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Monel 400 / (2.4360) / UNS N04400 / AMS 4675 / AMS 4544

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Monel 400 (2.4360 – 2.4361), also widely known as Alloy 400, is an alloy of nickel and copper. Belonging to the Monel family, this nickel-copper material is the most widely used and most readily available Monel grade. It has a very high degree of corrosion resistance, and its mechanical values are high as well. Able to withstand a wide range of corrosive environments, this nickel/copper alloy is chosen across a very broad range of applications.

Alloy 400 (Monel 400) is used in many marine fittings, in parts exposed to chemical processes, in valves, pumps, special shafts, connecting parts and screws, and in electrical components. It is also very suitable for welding, and is used from time to time in boat and yacht shafts. The material is also resistant to acids such as hydrochloric acid and phosphoric acid. It is weakly attracted by a magnet and is not a non-magnetic material.​‌​​‌​

Able to work across a very wide temperature range, this nickel-copper alloy operates at sub-zero temperatures and also retains most of its mechanical properties up to 450-500 °C. A much more readily machinable version of this material, Monel Alloy R-405 (Monel R405), is also available.

Standards by Product Form​‌​​‌​

Monel 400 (UNS N04400 / W.Nr. 2.4360 / EN NiCu30Fe) is a solid-solution Ni-Cu alloy. Two things on this page deserve particular care: magnetic behaviour (the alloy is sold as “non-magnetic” and is not) and flow regime in seawater (flowing seawater and stagnant seawater are not the same duty).​‌​​‌​

R-405 and K-500 — Separate UNS Numbers, Separate Products​‌​​‌​

Monel R-405 is UNS N04405, NOT N04400. The chemical difference is sulphur alone: S 0.025–0.060 % — a band with a minimum as well as a maximum — against S ≤0.024 % for N04400. Every other limit is identical. Forms: the originator states R-405 is “normally furnished only in the form of rod and bar“, and ASTM corroborates indirectly: B164 is the only ASTM product specification that lists N04405; B127, B165, B163, B725, B730 and B564 do not cover it. The mill’s own words: R-405 “is used chiefly for automatic screw-machine stock and is not generally recommended for other applications“. So “Monel R-405 sheet to ASTM B127” is a product that does not exist. And a grade whose sulphur is deliberately raised to 0.060 % is a poor choice for welded fabrication, given that nickel alloys embrittle with sulphur at temperature — specify N04400 for anything that will be welded.

Monel K-500 is UNS N05500 / 2.4375 and is age-hardenable: Al 2.30–3.15 % and Ti 0.35–0.85 % precipitate Ni₃(Ti,Al). Its specifications are ASTM B865, QQ-N-286 and AMS 4676 — not B127/B164/B165 and not QQ-N-281. Aged, it reaches 965–1310 MPa tensile and 27–38 HRC. And critically it is effectively non-magnetic (permeability ≈1.001–1.002). Alloy 400 cannot be heat-treated to K-500 strengths — it hardens only by cold work.​‌​​‌​

Composition (B127/B164/B165, identical across all three): Ni (+Co) 63.0 % min — a MINIMUM, not a maximum · Cu 28.0–34.0 % — a band with both ends specified · Fe ≤2.5 % · Mn ≤2.0 % · C ≤0.30 % · Si ≤0.50 % · S ≤0.024 %. Why nickel is a floor and copper a band: nickel is the corrosion-controlling element and the matrix former, and specifying it as a minimum guarantees the reducing-acid and caustic performance. Copper is bounded at both ends: too little and you lose the seawater and HF behaviour; too much and the alloy drifts toward the copper-rich cupronickels, dropping the Curie point and the nickel content and degrading reducing-acid resistance.

What the small iron and manganese do. Iron (≤2.5 %) is a deliberate residual, not a tramp element — the EN/DIN name is NiCu30Fe. It improves impingement and erosion resistance in flowing seawater and contributes solid-solution strengthening. Manganese (≤2.0 %) is a deoxidiser and, critically, a sulphur scavenger: it ties up residual sulphur as MnS and suppresses the low-melting nickel-sulphide grain-boundary films that cause hot-shortness in hot working and welding. The filler metal takes this further — ERNiCu-7 carries up to 4 % Mn and 1.5–3.0 % Ti.​‌​​‌​

Two real chemistry divergences — a dual-certification trap. (1) Carbon: ASTM/ASME allow 0.30 % max; one European mill runs 0.15 % max on its own product. Both are “to specification”. (2) Iron minimum: ASTM sets NO iron minimum; EN/DIN NiCu30Fe requires Fe ≥1.0 %. Material fully compliant with ASTM B127 at Fe = 0.4 % would fail an EN 2.4360 order.

