Monel 400 Tubing (UNS N04400)

Monel 400 tubing is OD-sized, round instrumentation tube drawn from nickel-copper Alloy 400 (UNS N04400, W.Nr. 2.4360), a solid-solution grade of roughly 67% nickel and 30% copper. It carries process fluid or gas between components and seats into a compression fitting's ferrule set to make a leak-tight, make-and-break joint. Crestflo supplies 400 tube seamless to ASTM B165 and welded to ASTM B725, in both imperial and metric sizes, shipped in the annealed condition so it bends and cones cleanly into a ferrule. You reach for Monel 400 tube when a line runs seawater, brine, hydrofluoric acid, or chloride-heavy media, the service where 316 stainless pits or stress-corrosion cracks and copper alloys erode. It is the benchmark marine and HF-service tube, immune to the chloride stress-corrosion cracking that limits austenitic stainless, and it stays tough down to cryogenic temperatures. Standard items ship from stock; specials are built to order, with delivery running 6-8 weeks. Request a quote to price your run.

Reviewed by the Crestflo Engineering Team

Monel® is a registered trademark of Special Metals Corporation; Crestflo supplies UNS N04400 to standard chemistry and is not affiliated with or endorsed by Special Metals Corporation. Swagelok® and Parker® are trademarks of their respective owners; Crestflo is not affiliated, and interchange is dimensional.

Why Specify Monel 400 Tubing

Tube is the long, continuous wetted surface of an instrument hookup. Unlike a fitting or a valve, it has no dead legs or packing bores, but it has length, every inch of it exposed to the media, and it has to survive the cold work of bending and ferrule make-up without cracking. On a seawater or chloride line that rules out a lot of materials. A stainless tube that pits or stress-corrosion cracks midway along a run is as dead as one that fails at the fitting.

Alloy 400 is a binary nickel-copper solid solution whose chemistry is tuned for exactly the media that defeat stainless. Nickel and copper are mutually soluble across the full range, which gives the alloy a single-phase structure and its standout immunity to chloride stress-corrosion cracking, the failure mode that limits austenitic stainless in seawater and brine. That same chemistry makes it the industry benchmark alloy for hydrofluoric acid, and it holds up strongly in brine, neutral and alkaline salts, caustics, and dilute reducing acids such as sulfuric and hydrochloric. For a tube run on an offshore seawater system, an HF alkylation unit, or a chloride-laden brine line, that is the whole reason to specify the nickel-copper grade over stainless.

Monel 400 has one honest limit worth stating up front: it is attacked by strongly oxidizing media. Nitric acid, oxidizing salts, wet chlorine, and ferric or cupric chlorides will corrode it, so where the stream turns oxidizing, a stainless or a Hastelloy grade is the right call. And because 400 is a moderate-strength, non-age-hardenable alloy, where a run needs high mechanical strength the age-hardenable nickel-copper grade Monel K500 is the better base.

The full metallurgical picture, composition ranges, mechanical and physical properties, corrosion-by-media data, and the note on why PREN does not apply to a chromium-free alloy, lives on the Monel 400 alloy grade page. This page stays on the tube: how it's made, sized, finished, tested, and matched to the fitting.

Also Specified As: Alloy 400 Tube, Monel 400 Seamless Tubing, UNS N04400 Tube

Buyers and specifiers call this tube several ways, and each one points to the same Ni-Cu material. A requisition might read "Alloy 400 tubing," "Monel 400 seamless tube," "N04400 instrument tube," "Nickel-Copper Alloy 400 tube," or "Monel tube," and every one of them lands here. Monel® 400 is the trademarked trade name; Alloy 400, Nickel-Copper Alloy 400, and 400 are the generic forms; the material is registered as UNS N04400 and Werkstoff 2.4360, with the German descriptive designation NiCu30Fe and the British designation BS NA13. The welded form is often written "welded tube," the seamless form "cold-drawn seamless tube."

Scope: this is round, OD-sized instrumentation and fluid-system tube for compression and tube-fitting connections. It is not oilfield coiled tubing for downhole well intervention, not marine propeller shafting or fastener stock, not structural or ornamental tube, and not pipe specified by nominal pipe size (NPS) and schedule. Crestflo supplies Monel 400 in instrumentation tube, fitting, and valve forms only, not mill bar, sheet, plate, or pipe.

