904L Tube Fittings (UNS N08904 / W.Nr. 1.4539, Super-Austenitic Stainless Instrumentation Fittings)
904L tube fittings are mechanical-grip compression fittings machined from UNS N08904, a super-austenitic stainless steel running roughly 20Cr-25Ni-4.5Mo with a deliberate 1.5% copper addition and one of the lowest carbon limits of any austenitic grade. One or two ferrules swage onto the tube's outside diameter for a leak-tight, gas-tight seal and a firm mechanical grip. That high-nickel, high-moly, copper-bearing chemistry is the whole reason 904L gets specified for instrument lines: it resists reducing acids, sulfuric and phosphoric in particular, along with chloride pitting and chloride stress-corrosion cracking far better than 316, and it stays fully austenitic and non-magnetic while doing it. Crestflo carries these fittings from 1/8-inch through 2-inch (50 mm) tube OD, in single and double (twin) ferrule versions, 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.
Swagelok®, Parker®, and Hy-Lok® are trademarks of their respective owners. Crestflo supplies UNS N08904 to standard chemistry, is not affiliated with or endorsed by any brand or mill referenced here, and all interchange is strictly dimensional.
What 904L Compression Tube Fittings Are
A 904L compression tube fitting is built for fluid-system instrumentation work: a body, a nut, and one or two ferrules that bite onto the tube's outer diameter as the nut is pulled up. 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 itself, not from an O-ring giving out first.
Scope matters here. These are instrumentation compression tube fittings in UNS N08904 super-austenitic stainless, not mill bar, not sheet or plate, not watch-case steel, and not electrical wire ferrules. ("Oystersteel" is a watch-industry trade name for a 904L variant; that is a different world entirely.) Crestflo's 904L fittings land on small-bore instrument tubing for transmitters, analyzers, sample lines, chemical-injection systems, and process taps across acid-handling, flue-gas, seawater, and chloride-bearing service, the reducing-acid and chloride duty where 316 pits, cracks, or simply corrodes away.
Also Known As: UNS N08904, 1.4539, SAE 904L & X1NiCrMoCu25-20-5
Buyers and specifiers write this grade several ways, but they all point to the same super-austenitic stainless. On drawings and in material call-outs it shows up as 904L, Alloy 904L, Type 904L, SAE 904L, 904 L, and grade 904L. Its formal designations are UNS N08904 (the US number) and EN / Werkstoff 1.4539 (the European one, EN steel name X1NiCrMoCu25-20-5). Ask for "904L ferrule fittings," "UNS N08904 tube fittings," "1.4539 fittings," or "904L compression fittings" and you get exactly what's described on this page, the terms are interchangeable.
| Designation type | Value |
|---|---|
| Common names | 904L, Alloy 904L, Type 904L, SAE 904L, 904 L, grade 904L |
| UNS number | N08904 |
| EN / Werkstoff (W.Nr.) | 1.4539 |
| EN steel name | X1NiCrMoCu25-20-5 |
| Alloy family | Super-austenitic (high-alloy austenitic) stainless steel |
UNS N08904 and Werkstoff 1.4539 describe one and the same alloy, so no matter which designation shows up on a spec sheet, you are looking at the same 904L super-austenitic stainless. A fitting labeled "904L" is routinely certified to both at once.
Why Choose 904L for Instrumentation Tube Fittings
904L is a copper-bearing super-austenitic stainless, nominally 20% chromium, 25% nickel, 4.5% molybdenum, with about 1.5% copper, and that chemistry is what makes it the right pick for a fitting in acid and chloride service. Two things drive the choice. First, reducing acids: the high nickel and the copper addition give 904L a genuine edge in dilute-to-moderate sulfuric acid and in phosphoric acid, environments that defeat 316 and plain 304. Second, chlorides: with a PREN near 34, 904L resists chloride pitting and crevice corrosion well beyond 316L (≈24), and its high-nickel austenitic structure gives it strong resistance to chloride stress-corrosion cracking, the failure mode that most often limits 304 and 316 in warm chloride service.
Be precise about where 904L sits, though. It is a fully austenitic grade, so it is non-magnetic in the annealed condition, unlike a duplex, which is ferromagnetic. That non-magnetic character is itself a reason to specify 904L where magnetic permeability matters. Its strength is moderate: 904L is not a high-strength grade like duplex, and its yield is comparable to 316 rather than double it, so the case for 904L is corrosion, not pressure-per-wall. Fully austenitic also means it keeps its toughness down to cryogenic temperatures, with no ductile-to-brittle transition.
