietly supplied as 316 or a leaner grade looks identical but pits in service. Datasheets and CAD models are available to customers on request.
Drop-In Interchange with Swagelok, Parker & Hy-Lok
Crestflo UNS S31254 fittings are built as dimensional interchange / equivalent fittings for the major instrumentation brands, and that claim rests on ASTM F1387 performance qualification, not on marketing language. These aren't "compatible" or "intermix" parts. Brand names such as Swagelok®, Parker®, and Hy-Lok® remain trademarks of their respective owners, and Crestflo is not affiliated with them; the interchange is strictly dimensional. We don't publish a part-number table. Instead, we prepare interchange references per project: request your cross-reference, and our engineering team will map your existing part numbers to Crestflo 254 SMO equivalents.
Origin & US-Serving Positioning
Crestflo manufactures for the US market. Standard product comes from India; US or European melt is available on request, with full material traceability. Parts are engineered as US drop-in replacements and ship in 6-8 weeks. TAA/DFARS and domestic-melt requirements are handled straight, no hedging, on our US-origin compliance page. If a project calls for a specific melt origin, just specify it on the RFQ.
Applications & Industries for 254 SMO Tube Fittings
254 SMO fittings show up wherever chlorides outrun what 316 or even 904L can handle, the seawater-grade duty where pitting and crevice attack are the governing failure modes:
- Offshore oil & gas and subsea, platform instrument hook-ups, hydraulic control lines, and chemical-injection systems exposed to seawater, sea spray, and chloride-bearing produced fluids.
- Desalination and seawater cooling, SWRO, MSF, and MED plants, and seawater cooling-water instrumentation, where raw chloride service defeats standard stainless.
- Pulp & paper bleaching, chlorine-dioxide and bleach-plant lines, among the most aggressive chloride environments in process industry.
- FGD and pollution control, flue-gas desulphurisation absorbers and scrubber-condensate instrumentation, where chloride-bearing acidic condensate attacks lower alloys.
- Chemical and coastal/marine processing, mixed chloride and reducing-acid streams, and coastal plant instrumentation where the ambient environment alone is corrosive.
Crestflo supplies UNS S31254 instrumentation components into offshore and seawater and chemical processing applications, among others.
Why Crestflo for 254 SMO Tube Fittings
Crestflo operates as the instrumentation division of an export house with four decades behind it and formal recognition from the Government of India. That parent company has a proven delivery record across the Middle East. Its backing grounds a new, focused instrumentation brand built on a handful of clear advantages:
- Lead time of 6-8 weeks versus 20+ weeks typical of the majors.
- ASTM F1387 drop-in interchange proof that mill and trading suppliers rarely publish.
- Small MOQ and private label for distributors and OEMs.
- Both imperial and metric on every fitting, single or double ferrule, hardened or non-hardened.
- Heat-number marking plus MTC 3.1/3.2 on every order, PMI-confirmed, the under-alloying guard on a 6-moly.
- Cost-competitive after US duties and tariffs, ISO 9001 / 14001 / 45001 & PED certified.
We Engineer Confidence.
Request a Quote for 254 SMO Fittings
Give us the details: size, shape, ferrule, thread, and quantity. We'll price it out and get your order scheduled:
- Request a quote, the fastest path to pricing and lead time.
- Request a sample for qualification.
- Become a distributor for line-card and stocking programs.
Datasheets and CAD models are available to customers on request. Reach Crestflo's US sales desk at sales@crestflousa.com or +1 346 594 9005 (Houston, TX). Round out your system with the related 254 SMO instrumentation valves and 254 SMO tubing, and serve the chloride-critical lines in offshore and seawater and chemical processing.
Frequently Asked Questions
Are 254 SMO, UNS S31254, and 1.4547 the same thing?
Yes. "254 SMO," "Alloy 254," "SMO 254," "6Mo," and "6-moly stainless" all point to the same material: a 6% molybdenum, nitrogen-alloyed super-austenitic stainless steel running roughly 20Cr-18Ni-6Mo with about 0.2% nitrogen and a small copper addition, designated UNS S31254 and EN / Werkstoff 1.4547, with ASTM forging grade F44 (A182 F44). UNS S31254 is the US number and 1.4547 is the European one; they describe one and the same alloy, and a fitting labeled "254 SMO" is routinely certified to both at once. Note it is not the same as 253 MA, a different, heat-resistant grade.
Why use 254 SMO tube fittings instead of 316 or 904L?
254 SMO is the choice where chlorides defeat lower-alloy stainless. Its 6% molybdenum plus a nitrogen addition pushes PREN to about 43, well above 316L's ~24 and a clear step over 904L's ~34, so it resists chloride pitting and crevice corrosion at temperatures where those grades fail, and its high nickel resists chloride stress-corrosion cracking. That makes it a seawater-grade alloy: offshore, desalination, brine, bleaching, and FGD service. Step down to 904L for lighter chloride or reducing-acid duty that is beyond 316 but short of seawater-grade; step across to super-duplex 2507 when you need the same chloride resistance with far higher strength; and consider AL-6XN as the sister 6-moly for the harshest chloride loads.
