Non-Magnetic Spring Materials: What "Non-Magnetic" Really Requires
"Make it non-magnetic" hides a trap: the obvious choice — stainless steel — usually isn't, once it has been cold-worked into a spring.
Why stainless disappoints
Austenitic stainless (302/304/316) is non-magnetic in the annealed state. But spring wire is heavily cold-drawn, transforming part of the austenite to martensite — magnetic. Spring-temper 304 typically shows relative permeability of 1.5–7; a compass notices, MRI environments and magnetic sensors certainly do. 316 behaves better (often µr < 1.1 in spring temper) but is not guaranteed without testing.
The genuinely non-magnetic list
| Material | µr (typical) | Notes |
|---|---|---|
| Phosphor bronze | ~1.000 | Economical, moderate strength |
| Beryllium copper | ~1.000 | Near-steel strength after aging |
| Titanium (Beta-C) | ~1.00005 | Strength + total corrosion immunity |
| Inconel X-750 | ~1.002–1.01 | Also heat; verify spec limit |
| 316 spring temper | ~1.02–1.1 | Often acceptable; test to confirm |
Applications that care
MRI-compatible medical devices, magnetometers and compasses, mine countermeasure equipment, particle physics and vacuum instruments, watch movements, magnetic-sensor assemblies, aerospace navigation hardware.
How to specify
Write a number: "relative permeability ≤ 1.01 (or ≤ 1.05) after forming, measured per ASTM A342." Include *after forming* — the wire certificate alone does not prove the finished spring, and forming adds cold work. Our factories measure finished parts with calibrated permeability indicators on request; note the limit in your RFQ.
Frequently Asked Questions
Can 304 springs be made non-magnetic again by annealing?
Full annealing restores low permeability but destroys spring properties — not an option. Stress relieving at normal spring temperatures reduces magnetism only slightly.
What is the strongest truly non-magnetic spring material?
Aged beryllium copper leads in strength-per-volume with µr essentially 1.0; Beta titanium follows closely and wins where weight or corrosion also matter.
Is "no magnetic pull by hand magnet" a valid spec?
It is a crude screen (~µr < 1.5) that spring-temper 304 can fail and 316 usually passes. For instruments, specify a numeric permeability limit and the test method instead.