Know the step-ups too. 904L is a corrosion step above 316 but below the 6-molybdenum super-austenitics. Where the duty is hotter seawater or higher-chloride service than 904L comfortably handles, 254 SMO or AL-6XN (PREN ~43) is the move. Where the driver is strength plus chloride resistance, a duplex such as 2205 or super duplex 2507 fits better. Where it is concentrated or hot sulfuric acid, Alloy 20 is the specialist. Match the alloy to the media and don't over-specify.
For the full metallurgical picture, composition, mechanical and physical properties, and corrosion-by-media data, see the 904L 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 | Min % | Max % |
|---|---|---|
| Chromium (Cr) | 19.0 | 23.0 |
| Nickel (Ni) | 23.0 | 28.0 |
| Molybdenum (Mo) | 4.0 | 5.0 |
| Copper (Cu) | 1.0 | 2.0 |
| Manganese (Mn) | — | 2.00 |
| Silicon (Si) | — | 1.00 |
| Carbon (C) | — | 0.020 |
| Phosphorus (P) | — | 0.045 |
| Sulfur (S) | — | 0.035 |
| Iron (Fe) | Balance | |
Copper is the element that separates 904L from other stainless grades in reducing acids, while the high molybdenum and chromium drive pitting resistance and the high nickel underwrites chloride SCC resistance. The very low carbon limit keeps corrosion resistance intact in and around welds. The key strength and corrosion figures that make 904L a fitting-grade super-austenitic are summarized here.
| Property | 904L (UNS N08904) |
|---|---|
| Tensile strength (min) | 490 MPa (71 ksi) |
| Yield strength (0.2%, min) | 220 MPa (32 ksi) |
| Elongation (min) | 35% |
| Hardness | ≤ ~90 HRB |
| Density | 7.95 g/cm³ (0.287 lb/in³) |
| Magnetic response | Austenitic; essentially non-magnetic in the annealed condition |
| PREN (Cr + 3.3×Mo + 16×N) | ≈ 34–36 |
| Max continuous corrosion service | ≈ 350–400 °C (660–750 °F), environment-dependent |
Those figures are standard published minimums and nominal physical data for the annealed condition; actual certified properties for a given heat are stated on the material test certificate. For the head-to-head against the baseline austenitic, see 904L vs 316; for the step up in chloride duty, compare 254 SMO and, where strength matters, Duplex 2205.
904L Tube Fitting Sizes (1/8-inch to 2-inch OD): Imperial & Metric
Crestflo machines UNS N08904 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 |
904L 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 N08904 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. 904L is a relatively soft, ductile austenitic that work-hardens quickly, so ferrule selection and surface treatment are worth getting right, Crestflo applies anti-galling measures such as silver plating on request where repeated make-up or high torque is expected, which matters on a high-nickel austenitic prone to galling.
| 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 904L Fitting Shapes & Connectors
UNS N08904 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)
904L 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: Sulfuric & Phosphoric Acid, Chlorides, Chloride SCC & Seawater
What a fitting actually sees in service is the whole reason to specify 904L over 316. The table below sums up how UNS N08904 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 | 904L behavior | Note |
|---|---|---|
| Sulfuric acid (H₂SO₄), dilute–moderate | Good–Excellent | Copper addition is the defining advantage over 316L |
| Phosphoric acid (H₃PO₄) | Good | Including contaminated process acid |
| Acetic / formic & organic acids | Excellent | Organic-acid process streams |
| Warm chlorides / pitting & crevice | Good | ≈ 2× the molybdenum of 316L; PREN ~34 |
| Chloride stress-corrosion cracking | Highly resistant | High nickel, the key advantage over 304/316 |
| Seawater / brackish water (ambient) | Good | For hotter/higher-chloride seawater, step up to 254 SMO / AL-6XN |
| Flue-gas / FGD scrubber condensate | Good | Reducing, chloride-bearing condensate |
| Concentrated / hot sulfuric acid | Limited | Step up to Alloy 20, the sulfuric specialist |
Where the driver is hotter chloride, seawater, or stronger pitting risk than 904L handles, the 6-molybdenum route is the better fit, move to 254 SMO or AL-6XN. Where the service needs strength plus chloride resistance, a duplex is the call; where it is concentrated or hot sulfuric, Alloy 20 is purpose-built for it.
Standards, Testing & NACE MR0175 Sour-Service Compliance
Crestflo qualifies its compression tube fittings to ASTM F1387, the performance standard for mechanically attached fittings. That's the basis for the dimensional interchange and mechanical-integrity claims below. For sour service, UNS N08904 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 B-series for high-alloy stainless, B677 (seamless pipe and tube), B673 (welded tube), B674 (welded pipe), B649 (bar), and B625 (plate, sheet and strip), each paired with a matching ASME SB designation; we hold the standard approvals that apply to instrumentation tube fittings.
| 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 B677 / B673 / B649 | N08904 seamless tube, welded 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 high-alloy super-austenitic, where the nickel, molybdenum, and copper content is exactly what buys the corrosion 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 N08904 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 acid-handling, chemical, seawater, and sour-service work, the jobs where a mis-specified or under-alloyed fitting is not a small problem. Datasheets and CAD models are available to customers on request.
