A container of A2-70 stainless bolts arrives, someone touches a magnet to one, it sticks — faintly, but it sticks — and a quality claim lands in your inbox: “these are not real stainless.” It is one of the most common misunderstandings in fastener procurement, and it costs real time on real projects. This article explains where the magnetism comes from, what the numbers look like, and how to actually verify the grade you received.
Table of Contents
- The complaint we hear
- Where the magnetism comes from
- Typical permeability values
- Does it affect corrosion resistance?
- How to verify what you received
- When to specify 316/A4 instead
- Quick FAQ
The complaint we hear
“Your 304 bolts attract a magnet, so they must be carbon steel or a fake grade.” The logic feels sound — everyone learns that stainless steel is non-magnetic — but the premise is wrong. Annealed austenitic stainless steel is essentially non-magnetic. A finished fastener is not annealed: it has been cold-formed, and cold forming changes the microstructure.
Where the magnetism comes from
Grades 304 (A2) and 316 (A4) are austenitic: their crystal structure, austenite, is paramagnetic — a magnet does not stick to it. But austenite in these alloys is metastable. When the metal is plastically deformed at room temperature — thread rolling, cold heading of the bolt head, wire drawing — part of the austenite transforms into strain-induced martensite, which is ferromagnetic.
A bolt is one of the most heavily cold-worked shapes in industry. The thread flanks and the head, formed under enormous local strain, carry the highest martensite fraction — which is why the magnet clings most noticeably at the threads and head, and barely at the shank. The effect is stronger in 304 than in 316, because 316’s higher nickel and molybdenum content stabilizes the austenite against transformation.
Typical permeability values
Magnetism is quantified by relative magnetic permeability (µr); µr = 1.0 means fully non-magnetic. Typical values reported for fastener materials:
| Material / condition | Relative permeability (µr, typical) | Magnet response |
|---|---|---|
| Austenitic 304/316, annealed | ≈1.0 | None perceptible |
| 304 / A2 fastener, cold-formed | up to ≈1.8 | Faint attraction at threads and head |
| 316 / A4 fastener, cold-formed | ≈1.015 | Barely detectable |
| 316L, cold-formed | ≈1.005 | Effectively none |
| Carbon steel / martensitic stainless (410) | hundreds+ | Snaps firmly to the magnet |
The practical takeaway: a faint pull on a cold-formed 304 bolt is normal physics. A bolt that snaps hard onto a magnet like a paper clip is a different conversation — that is worth investigating.
Does it affect corrosion resistance?
Not in any way that matters for a mounting system. Corrosion resistance in austenitic stainless comes from the chromium-rich passive layer, which depends on composition — chromium, nickel, molybdenum — not on whether some grains are austenite or strain-induced martensite. A fastener manufactured to ISO 3506-1, from conforming 304 wire, performs like 304 regardless of how it responds to a fridge magnet.
What genuinely does degrade stainless fasteners is different: chloride pitting and crevice corrosion in coastal environments, iron contamination from carbon-steel tooling or drilling swarf left on the surface, and galling during installation. Those are specification and installation issues — none of them are detected by a magnet.
How to verify what you received
A magnet is not a grade test. If grade matters — and on a 25-year rooftop asset it does — verify properly:
- Head markings: ISO 3506-1 requires property marking on bolts M5 and larger — look for “A2-70” (or “A4-70”) on the head.
- Material certificate: ask for the mill or third-party certificate with chemistry and mechanical properties. As an example, the third-party certificate for our DIN 912 M8×25 A2-70 fasteners reports yield strength 584 N/mm², tensile strength 730 N/mm², chromium 18.25% and nickel 8.01% — squarely within 304/A2-70 requirements. You can download that certificate here.
- Spectrometry (PMI): for critical lots, a handheld XRF test at goods-in settles composition in seconds.
Every Solaracks delivery can include material certificates as part of the standard documentation pack — see our certifications and test reports.
When to specify 316/A4 instead
Choose A4 where chlorides are part of the service environment: coastal sites (roughly C4–C5 corrosivity), swimming-pool surroundings, de-icing-salt exposure. A4’s molybdenum addition markedly improves pitting resistance — and as a side effect, its more stable austenite means it stays essentially non-magnetic even after cold forming. If a project has an instrumentation-driven requirement for genuinely low permeability, specify it explicitly; do not rely on grade alone.
How we map fastener grades to site corrosivity categories — including when A2 is enough, when A4 is warranted, and the procurement wording to use — is covered in our Corrosion Protection Guide (PDF).
Quick FAQ
Q: The magnet sticks to some bolts in the box more than others. Mixed grades?
A: Not necessarily — martensite fraction varies with forming strain, which varies bolt to bolt and even along one bolt. Composition testing, not magnetism, answers the mixed-lot question.
Q: Will the magnetism increase over time in service?
A: No. The transformation happens during cold forming. In service on a roof, nothing continues to deform the fastener, so nothing changes.
Q: Can I demagnetize them?
A: The response is a property of the microstructure, not a stored magnetization — a full solution anneal would remove it, but would also destroy the fastener’s property class. Leave it alone; it is harmless.