Is stainless magnetic? The honest answer has three parts, and the one buyers need is the third. Austenitic grades like 304 and 316 are essentially non-magnetic in the annealed condition — their crystal structure will not align with a field. Ferritic and martensitic grades are magnetic — same iron, different lattice. And the part at your bench that pulls toward a magnet despite its 304 certificate is probably not fake: cold work drags austenite toward martensite, and the magnet is reading the transformation, not the fraud.
Key takeaways
- Crystal structure decides: face-centered-cubic austenite is non-magnetic; body-centered ferrite and tetragonal martensite are magnetic.
- Annealed austenitic permeability sits near 1.0 — published figures run 1.003 to 1.02 — which is functionally non-magnetic.
- Cold working 304 partially transforms austenite to martensite: drawn bar runs a relative permeability around 1.1–2, and heavy cold rolling reaches the majority of the structure — published research puts 80% cold-rolled 304 near 95% martensite versus about 52% for 316.
- 316’s higher nickel plus its molybdenum stabilize austenite, so the same cold work magnetizes it less.
- Magnetism does not grade stainless — it reads cold work, not composition. The field test that separates 304 from 316 is chemical, not magnetic.
The three families
Magnetism in stainless is a crystal-structure story. Iron atoms arranged in the body-centered cubic lattice of ferrite — or the body-centered tetragonal lattice of martensite — align with a magnetic field, and those two families are ferromagnetic: the ferritic 430s of the appliance world, the martensitic 410 and 420 of cutlery and fasteners. Add roughly 8% nickel and 18% chromium, as 304 does, and the lattice switches to face-centered cubic austenite, where the atomic moments cancel and the steel answers a magnet with a polite shrug. That is the entire taxonomy: austenitic non-magnetic, ferritic and martensitic magnetic, and the grades cataloged by family in the stainless steel grades chart. The grade-level differences within the austenitics are covered in 316 vs 316L and the selection logic in the grades field guide.
Why cold-worked 304 attracts
The austenite-to-martensite transformation is strain-driven, and cold work is the strain. Bending, drawing, threading and pressing all shift part of the austenitic structure toward the magnetic martensitic one, and the fraction tracks the severity of the work — published figures put annealed austenitic relative permeability at 1.003–1.02, rising to roughly 1.1–2 on 10–20% drawn bar, and at extreme reductions the research literature reports 80% cold-rolled 304 reaching about 95% martensite by volume. A pressed fitting shoulder, a swaged tube end, a heavily threaded run: each shows a magnetic pull the annealed bar beside it does not. Hence the receiving-dock paradox — the bar passes the magnet check and the bent fitting beside it fails, both from the same heat. 316 transforms less for the same strain — the published comparison puts 80% cold-rolled 316 near 52% martensite, attributed to the austenite-stabilizing nickel and molybdenum — so it stays cooler under the magnet, not immune, just slower to convert.
What magnetism does NOT change
The magnet reads structure, not composition, and the properties buyers care about ride composition. Corrosion behavior comes from the chromium, nickel and molybdenum percentages, which cold work leaves untouched — the corrosion references state plainly that magnetic response and quality are independent properties, and that cold work alone costs little corrosion resistance. So a magnetic 304 elbow is not a fake, not a downgrade and not a pitting sentence. The honest hedge for extreme cases: very heavy cold work can leave residual stresses that matter in specific environments — the chloride-stress regime of chloride stress corrosion cracking among them — but that is a stress story, not a magnetism story, and it belongs in the service review, not the receiving inspection.
The field test that works
If the magnet cannot grade stainless, what can? Chemistry — and the practical field test is the molybdenum spot test: a reagent drop on an abraded, degreased surface turns brown where molybdenum is present and stays clear where it is absent. The published reading is that the brown threshold sits around 0.15% molybdenum — so 316 browns, 304 stays clear, and the two grades separate in minutes at the bench. The reagents are corrosive acids and the test leaves a mark, so it belongs on scrap-off coupons, not show surfaces. Combined with the caliper-and-gauge routine from the thread-identification post, a buyer can verify grade and standard in the receiving bay; the pitting corrosion post covers why 304 versus 316 is worth that minute in chloride country.
RFQ lines
magnetic requirements— if the installation forbids magnetic response, state it: annealed condition, low-permeability requirement, or a defined permeability ceiling.cold-work state— as-drawn or annealed; the words that predict the magnet behavior.grade verification— PMI or spot-test on receipt, where substitution risk is real.
FAQ
Will annealed 316 be non-magnetic?
Essentially yes — near-zero magnetic response in the fully annealed condition, with published relative permeability figures starting at about 1.003 against 1.0 for a vacuum. Annealed 316 is more austenite-stable than 304 to begin with, thanks to the nickel and molybdenum, so it stays quiet under a magnet until the cold work adds up. Non-magnetic in practice, not a mathematical zero.
Does a magnet sticking to my 304 fitting mean it is fake?
No — it means the fitting saw cold work. Drawing, bending, threading and swaging all transform part of the austenite to magnetic martensite, and the magnet reads the transformation, not the composition. The chemistry that decides corrosion resistance is unchanged. Verify grade with a molybdenum spot test if substitution worries you; do not reject on magnetism alone.
Which stainless is magnetic?
Ferritic grades like 430 and martensitic grades like 410 and 420 are magnetic — think appliance doors and hardened fasteners. Austenitic 304 and 316 are non-magnetic in the annealed condition, and partially magnetic after heavy cold work, because strain converts part of the structure to martensite. Same periodic table, different lattice, different answer to the magnet.
Why is 316 less magnetic than 304 after bending?
Austenite stability. The higher nickel and the molybdenum in 316 hold the austenite lattice against strain-driven transformation, so the same bending or drawing produces less martensite — the cold-work research puts 80% rolled 304 near 95% martensite against about 52% for 316. 316 still magnetizes under enough work; it just asks for more of it.
How do I check 304 versus 316 at the bench?
Magnetism cannot do it — both are austenitic and both can show pull after cold work. The field test is chemical: the molybdenum spot test turns brown on 316, stays clear on 304, with the brown threshold near 0.15% molybdenum. Abrade a spot, degrease, drop, read the color in minutes. Use a coupon, not the show surface — the reagent is acidic and leaves its mark.