Drop-In Interchange with Swagelok, Parker & Hy-Lok
Crestflo UNS N08904 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 904L 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 904L Tube Fittings
904L fittings show up wherever reducing acids or chlorides outrun what a 316 austenitic can handle, but the duty doesn't yet justify a 6-moly or nickel alloy:
- Chemical and petrochemical acid handling, dilute-to-moderate sulfuric and phosphoric acid lines, mixed-media and organic-acid streams, and sampling systems on those services.
- Pollution control and FGD, flue-gas desulphurisation and scrubber-condensate instrumentation, where reducing, chloride-bearing condensate attacks standard stainless.
- Pickling plants and metal processing, acid-pickling and process lines where sulfuric and mixed acids are the daily environment.
- Oil & gas, sour and chloride-bearing production and process instrumentation, supplied to NACE MR0175 / ISO 15156 where qualified.
- Seawater, brine and pulp & paper, ambient-chloride cooling and process lines, desalination service, and bleach-plant instrumentation.
Crestflo supplies UNS N08904 instrumentation components into chemical processing and oil and gas applications, among others.
Why Crestflo for 904L 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.
- Cost-competitive after US duties and tariffs, ISO 9001 / 14001 / 45001 & PED certified.
We Engineer Confidence.
Request a Quote for 904L 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 904L instrumentation valves and 904L tubing, and serve the corrosion-critical lines in chemical processing and oil and gas.
Frequently Asked Questions
Are 904L, UNS N08904, and 1.4539 the same thing?
Yes. "904L," "Alloy 904L," "Type 904L," and "SAE 904L" all point to the same material: a super-austenitic stainless steel running roughly 20Cr-25Ni-4.5Mo with a 1.5% copper addition, designated UNS N08904 and EN / Werkstoff 1.4539 (EN steel name X1NiCrMoCu25-20-5). UNS N08904 is the US number and 1.4539 is the European one; they describe one and the same alloy, and a fitting labeled "904L" is routinely certified to both at once.
Why use 904L tube fittings instead of 316, and when should I step up to a 6Mo alloy?
904L is the better choice where the stream carries reducing acids, sulfuric or phosphoric, or chlorides that would pit or crack 316. It carries roughly double the molybdenum of 316L plus higher nickel and a copper addition, pushing PREN to about 34 versus about 24 for 316L, and the copper specifically buys resistance in sulfuric and phosphoric acid. Its high-nickel structure resists chloride stress-corrosion cracking too. Step up to a 6-molybdenum super-austenitic like 254 SMO or AL-6XN (PREN ~43) when the duty is hotter seawater or higher chloride than 904L handles; step to Alloy 20 for concentrated or hot sulfuric acid; and to a duplex where you also need the strength.
How much do 904L tube fittings cost?
904L fittings cost more than 316 because of the higher nickel, molybdenum, and copper, and because the grade work-hardens and machines tougher than 316L. 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 904L tube fittings used for?
They handle acid- and chloride-driven instrumentation lines wherever 316 is under-alloyed but a 6-moly or nickel alloy is over-specified: chemical and petrochemical acid handling (dilute-to-moderate sulfuric and phosphoric acid), flue-gas desulphurisation and pollution-control condensate, pickling plants, oil-and-gas sour and chloride service, and seawater, brine, and pulp-and-paper bleach-plant lines. In instrumentation terms, that's the fittings on impulse lines, sample systems, analyzer panels, and gauge hook-ups on those services.
Are Crestflo 904L fittings NACE-compliant?
904L 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 904L magnetic?
No. 904L is fully austenitic and essentially non-magnetic in the annealed condition, unlike a duplex such as 2205, which is ferromagnetic. That makes 904L a sensible pick where low magnetic permeability matters. Heavy cold work can induce a trace of magnetism, as with other austenitic stainless steels, but a 904L fitting as supplied is non-magnetic.
What is the maximum service temperature for a 904L fitting?
904L is typically used for corrosion service up to around 350–400 °C (660–750 °F), depending on the environment. Because it is fully austenitic it also keeps its toughness down to cryogenic temperatures, with no ductile-to-brittle transition, so unlike a duplex, it is not ruled out of low-temperature duty. The rated pressure of the finished fitting still depends on size, wall, and temperature, confirmed on the quotation.
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 904L is a high-nickel 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 N08904, which matters on a super-austenitic, where the nickel, molybdenum, and copper content is what buys the corrosion performance.
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.