Is Monel K500 Non-Magnetic? Magnetic Behavior by Heat Treatment Condition

Monel K-500 (UNS N05500 / W.Nr. 2.4375 / ASTM B865) is a precipitation-hardenable Ni-Cu alloy (≈63–67% Ni, 27–33% Cu, 2.3–3.15% Al, 0.35–0.85% Ti). It is widely described as "virtually non-magnetic"​ in engineering specifications, but this statement requires qualification against its metallurgical condition.

The short answer: Monel K-500 is paramagnetic (μ ≈ 1.0006–1.002) at room temperature in all standard mill conditions—solution-annealed, age-hardened, and moderately cold-worked. It is not ferromagnetic like carbon steel or pure nickel.​ However, surface oxidation films, excessive cold work, or cryogenic temperatures can produce detectable weak magnetic response, and the age-hardening precipitate (γ′ Ni₃(Al,Ti)) has a minor effect on measured permeability.

Metallurgical Basis of Magnetic Behavior

Pure nickel is ferromagnetic below its Curie temperature (T_c≈ 354°C / 631°F). When alloyed with 27–33% copper to form a Ni-Cu face-centered cubic (FCC) γ-solid solution, the long-range magnetic ordering of nickel atoms is disrupted. The Cu-Ni solid solution in the Monel composition range has its Curie point suppressed well below room temperature.

• Solution-annealed (SA) Monel K-500: Homogeneous γ-matrix → paramagnetic at 20°C, Curie temperature T_c≈ –90 to –100°C (–130 to –148°F) depending on exact Ni/Cu ratio.

• Age-hardened condition: Precipitation of ordered γ′ phase (Ni₃(Al,Ti), L1₂ structure) does not restore bulk ferromagnetism. The γ′ volume fraction is only ~3–8% in peak-aged condition, insufficient to alter the paramagnetic character of the matrix, though it can raise relative permeability marginally.

• Contrast with Monel 400 (N04400): Monel 400 has a Curie point closer to or just below room temperature (~–20 to +30°C depending on source), making it slightly more prone to weak magnetic response after cold work or at low ambient temperature. K-500's higher Al/Ti addition and tighter solution control generally depress T_cfurther.

Magnetic Permeability by Heat Treatment & Working Condition

Measured relative permeability (μ, referenced to μ_air=1.0) per Special Metals / ASTM B865 typical data at H=15.9 kA/m (200 Oe), room temperature:

Interpretation: All values are μ_r< 1.01. For comparison, 304 SS annealed μ_r≈ 1.02–1.10 (can be higher if worked), 316L μ_r≈ 1.003–1.05, carbon steel μ_r≈ 200–5000. Monel K-500 is therefore classified as non-magnetic / negligibly magnetic for oilfield MWD/LWD tools, NMR-adjacent equipment, and compass-safe marine hardware, provided surface effects are controlled.

Effect of Precipitation Hardening (Aging) on Magnetism

Standard age-hardening cycle per AMS 2759/ASME SB-865:

• Solution anneal: 980–1040°C (1800–1900°F), hold, rapid water quench

• Age: 480–510°C (900–950°F), 4–16 h, air cool

The γ′ (Ni₃(Al,Ti)) phase that provides precipitation strengthening has a small ordered magnetic moment contribution. In practice:

• Permeability increases from ~1.0011 (SA) to ~1.0018–1.0020 (peak aged)—a change measurable in a lab but insignificant for field applications.

• Age hardening does not​ make Monel K-500 ferromagnetic or "magnetic" in the sense of attracting a magnet.

• For critical non-magnetic instrument applications (e.g., drill collar non-mag spacer, gyro housings), both SA and age-hardened K-500 are acceptable if μ_r≤ 1.01 is specified and verified.

Effect of Cold Working

Cold working introduces dislocation density and lattice strain, which can promote minor short-range ordering of Ni atom spins in the FCC matrix:

• Light–moderate cold work (≤30% RA): μ_rremains ≤ 1.0015, indistinguishable from annealed.

• Heavy cold work (>50% RA): μ_rmay rise to 1.002–1.005. Still paramagnetic.

• Re-solution annealing + water quench removes cold-work-induced permeability increase completely.

If a part shows attraction to a hand magnet after machining, the cause is almost always a surface ferromagnetic contaminant​ (steel filings embedded in the work-hardened surface) rather than the alloy itself.

Surface Magnetic Film — A Common Pitfall

During high-temperature exposure in air (especially 650–870°C without protective atmosphere), selective oxidation of Cu and Al can leave a Ni-enriched surface layer that is faintly ferromagnetic. This affects thin sections (wire, foil) disproportionately.

Remedy: Pickle in HNO₃+HF or bright dip to remove the surface layer. The underlying bulk material remains paramagnetic with μ_r≈ 1.001.

Temperature Dependence — Cryogenic & Elevated

• Room temperature to ~300°C: Paramagnetic behavior, permeability essentially flat or decreases slightly with rising T.

• Below Curie point (~–90°C for K-500, varies with exact chemistry): Very weak induced magnetization may appear—still orders of magnitude below ferromagnetic materials. Special Metals data confirms "virtually nonmagnetic even at quite low temperatures".

• Cryogenic service (LNG, –162°C): Remains suitable for non-magnetic requirements; verify μ_rif specification demands <1.005 at operating T.

Practical Selection for Non-Magnetic Service

Procurement note: If non-magnetic certification is contractually required, specify "Relative magnetic permeability μ_r≤ 1.005 (or ≤1.002) at 15.9 kA/m, measured per ASTM A342 Method 4 or equivalent" and request the test report with MTR.

Summary

• Monel K-500 is paramagnetic, not ferromagnetic, in all standard conditions (annealed, age-hardened, cold worked ≤50%).

• Relative permeability: μ_r= 1.0010–1.0020​ at room temperature—effectively non-magnetic for engineering purposes.

• Age hardening causes a negligible permeability rise (Δμ ≈ 0.0005–0.0008), insufficient to reclassify the material.

• True ferromagnetic response is only possible from surface Ni-rich oxide films (removable by pickling) or steel particle contamination—never from the bulk alloy in any heat treatment condition.

• Curie temperature ≈ –90 to –100°C, ensuring stable low-permeability behavior across normal and cryogenic service ranges.