Specifications
Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175/ISO 15156-2
- NORSOK M-650
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
Monel 400 Magnetic Permeability - Curie Behaviour & Non-Magnetic Hardware
Monel 400 magnetic permeability (also is Monel 400 magnetic, Monel magnetic permeability, Monel 400 non-magnetic, UNS N04400 magnetic properties) sits in a thin band that is genuinely useful: weakly ferromagnetic below the Curie point near 35 to 50 degrees Celsius and effectively non-magnetic above. Annealed Monel 400 measured at room temperature reports a relative permeability of about 1.0 to 1.05; cold-worked material runs higher because dislocation density and residual stress raise the permeability. Above the Curie point any compliant heat is paramagnetic at relative permeability near 1.001. The Curie temperature moves with iron content: heats near the 2.5 percent iron ceiling of ASTM B164 carry the Curie point near 49 degrees Celsius; heats at 1.0 percent iron sit near 21 degrees Celsius. The same heat behaviour is why Monel 400 (UNS N04400) is the default for naval minesweeper bolts and nuts, submarine compass and gyroscope housings, MRI room fasteners and other low-signature hardware. Monel® is a registered trademark of Special Metals Corporation; TorqBolt is not authorized by, affiliated with or endorsed by Special Metals.
Curie Point and Magnetic Behaviour
Monel 400 is a single-phase FCC nickel-copper solid solution. Pure nickel is ferromagnetic with a Curie temperature of 358 degrees Celsius; copper is diamagnetic. Alloying copper into nickel suppresses the Curie point. At the Monel 400 chemistry of 63 to 70 percent nickel and 28 to 34 percent copper, the Curie temperature drops to roughly room ambient. Iron, present at up to 2.5 percent residual, partly counteracts the copper effect and raises the Curie point.
Below the Curie temperature the alloy is weakly ferromagnetic with relative permeability that depends on field strength, prior cold work and temperature. Above the Curie point the spin alignment breaks down and the material becomes paramagnetic with relative permeability essentially equal to 1.001. The transition is not a sharp step; permeability falls gradually across a roughly 20 degree window around the nominal Curie temperature. Naval and MRI specifications therefore call out a test temperature and a permeability ceiling together.
Why Iron Content Matters
Iron is the single variable that moves the Curie point inside the ASTM bracket. ASTM B164, ASTM B127, ASTM B564 and ASTM F468 all allow iron up to 2.5 percent maximum, but the heat-actual iron percentage on the EN 10204 type 3.1 or type 3.2 MTC is what governs magnetic behaviour. Higher iron pushes the Curie point up; lower iron pulls it down. For naval non-magnetic service the procurement convention is to specify iron at the lower half of the bracket and to require the heat-actual figure on the certificate.
| Iron content (heat actual) | Approximate Curie point | Room-temperature behaviour |
|---|---|---|
| 1.0 percent | 21 degrees Celsius | Non-magnetic above normal ambient |
| 1.5 percent | 30 degrees Celsius | Weakly magnetic in winter, non-magnetic in summer |
| 2.0 percent | 40 degrees Celsius | Weakly ferromagnetic at room temperature |
| 2.5 percent (ASTM ceiling) | 49 degrees Celsius | Weakly ferromagnetic in most service |
Cold work raises permeability further at any given iron content because dislocation tangles act as additional pinning sites for magnetic domains. Annealed material is therefore the standard for non-magnetic hardware: cold-worked Monel 400 bolts measured at room temperature can carry permeability above 1.1 even at 1.0 percent iron, while the same heat in the annealed condition stays below 1.01.