Mechanical Properties — Minimums and Typicals​‌​​‌​

Typical values (NOT minimums, not enforceable): annealed rod 517–620 MPa tensile / 172–345 MPa yield / 60–35 % elongation / 110–149 HB · cold-drawn and stress-relieved rod 579–827 MPa / 379–690 MPa / 40–22 % / 160–225 HB · annealed sheet 482–586 MPa / 207–310 MPa / 45–35 % / 65–80 HRB. Two publishing traps: (1) the originator’s typical table is very widely reproduced with the tensile and yield columns transposed (giving the absurd “annealed rod: 75–90 ksi yield, 25–50 ksi tensile”) — the correct reading is tensile 75–90 ksi, yield 25–50 ksi. (2) Some pages publish typical figures as minimums (“UTS min 82,000 psi, elongation 48 % minimum”) — the enforceable ASTM minimums are 70 ksi / 28 (or 25) ksi / 35 %, and quoting typicals as minimums produces rejectable certificates.​‌​​‌​

Magnetic Behaviour — the Commercially Dangerous Point

Monel 400 is not a non-magnetic alloy, and almost every distributor gets this wrong.​‌​​‌​

Its Curie temperature is approximately 21–49 °C — inside the ambient range. The originator’s own words: “The Curie temperature lies within the ambient range. It is affected by variations in chemical composition. The values shown represent the range which can be expected from normal production; therefore, some heats will be magnetic at room temperature and others not.” And its own recommendation: “If there is a strong requirement for nonmagnetic characteristics, other MONEL alloys should be considered.” Two distributors state plainly that alloy 400 is “slightly magnetic at room temperature”.

What that means in practice: (1) Two conforming heats from the same mill can behave differently on a magnet — the Ni:Cu balance and the iron level move the Curie point across room temperature. (2) It is temperature-dependent within the service range: a part that is magnetic in a cold warehouse in January may be non-magnetic in a 55 °C process line — and no heat treatment fixes this. (3) There is no permeability limit in B127/B164/B165; a magnetic-permeability requirement is not a standard property and must be negotiated as a supplementary requirement with heat selection and measurement — and even then cannot be reliably met. (4) If the customer genuinely needs non-magnetic, sell them K-500 (permeability ≈1.001–1.002), not alloy 400. No numeric permeability for N04400 could be verified — no mill publishes one, precisely because it is not a stable property. Do not put a permeability figure on the page.​‌​​‌​

Welding and Heat Treatment

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Welding

Processes: GTAW (including hot-wire), GMAW (spray, short-circuit, pulsed), SMAW, FCAW, SAW, plasma. GTAW is the reference process for thin sections and root passes. Preheat is not required — the only exception: if the base metal is ≤2 °C, warm the metal within about 300 mm of the joint to a few degrees above ambient, solely to stop condensation causing porosity, not for metallurgical reasons. Interpass temperature max 150 °C (single-sourced). Heat-input maxima (single-sourced): manual GTAW ≤8 kJ/cm, plasma ≤10 kJ/cm, GMAW ≤11 kJ/cm, SAW ≤7 kJ/cm. No post-weld heat treatment is required: “Neither pre- nor post weld heat treatments are normally required.” A stress relief or anneal may be desirable for heavily stressed structures going into SCC-inducing environments.​‌​​‌​