Seamless vs Welded Monel 400 Tubing

Both constructions are built to their own nickel-copper tube specification, and both accept a compression fitting the same way. Seamless Monel 400 tube is cold-drawn from a hollow with no longitudinal weld, which makes it the default for instrument work and the pick where you want maximum integrity for higher-pressure, cyclic, or critical service. Welded Monel 400 tube is formed from strip, seam-welded, then drawn to size; it is dimensionally consistent and cost-effective for general fluid-system work. Alloy 400 needs no post-weld heat treatment to restore corrosion resistance, because it is a single-phase solid solution that does not sensitize, so a sound weld in the nickel-copper base keeps the same resistance as the parent metal.

Seamless is the primary instrument form and the one most Monel 400 tube is specified in; welded is the economical choice for longer, lower-duty runs. Not sure which construction to call out? The deep dives on seamless tubing and welded tubing walk through the trade-off before you commit a spec.

Monel 400 Tube Attributes at a Glance

Monel 400 Tube Sizes: OD, Wall & Imperial/Metric

Instrument tube is specified by its actual outside diameter and wall thickness, given in both imperial and metric units, never by NPS and schedule. The common instrument range runs fractional 1/8″ to 1″ and metric 6 to 12 mm; smaller 1/16″ small-bore tube and larger diameters up to 2″ (50 mm) are available to order. Wall thickness is chosen to carry the pressure and to suit the ferrule, and both OD and wall are held to the tolerances of ASTM B165 (seamless) or B725 (welded). Working pressure is not a single blanket figure: it is calculated from OD, wall thickness, and the alloy's allowable stress per ASME B31.3, so it is stated per size and wall rather than as one rating for the grade.

Values per applicable standard; customizable to requirement. For how to work back from a target pressure to a wall thickness, see the tube sizing and wall thickness guide, and for the full dimensional matrix across the range, the tubing size chart.

Temper & Hardness: Annealed for Bending and Ferrule Seating

The single most important mechanical property of instrument tube is not its peak strength but its softness. Alloy 400 is a solid-solution grade that does not age-harden, so it comes in one working condition, annealed, and that condition does two jobs at once. First, it is what lets the tube bend around a radius and cone down under a ferrule without cracking, so the joint seats and seals the way a compression fitting is designed to. Second, the annealed condition is the one NACE MR0175 / ISO 15156 accepts for UNS N04400 in sour service, so the temper that makes the tube workable is also the temper that makes it qualified. In the annealed condition Monel 400 carries a tensile strength around 480–550 MPa (70–80 ksi) with a 0.2% yield near 170–240 MPa (25–35 ksi) and elongation above 35%, with hardness in the annealed band (roughly 110–150 HB, about 65–80 HRB). It is a moderate-strength alloy rather than a high-strength one, but it work-hardens as it is cold-drawn, so a finished tube carries strength above the annealed base values. Where sustained high mechanical strength governs, the age-hardenable Monel K500 is the grade to reach for. Specify the hardness or condition to your own standard on the RFQ and it is confirmed on the certificate.

Finishes & Cleanliness: Bright-Anneal, Electropolish & Oxygen-Clean

Monel 400 tube ships in a mill / bright-annealed finish as standard. Because this is a nickel-copper alloy rather than a chromium stainless, the finishing route is cleaning and degreasing rather than acid passivation; for high-purity gas, analytical, and oxygen service, electropolishing (EP) and oxygen / UHP cleaning with capped-and-bagged packaging are available. Electropolishing lowers surface roughness and smooths the inner diameter, which matters where particle generation, moisture dry-down, or deposit build-up is the concern. Standard UHP electropolish surface-roughness targets can be specified right on the RFQ to your own standard; electropolished tubing and oxygen and UHP cleaning cover the options in detail. Finish and cleanliness figures follow the applicable standard and are customizable to requirement.

Monel 400 Tube Standards: ASTM B165, B725 & ASME B31.3

Monel 400 tube is covered by the wrought nickel-copper tube specifications, not the austenitic-stainless ones, and the construction decides which applies: seamless falls under ASTM B165, welded under ASTM B725, each with its matching ASME boiler-code SB- designation. Allowable working pressure for a given size and wall is calculated per ASME B31.3. For sour service, the material is offered to NACE MR0175 / ISO 15156. Crestflo holds the standard approvals that apply to instrumentation tube and is ISO 9001 / 14001 / 45001 and PED certified.

