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Monel 400 (UNS N04400 / W.Nr. 2.4360) is frequently described in sales literature as "non-magnetic" or "essentially non-magnetic." This is technically incomplete. Monel 400 is a single-phase, face-centered cubic (FCC) Ni–Cu solid solution (63–70% Ni, 28–34% Cu, ≤2.5% Fe), but its magnetic behavior is temperature- and composition-sensitive because its Curie temperature (Tc) lies within or just below the ambient range (≈ 21–49°C / 70–120°F depending on Ni:Cu ratio and Fe content).
The short answer:
In the annealed condition at temperatures above its Curie point (typically > 25–50°C depending on heat chemistry), Monel 400 is paramagnetic (μr≈ 1.0006–1.0012) — "non-magnetic" for engineering purposes.
At room temperature in cool climates or seasons (≤ 20°C), some production heats may fall below Tcand exhibit weak ferromagnetism (μr≈ 1.005–1.015), detectable by a hand magnet.
Heavy cold work can induce a slight increase in apparent permeability even above Tc, though the material remains paramagnetic in the thermodynamic sense.
For strict non-magnetic applications (MWD/LWD tools, gyro housings, NMR-adjacent equipment), Monel K-500 (UNS N05500) with Tc≈ –90°C is preferred and should be specified with a magnetic permeability certificate (μr≤ 1.005 or 1.002).
1. Crystal Structure & Origin of Magnetic Behavior
Monel 400 is a single-phase γ (FCC) Ni–Cu solid solution. Pure Ni is ferromagnetic below Tc≈ 354°C (631°F). When alloyed with Cu, the Ni–Ni exchange interaction is diluted because Cu atoms (also FCC, non-magnetic) break the continuity of the Ni 3d-electron spin alignment.
As Cu content increases, the Curie temperature of the Ni–Cu alloy decreases monotonically:
Ni–20%Cu → Tc≈ 200°C
Ni–30%Cu (typical Monel 400) → Tc≈ 21–49°C (70–120°F)
Ni–40%Cu → Tc< 0°C
Thus Monel 400 sits on the threshold: some lots are paramagnetic at 25°C, others weakly ferromagnetic — purely due to small Ni:Cu ratio shifts within spec (63–70% Ni).
Fe (≤2.5%) slightly raises Tc; high-purity low-Fe heats trend toward lower Tc.
2. Measured Relative Magnetic Permeability by Condition
Typical values measured per ASTM A342 Method 4 (or comparable permeameter) at H = 15.9 kA/m (200 Oe), room temperature (20–24°C):
|
Condition |
Typical μr(Room Temp, Heat Dependent) |
Observable Magnetic Response |
|---|---|---|
|
Solution Annealed, T > Tc(warm lab/summer) |
1.0006 – 1.0012 |
None — true paramagnet |
|
Solution Annealed, T < Tc(cool room/winter) |
1.002 – 1.015 |
Weak attraction to strong hand magnet |
|
Cold Drawn 20–30% |
1.0010 – 1.0030 (if T > Tc) |
None to barely perceptible |
|
Cold Drawn 50%+ , T < Tc |
up to 1.010 – 1.018 |
Weak but measurable |
|
Surface Ni-rich oxide (improper pickling) |
May read higher locally |
Artifact — not bulk property |
Comparison for context:
304 SS (annealed): μr≈ 1.003–1.010, paramagnetic — never ferromagnetic
Carbon steel: μr≈ 200–2000
Monel K-500 (annealed & aged): μr≈ 1.0008–1.0015 at all room temps (Tc≈ –90°C)
⚠️ If a magnet "sticks" weakly to Monel 400 bar stock in winter, this is normal physical behavior, not a sign of contamination by carbon steel — provided the MTR shows correct chemistry and PMI verifies Ni/Cu ratio.
3. Effect of Heat Treatment
Solution Annealing (704–982°C, rapid quench): Produces homogeneous single-phase γ. Magnetic response is determined solely by whether test temperature is above or below Tc. No aging/hardening phase forms in Monel 400.
No precipitation hardening: Unlike K-500, there is no γ′ (Ni₃(Al,Ti)) phase. Aging Monel 400 (e.g., 480–560°C × 4–16 h) does not change strength or magnetism meaningfully and is not a standard commercial practice.
Stress Relief (≤ 400°C): Does not affect magnetic state.
4. Effect of Cold Working
Cold working introduces high dislocation density and micro-strain, which can promote short-range magnetic ordering in an otherwise paramagnetic FCC Ni–Cu matrix:
Light cold work (≤ 20% RA): negligible effect on μrif T > Tc.
Heavy cold work (> 50% RA): may increase apparent μrby 0.002–0.005 even above Tc, due to stress-induced magnetic cluster alignment.
If T < Tc, cold work amplifies the weak ferromagnetic signal slightly.
This is reversible by re-solution annealing + water quench, which restores the annealed paramagnetic/weak-ferro state per the Curie point of that heat.
5. Surface Magnetic Film — A Common Pitfall
During high-temperature exposure in air (especially 650–870°C without protective atmosphere), selective oxidation of Cu can leave a Ni-enriched surface layer that is faintly ferromagnetic (Ni is ferromagnetic below 354°C). This surface film:
Is typically ≤ 10–30 µm thick
Can attract a magnet even if bulk T > Tc
Is removed completely by pickling (HNO₃ + HF) or mechanical abrasion
Always pickle Monel 400 after hot forming or welding if magnetic verification is required.
6. Practical Selection for Non-Magnetic Service
|
Application |
Recommended Material / Condition |
Rationale |
|---|---|---|
|
MWD/LWD non-mag subs, gyro housing, compass-safe marine hardware |
Monel K-500, solution ann. + aged, μr≤ 1.005 (certify) |
Tc≈ –90°C → always paramagnetic at service T |
|
Seawater pump shaft, valve stem, heat exchanger (no mag requirement) |
Monel 400 annealed or cold drawn per strength need |
Magnetism irrelevant |
|
Lab/test fixture needing low-μ near room T on budget |
Monel 400 from low-Tcheat (Ni ~63%, Cu ~34%, Fe min), annealed, pickled; specify μr≤ 1.005 test |
Possible but not guaranteed — verify per lot |
Procurement tip: If non-magnetic is contractually required, specify:
"Relative magnetic permeability μr≤ 1.005 when measured per ASTM A342 Method 4 at 15.9 kA/m and 20°C. Material: Monel K-500 (UNS N05500), solution annealed + aged. Test report to accompany MTR."
7. Summary
Monel 400 is a single-phase FCC Ni–Cu solid solution, not austenitic stainless steel and not ferritic.
Its Curie temperature (21–49°C) places it near/within the ambient range → some annealed bars are weakly magnetic in cool rooms, others are not — both are correct for spec-compliant material.
Relative permeability in normal conditions: μr= 1.0006 – 1.015 (far below ferromagnetic materials).
Cold work and surface Ni-rich oxide can slightly increase apparent μror produce local attraction — removed by annealing/pickling.
For guaranteed non-magnetic performance, use Monel K-500 with permeability certification, not Monel 400.
Shanghai COCESS Special Alloys Co., Ltd. supplies Monel 400 (UNS N04400) and Monel K-500 (UNS N05500) in bar, plate, seamless tube, forged flange/ring, and custom-machined blank forms. Magnetic permeability testing (ASTM A342) and EN 10204 3.1 MTR are available on request — particularly recommended for non-mag drill collar, sensor housing, and nav-system applications. Contact our technical department for alloy selection advice and stock availability.
Email: sales@cocessalloys.com