The number-one failure mode: sulphur, lead and phosphorus embrittlement. The originator’s joining handbook states it plainly: “Cleanliness is the single most important requirement for successfully welding nickel and nickel-based alloys. At high temperatures, nickel and its alloys are susceptible to embrittlement by sulfur, phosphorus, lead, and some other low-melting point substances.” The same handbook carries a photograph of cracking captioned “Typical effect of lead in MONEL alloy 400 welds“, and a failure case from a fatty-acid tank previously lined with lead and lined with Monel 400 without proper cleaning. Contamination sources named: grease, oil, paint, cutting fluids, marking crayons and inks, processing chemicals, machine lubricants, temperature indicating sticks, pellets and lacquers. Cleaning regime: vapour degrease or solvent swab for oils and crayon; alkaline cleaner for paint — but alkaline residues must themselves be removed, and wire brushing will not do it; a hot-water spray and scrub is required; embedded process chemicals need grinding, abrasive blasting or a 10 vol-% HCl swab followed by a thorough water wash. Clean a band of at least 50 mm either side of the joint, including plate edges and the bore of tubulars. The mill’s own illustration is the standard to hold a fabricator to: a root bend specimen cracked on the side wiped with a dirty cloth and sound on the side wiped with a clean one.

Porosity and flow. Ni-Cu weld metal is porosity-prone; the countermeasure is chemical — ERNiCu-7 carries 1.5–3.0 % Ti and up to 4 % Mn as deoxidiser and desulphuriser. Consequently: do not weld autogenously and do not use sheared strips of the base metal as filler — base-metal N04400 has no titanium, so the deposit has no deoxidiser and will be porous. The mill’s warning: “The molten pool must be kept as quiet as possible to prevent burning out of the deoxidizing elements“; keep the hot filler end inside the gas shield. Also: “Nickel alloy weld metal does not flow or spread as readily as steel weld metal.” The consequences: all beads must be slightly convex — “flat or concave beads such as those commonly encountered when joining stainless and carbon steels should be avoided”; bevel to an 80° included V (against about 60–70° for steel); use smaller root lands; and do not try to buy penetration with amperage — “increases in weld current will not significantly increase the penetration of the arc“, and excess current spalls the SMAW flux coating. Dilution: target at least 50 %, preferably 75 % filler metal in the completed weld.​‌​​‌​

Heat treatment

Monel 400 is a solid-solution alloy and is NOT hardenable by heat treatment. The originator: “MONEL alloy 400 is a solid-solution alloy that can be hardened only by cold working.” Verified in three independent sources; this is settled. Strength is set by the amount of cold or hot work plus the thermal cycle, and by nothing else.​‌​​‌​

Machining and Forming​‌​​‌​

Monel 400 work-hardens rapidly, at a rate that sits between mild steel and 304 stainless. The practical rules follow directly: positive rake angles, sharp tools, a rigid setup and a continuous cut — never dwell or rub, which glazes the surface and work-hardens it ahead of the tool. Carbide over HSS; TiC and TiAlN coatings handle the heat. Flood coolant. Intermediate annealing is required during multi-stage cold work, and a stress-relief anneal is recommended after forming. Cold-drawn and stress-relieved bar machines better than annealed bar. Cutting speeds diverge by about 10× across publishers: one gives turning at 46–61 m/min (150–200 sfm), another at 155–205 m/min with a 45 % machinability rating. These cannot be reconciled or averaged; 46–61 m/min is consistent with conventional carbide turning of a work-hardening nickel alloy, while 155–205 m/min is only plausible with advanced coated carbide or ceramic tooling and high-pressure coolant. Publish both with attribution, or neither. One outright error in circulation: a comparison article states that “Monel 400 cannot be hot worked” — that is false; three sources publish a hot-working range for N04400. Free-machining alternative: if the part is a screw-machine component, not welded and not a pressure boundary, R-405 (N04405) exists for exactly that — but in bar and rod only.

Corrosion — the Alloy’s Reason to Exist​‌​​‌​

Physical properties: density ≈8.8 g/cm³ · melting range 1300–1350 °C · modulus of elasticity at 20 °C 179–182 GPa · Poisson’s ratio 0.32 · electrical resistivity ≈51 µΩ·cm · thermal conductivity at 20 °C 21.8 or 23.0 W/m·K (sources about 5 % apart) · specific heat 427 or 452 J/kg·K (about 6 % apart) · mean thermal expansion 13.8 × 10⁻⁶/K.​‌​​‌​

Frequently Asked Questions

Monel 400, K-500, 70/30 copper-nickel or super duplex for seawater — when is each actually required?​‌​​‌​