Working Pressure: Calculated per ASME B31.3, Not Assumed

There is no single pressure rating for "Monel 400 tube," and any supplier who quotes one is skipping the engineering. Allowable internal pressure depends on three things: the tube OD, its wall thickness, and the alloy's allowable stress at the service temperature. ASME B31.3 sets the method, relating wall thickness to pressure, diameter, the allowable stress value S, the weld-joint quality factor E, and the coefficient Y. Seamless tube carries a higher joint factor than welded, which is part of why seamless is chosen for higher-pressure work. Alloy 400 is a moderate-strength, non-age-hardening grade, so its allowable stress sits below that of the high-strength nickel-chromium grades; the thin-wall instrument sizes still carry ample pressure for analyzer, sample, seawater, and injection duty, and because the alloy keeps useful strength up to roughly 480–540 °C its allowable stress holds reasonably as the line warms. Give us the OD, wall, temperature, and medium on the RFQ and the engineering team returns the calculated working pressure for your configuration. Values per the applicable standard; customizable to requirement.

Testing, NDT & NACE MR0175 Sour-Service Compliance

Every length of Monel 400 tube is verified before it ships. ASTM B165 and B725 require each tube to pass either a hydrostatic test or a nondestructive (eddy-current) electric test, and dimensional checks confirm OD, wall, and straightness against the ordered tolerances. Positive material identification (PMI) confirms the alloy on the material, which matters for a high-nickel grade where substitution is a real and costly risk, and it also distinguishes N04400 from the look-alike stainless and copper-nickel grades it can be mistaken for. NDT is done in-house; destructive testing, where a job calls for it, goes to ISO/IEC 17025-accredited labs, and third-party inspection is available on request.

For sour service, Monel 400 tube material is offered to NACE MR0175 / ISO 15156: in the annealed condition, nickel-copper alloy UNS N04400 is an accepted material for sour (H₂S-containing) environments under NACE MR0175 / ISO 15156-3. Confirm the hardness and environmental limits, H₂S partial pressure, temperature, and chloride content, in the standard against your application, and note the sour-service requirement on the RFQ.

Straight Lengths & Coiled Monel 400 Tube

Monel 400 tube ships as straight lengths for racked and rigidly supported runs, and as continuous coils for heat-tracing, long analyzer and sample lines, and chemical-injection and hydraulic bundles where a joint-free run is the whole point. Coiled tube keeps the count of mechanical joints down, which on a seawater, brine, or buried line is one fewer place to leak, and the alloy's chloride-SCC immunity makes it a natural choice for the injection and umbilical cores that run in seawater and produced-water service. Both forms are supplied annealed; call out length or coil length on the RFQ. For long-run and tracing applications, see coiled tubing; for thicker-wall high-pressure control lines, hydraulic tubing.

Material Traceability & Certification

Every length of Monel 400 tube carries its own heat/lot number, and each order ships with an EN 10204 MTC 3.1 certificate as standard, 3.2 available on request. PMI confirms the grade at the part level, every time. That documentation backs the audit trails that offshore, marine, HF-alkylation, and sour-service jobs demand, the kind of work where a mis-specified tube in a long run is an expensive thing to discover in commissioning. Full detail is on the traceability page.

Applications for Monel 400 Tubing

Monel 400 tube earns its place wherever seawater, chlorides, hydrofluoric acid, or reducing-acid brine rule stainless out of a long wetted run:

• Offshore and marine systems, seawater cooling, firewater, ballast, produced-water, and deck instrument lines, where immunity to chloride stress-corrosion cracking is the deciding factor. See the marine and shipbuilding industry page and offshore industry page.
• HF alkylation in refining, hydrofluoric acid is the alloy's signature service; instrument, sample, and analyzer lines on HF units are a classic N04400 application.
• Seawater and brine handling, desalination, coastal plant cooling, and brine-transfer instrument lines in high-chloride water.
• Chlor-alkali and reducing-acid process, caustic, and dilute sulfuric and hydrochloric service on dosing, sample, and process lines.
• Sour oil and gas, H₂S-bearing wellhead, platform, and chemical-injection instrument lines qualified to NACE MR0175 / ISO 15156. See the oil & gas industry page.
• LNG-adjacent and cryogenic duty, because Monel 400 stays tough and ductile to sub-zero temperatures, it suits cold instrument lines where many alloys lose toughness.