Typical Relative Permeability Values
Reported relative permeability for Monel 400 depends on condition (annealed or cold-worked), heat-actual iron, applied field strength and test temperature. The published reference values cluster in a tight range; the cold-worked figures are the practical ceiling for as-received bolting.
| Condition | Test temperature | Relative permeability (typical) | Notes |
|---|---|---|---|
| Annealed bar | 20 degrees Celsius | 1.0 to 1.05 | Heat-actual iron 1.0 to 1.5 percent |
| Annealed bar | 50 degrees Celsius | about 1.001 | Above Curie for any compliant heat |
| Cold-worked bar | 20 degrees Celsius | 1.05 to 1.5 | Permeability rises with cold reduction |
| ASTM F468 Ni 400 bolts (annealed) | 20 degrees Celsius | 1.0 to 1.05 | Naval procurement default |
| Air or vacuum (reference) | any | 1.0000 | Diamagnetic and paramagnetic ceiling |
Witnessed permeability testing is offered under Lloyd's Register, DNV and Bureau Veritas survey. The standard method uses a Severn-Gauge style permeability indicator at the bolt head or shank; the test temperature is recorded against the permeability reading on the EN 10204 type 3.2 MTC. Some Indian Navy and Royal Navy lots additionally require a magnetic susceptibility figure measured per ASTM A342 with the witness signature against the heat number.
Naval Non-Magnetic Hardware Applications
Monel 400 (UNS N04400) is on the approved-material lists of every major navy for low-signature hardware where ferromagnetic steel would couple into compass, magnetometer or mine-fuse circuits. The combination of a Curie point near room ambient and the seawater corrosion envelope of nickel-copper makes the alloy difficult to replace at any sensible budget.
| Application | Reason Monel 400 is specified |
|---|---|
| Minesweeper and mine countermeasure vessel (MCMV) hull bolting | Permeability near 1.0 above ambient avoids magnetic-influence mine triggers; seawater resistance covers the splash zone. |
| Submarine compass and gyroscope housing fasteners | Annealed permeability below 1.05 keeps stray field below compass tolerance. |
| MRI scanner room fasteners, cable trays and bolting | Paramagnetic permeability under 1.01 avoids artefact distortion at 1.5 T and 3 T field strength. |
| Magnetometer instrument enclosures (geophysical survey) | Low background permeability allows nT-level magnetic anomaly detection. |
| Naval acoustic array cable glands and fittings | Combines non-magnetic behaviour with chloride pitting resistance. |
| Cathodic protection anode bolting on warships | Galvanic compatibility with copper-nickel hull plating plus non-magnetic signature. |
The procurement convention for naval non-magnetic bolting is ASTM F468 Ni 400 in the annealed condition, supplied against an EN 10204 type 3.2 MTC that reports heat-actual iron percentage and witnessed permeability at the specified test temperature. Lloyd's Register, DNV and Bureau Veritas certification is the usual survey route.
Monel 400 vs SS 304 vs SS 316 vs Inconel 600 Magnetic Behaviour
The four nickel-bearing alloys most often considered for non-magnetic service split on phase and on cold-work response. Monel 400 is paramagnetic above its Curie point; austenitic stainless is acceptable only in the annealed condition; Inconel 600 is the safest fully paramagnetic answer at any temperature.
| Alloy | Phase | Relative permeability (annealed, 20 degrees Celsius) | Cold-work response | Magnetic behaviour |
|---|---|---|---|---|
| Monel 400 (UNS N04400) | FCC Ni-Cu solid solution | 1.0 to 1.05 | Rises moderately with cold work | Non-magnetic above 50 degrees Celsius; weakly ferromagnetic below |
| SS 304 (UNS S30400) | Austenite | 1.005 typical | Strain-induced martensite raises permeability above 1.5 | Non-magnetic annealed; magnetic if cold-formed |
| SS 316 (UNS S31600) | Austenite | 1.005 typical | Less martensite than 304 due to higher Ni and Mo | Non-magnetic annealed; mild rise on cold work |
| Inconel 600 (UNS N06600) | FCC Ni-Cr-Fe | 1.01 typical | Stays paramagnetic at any cold reduction | Fully paramagnetic; safest non-magnetic answer |
Detail comparison on the Monel vs stainless steel reference. For environments that combine non-magnetic service with hydrofluoric acid or seawater, Monel 400 remains the only practical option; for non-magnetic service in oxidising or high-temperature environments, Inconel 600 is the closer pick.
Frequently Asked Questions
Is Monel 400 magnetic? Monel 400 is weakly ferromagnetic below its Curie point near 35 to 50 degrees Celsius and effectively non-magnetic above. Annealed Monel 400 at room temperature carries a relative permeability of about 1.0 to 1.05; cold-worked material runs higher. Above the Curie point the relative permeability drops to about 1.001.