These four are not interchangeable, and the deciding factor is rarely corrosion rate — it is flow regime, strength and price. 70/30 copper-nickel (C71500) is the default for seawater piping and condenser tubing, and the cheapest. Its constraint is velocity: design maxima of roughly 2.9 m/s once-through, 2.4 m/s two-pass and 2.3 m/s in 4–8 inch piping with short-radius bends, plus a minimum of about 0.9 m/s to prevent under-deposit attack. Above those velocities it erodes. Monel 400 is what you buy when velocity or cavitation exceeds what copper-nickel tolerates — pump impellers, valve trim, propeller shafts, seawater-lift components. It has no published critical velocity in clean seawater and superior cavitation resistance. Its weakness is the mirror image: stagnant, creviced or fouled seawater causes pitting and crevice corrosion, and annealed it is only a ~170–195 MPa yield material. K-500 enters when you want the same Ni-Cu corrosion behaviour but need strength: aged, 965–1310 MPa tensile and 27–38 HRC — shafts, bolting, stems. It is also the genuinely non-magnetic one. You pay in price and in hydrogen-embrittlement risk; and it is not freely approved under NACE — it is restricted to wellhead and Christmas-tree components and non-pressure-containing valve internals, and is barred from valve shafts and stems. Super duplex beats both on strength (≥550 MPa yield) and pitting (PREN ≥40) and is usually cheaper — but it lacks Monel’s high-nickel margin against chloride SCC and is limited in sour service to 232 °C and 0.20 bar H₂S. In short: low velocity → copper-nickel; high velocity or cavitation → 400; strength as well → K-500; strength and pitting first with low stagnant-chloride SCC risk → super duplex.

“Exceptional resistance to HF at all concentrations to boiling” — is that really true?​‌​​‌​

You have to read the whole sentence. The mill’s statement is real and correct: Monel 400 offers “exceptional resistance to hydrofluoric acid in all concentrations up to the boiling point“; this is the alloy’s signature duty and HF alkylation units are built on it. But the qualifier is aeration, and it is dropped almost everywhere. That rating is for un-aerated acid: “aeration increases the corrosion rate”, and oxidising salts increase it further. More pointedly, one European mill lists “moist, aerated HF vapours” under NOT RESISTANT, identifies it as a stress-corrosion cracking risk, and requires stress-relief annealing before exposure to HF vapour. Field experience from an instrument manufacturer points the same way: HF is “both highly corrosive and prone to triggering stress corrosion cracking, especially when oxygen is present in the vapor phase“; “even a small amount of oxygen in the tube can increase SCC risk, as it becomes trapped in the most highly stressed area”. The consequences for the order and the design: (1) for liquid-phase, air-free HF, 400 is the standard material with no concentration limit; (2) vapour phase and aerated HF are a different duty — components must be stress-relieved at 538–650 °C and stress levels kept to a minimum; (3) oxidising impurities in the acid (ferric salts, chromates, peroxides) will spoil a good selection, so ask about them as part of the plant chemistry. A footnote: alloy 400’s HF-vapour SCC resistance is slightly better than K-500’s, “however, this advantage is minimal”.

We ordered “non-magnetic Monel” and a magnet sticks to it — is it the wrong material?​‌​​‌​

Most likely the material is right and the order was wrong. Monel 400’s Curie temperature is approximately 21–49 °C — inside the ambient range. The originator’s own bulletin says so plainly: “The Curie temperature lies within the ambient range… therefore, some heats will be magnetic at room temperature and others not“, and adds: “If there is a strong requirement for nonmagnetic characteristics, other MONEL alloys should be considered.” So two heats from the same mill, both fully compliant with ASTM B127/B164/B165, can behave differently on a magnet; what decides it is the Ni:Cu balance and the iron level. It is also temperature-dependent: a part that is magnetic in a cold warehouse in January may be non-magnetic in a 55 °C line — and no heat treatment fixes it. Third, B127, B164 and B165 contain no magnetic-permeability requirement; it is not a standard property. A permeability requirement can only be negotiated as a supplementary requirement with heat selection and actual measurement, and even then cannot be reliably guaranteed. What to do: if the requirement is real, specify Monel K-500 — permeability ≈1.001–1.002, and it stays that way at low temperature. Finally: no mill publishes a numeric permeability for N04400, precisely because it is not a stable property, so a datasheet that gives you a figure is inventing it.

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