Pairing Monel 400 Tube With Monel 400 Fittings and Valves

A tube is one part of the hookup, and the joint is only as corrosion-resistant as its weakest metal. For a matched instrument run, pair this tube with Monel 400 tube fittings and Monel 400 instrumentation valves so the whole assembly, tube, ferrules, and valve body, carries the same resistance and the same traceability. Crestflo supplies the tube alongside compression fittings tested to ASTM F1387, so the ferrule set seats correctly on the exact OD and wall you ordered, and the joint is a dimensional match rather than a mix of two suppliers' tolerances. One trait to note: Monel 400 can gall against itself, so on the fitting side Crestflo applies anti-galling measures such as silver plating on request to keep make-up smooth. Putting a Monel 400 tube into a 316 fitting just moves the failure point to the ferrule.

Drop-In Interchange and the Tube-Plus-Fitting System

Crestflo Monel 400 tube is cut and finished to standard OD and wall, so it drops into the compression fittings you already run, and the matching 400 fittings are built as dimensional interchange / equivalent parts for the major brands, with the fitting ends qualified to ASTM F1387. These are not "compatible" or "intermix" parts. Swagelok® and Parker® remain trademarks of their respective owners, and Crestflo is not affiliated; the interchange is strictly dimensional. Moving a run off a major brand? Request your Swagelok®/Parker® cross-reference and the engineering team maps your tube and fitting part numbers to Crestflo Monel 400 equivalents. We don't publish a part-number table.

Origin & US-Serving Positioning

Crestflo manufactures for the US market. Standard product is of Indian origin; US or European melt is available on request, with full material traceability. Tube is engineered as a US drop-in and ships in 6-8 weeks, well under the 20+ weeks typical of the majors. If a project calls for a specific melt origin or TAA/DFARS handling, specify it on the RFQ and we'll address it straight, no hedging. See the US-origin compliance page for how we document it.

Why Crestflo for Monel 400 Tubing

Crestflo operates as the instrumentation division of an export house with four decades behind it and formal recognition from the Government of India, with a proven Middle-East delivery record. That backing grounds a focused instrumentation brand built on a handful of clear advantages:

• Lead time of 6-8 weeks versus 20+ weeks typical of the majors, on standard stock and made-to-order specials alike.
• A matched system, tube plus F1387-tested fittings and ferrules that seat on the exact OD and wall you ordered.
• Both seamless and welded Monel 400 tube, in both imperial and metric, straight or coiled.
• Heat-number marking plus MTC 3.1/3.2 on every order, PMI-confirmed, so a high-nickel grade is never taken on trust.
• NACE MR0175 / ISO 15156 material options for sour service, in the annealed condition.
• ISO 9001 / 14001 / 45001 & PED certified, cost-competitive after US duties and tariffs, with small MOQs and private-label available.

We Engineer Confidence.

Request a Quote for Monel 400 Tubing

Give us the details: seamless or welded, OD and wall, straight length or coil, finish and cleanliness, and the line conditions (media, pressure, temperature, sour-service requirement). We'll calculate the working pressure, confirm the spec, and schedule the run.

• Request a quote, the fastest path to pricing and lead time.
• Request a sample for qualification.
• Request your Swagelok®/Parker® cross-reference and we'll map your tube and fitting part numbers.

Datasheets and CAD 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 the system with Monel 400 tube fittings and Monel 400 valves, browse the tubing pillar for other grades and constructions, and see where this tube serves in marine and shipbuilding and offshore.

Frequently Asked Questions

Related Pages

• Monel 400 alloy grade & corrosion data · Monel K500 (higher-strength nickel-copper)
• Monel 400 tube fittings · Monel 400 instrumentation valves
• Tubing pillar · Instrumentation tubing · Seamless tubing · Welded tubing
• ASME B31.3 · NACE MR0175 · EN 10204 MTC · Marine & shipbuilding · Offshore · Oil & gas