At what temperature does Monel 400 become non-magnetic? The Curie temperature of Monel 400 sits between 21 and 49 degrees Celsius depending on heat-to-heat iron content. Heats near the 2.5 percent iron ceiling carry the Curie point near 49 degrees Celsius; heats at 1.0 percent iron sit near 21 degrees Celsius. Above 50 degrees Celsius any compliant heat is effectively non-magnetic with relative permeability close to 1.001.
Why use Monel 400 for naval non-magnetic hardware? Monel 400 combines a Curie point near ambient with the seawater corrosion envelope of nickel-copper. Annealed Monel 400 above its Curie point gives a permeability close to 1.001 that minesweeper hulls, submarine compass and gyroscope housings and MRI room fasteners require. SS 304 and SS 316 are non-magnetic only in the annealed condition; cold work induces strain-induced martensite and lifts permeability above 1.05. Monel 400 has no austenite phase to transform.
What is the relative magnetic permeability of Monel 400? Typical relative magnetic permeability for annealed Monel 400 is 1.0 to 1.05 at room temperature, rising to roughly 1.05 to 1.5 for cold-worked material at the same temperature. Above the Curie point (above 50 degrees Celsius for any compliant heat) the relative permeability drops to about 1.001.
How does iron content affect Monel 400 Curie point? Iron raises the Curie temperature of the nickel-copper matrix. ASTM B164, B127, B564 and F468 allow iron up to 2.5 percent maximum. Heats near 2.5 percent iron carry a Curie point near 49 degrees Celsius and behave as weakly ferromagnetic at room temperature. Heats at 1.0 percent iron carry a Curie point near 21 degrees Celsius and are non-magnetic above any normal industrial ambient.
Is Monel 400 K-500 also non-magnetic? Monel K-500 (UNS N05500) carries the same nickel-copper backbone and a similar Curie behaviour, but the aluminium and titanium additions for age-hardening raise the Curie point slightly. K-500 in the age-hardened condition is still classified as non-magnetic above 50 degrees Celsius. K-500 is preferred where strength is required alongside non-magnetic service; Monel 400 is preferred where strength is not the primary requirement.
Related Pages
Properties: chemical composition, mechanical properties, density, corrosion resistance, heat treatment, machinability.
Standards: ASTM B164, ASTM B127, ASTM B564, ASTM F468, ASTM F467, DIN 17743, NACE MR0175.
Alloy comparison: Monel vs stainless steel.
Marine and naval: marine applications, bolts, hex bolts, anchor bolts, round bar, Monel 400 reference.
Request a Quote
For Monel 400 (UNS N04400) non-magnetic hardware against ASTM F468 Ni 400 annealed with witnessed permeability testing under Lloyd's Register, DNV or Bureau Veritas survey, send drawing, quantity, certification class (EN 10204 type 3.1 or 3.2) and target permeability ceiling to info@torqbolt.com or WhatsApp +91-22-66157017. Ex-Mumbai price plus heat-numbered MTC with heat-actual iron percentage inside one working day. Domestic India 3 to 7 working days; export through JNPT.
References
- ASTM B164 Nickel-Copper Alloy Rod, Bar, and Wire (UNS N04400, Table 1 chemistry, Fe 2.5 max)
- ASTM B127 Nickel-Copper Alloy Plate, Sheet, and Strip
- ASTM B564 Nickel Alloy Forgings, Table 1, UNS N04400 Class A
- ASTM F468 / F468M Nonferrous Bolts, Hex Cap Screws, Socket Head Cap Screws, and Studs (Ni 400)
- ASTM A342 Permeability of Feebly Magnetic Materials
- DIN 17743 Wrought Nickel-Copper Alloys, NiCu30Fe, Werkstoff 2.4360
- EN 10204 Metallic Products, Types of Inspection Documents (3.1 and 3.2 MTC)
- Lloyd's Register Rules for Naval Ships, magnetic permeability survey
- DNV-RU-NAVAL-Pt1 Surveyor witnessing for low-signature materials
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