How much do 254 SMO tube fittings cost?
254 SMO fittings cost more than 316 or 904L because of the higher molybdenum, nitrogen, and nickel and because the grade work-hardens and machines tough. In high-chloride service, though, it is often cheaper than the nickel alloys or titanium that would otherwise be needed. Price shifts with size, shape, ferrule type, and quantity ordered. Crestflo stays cost-competitive against the majors after US duties and tariffs, but it quotes per configuration. There's no live pricing posted anywhere. Request a quote to get a firm number.
What are 254 SMO tube fittings used for?
They handle chloride-driven instrumentation lines wherever 316 and 904L are under-alloyed: offshore and subsea instrument hook-ups and chemical-injection lines, seawater cooling and desalination systems, pulp-and-paper bleach plants, FGD scrubbers and pollution-control condensate, and coastal or marine chemical processing. In instrumentation terms, that's the fittings on impulse lines, sample systems, analyzer panels, and gauge hook-ups on those services.
Are Crestflo 254 SMO fittings NACE-compliant?
254 SMO can be supplied to NACE MR0175 / ISO 15156 for sour H₂S service where the application qualifies, with material traceability backing project qualification. Confirm the exact service envelope, H₂S partial pressure, chloride level, and temperature, and the hardness and material-condition limits in the standard against your application before committing.
Do they interchange with Swagelok, Parker, or Hy-Lok fittings?
They're built as dimensional interchange and equivalent fittings, with performance backed by ASTM F1387 qualification. We don't call them "compatible" or "intermix," and every brand name mentioned belongs to its respective owner; Crestflo is not affiliated. Send us your cross-reference request and we'll map your part numbers.
What sizes are available?
Tube-OD sizes run from 1/8-inch up to 2-inch (50 mm), covering both imperial fractional and metric series, in single or double ferrule. Standard sizes span 1/8" to 1" and 6 mm to 25 mm; anything larger is made to order.
Is 254 SMO magnetic?
No. 254 SMO is fully austenitic and essentially non-magnetic in the annealed condition, unlike super-duplex 2507, which is ferromagnetic. That non-magnetic character, alongside its chloride resistance, is a reason to choose it over a duplex where magnetic permeability matters. Heavy cold work can induce a trace of magnetism, as with other austenitic stainless steels, but a 254 SMO fitting as supplied is non-magnetic.
How does 254 SMO compare to super-duplex 2507 and AL-6XN for fittings?
254 SMO and super-duplex 2507 sit at about the same PREN (~43), so chloride pitting resistance is comparable, but 2507 is a ferromagnetic duplex with roughly double the yield strength, while 254 SMO is a non-magnetic austenitic that welds more forgivingly and stays tough at cryogenic temperatures. Pick 2507 when you need strength-per-wall; pick 254 SMO when you want austenitic behavior and weldability. AL-6XN (UNS N08367) is the sister 6-moly super-austenitic, running slightly higher molybdenum and nitrogen (PREN ~45-47), and is the close alternative for the harshest chloride duty.
Are single and double ferrule versions both offered?
Yes. Both single-ferrule and double (twin) ferrule designs are available, with hardened or non-hardened ferrules supplied depending on what's required. Because 254 SMO is a tough, work-hardening super-austenitic that can gall, anti-galling measures such as silver plating are offered on request where repeated make-up or high torque is expected.
What material certification comes with the fittings?
Standard practice is an EN 10204 MTC 3.1 certificate, though 3.2 is available on request. Every part gets a heat/lot number stamped on it, and PMI confirms the alloy is genuinely S31254, which matters on a 6-moly, where the molybdenum, nitrogen, and nickel content is what buys the chloride performance, and where an under-alloyed substitute looks identical but pits in service.
Are these made in the USA?
Crestflo manufactures for the US market. Standard product comes from India, though US or European melt is available on request, complete with full material traceability. It's engineered for a US drop-in fit and ships in 6-8 weeks. For TAA and origin specifics, see the US-origin compliance page.
What thread and end connections are available?
Tube-OD compression ends come standard, alongside NPT, NPTF, BSPT, BSPP, and ISO parallel threads. JIC 37° and SAE ORB fittings round out the lineup, and weld ends are available in socket, butt, and orbital styles. National, international, or custom thread, they're all supported.
What is the minimum order and lead time?
Small minimum order quantities apply, and lead time for US orders runs 6 to 8 weeks. Standard items ship from ready stock; specials are made to order. Private-label and custom or made-to-print options are also available.
Related Pages
- 254 SMO alloy grade & corrosion data
- 254 SMO instrumentation valves
- 254 SMO tubing
- 254 SMO vs Super Duplex vs AL-6XN · Super Duplex 2507 · Super Duplex 2507 tube fittings · 904L
- Tube-fittings product hub · single-ferrule fittings · double-ferrule fittings · ASTM F1387 standard
- Offshore & seawater · chemical processing