Key Takeaways
Quick rule of thumb
In everyday hardware, a magnet that sticks hard usually points to carbon steel; a weak or no pull often points to common stainless grades like 304 or 316.
Why the rule works
Magnetism follows crystal structure, not rust resistance. Austenitic stainless (304/316) is typically non-magnetic; ferritic and martensitic stainless can be magnetic.
Markings matter
Codes like “A2-70” identify stainless type and strength and are more reliable than a magnet alone.
Use with care
Cold-worked austenitic parts can show a faint pull. A magnet is a fast screen, not final proof.
Story & Details
The shop shortcut
The fastest field check is the magnet test. In most workshops, it sorts parts well enough to move the job forward.
The science behind the pull
Austenitic stainless steels—304 and 316—have a face-centred cubic lattice that does not support strong magnetic domain alignment, so they tend to show little or no attraction. Ferritic and martensitic stainless steels use structures that do allow domain alignment, so they can be plainly magnetic. None of this says anything about corrosion; rust resistance comes from chromium forming a protective film.
The “eight-out-of-ten” reality
Because austenitic fasteners dominate general hardware, “no pull ≈ likely stainless” is often right. But it’s not universal: ferritic grades (such as 430) are stainless and magnetic; a cold-rolled 304 bracket may “bite” slightly due to martensite formed during forming or threading.
Read the head, not just the feel
Fastener markings complete the picture. An “A2-70” nut signals type-304 stainless with property class 70. Standards require such markings on many screws and nuts, making identification more reliable than a magnet alone.
Conclusions
A smart first step
Keep the magnet in your pocket. It’s a quick, useful screen that aligns with how most fasteners are made and sold.
Not a verdict
Treat the result as a clue. Confirm with markings or documentation when the environment or liability demands certainty.
The memory hook
“Stainless often doesn’t pull; carbon steel usually does.” It’s simple—and it works, most of the time.
Sources
- British Stainless Steel Association — “Is stainless steel non-magnetic?” https://bssa.org.uk/bssa_articles/is-stainless-steel-non-magnetic-2/
- TWI (The Welding Institute) — “Why are my 300 series austenitic stainless steel welds magnetic?” https://www.twi-global.com/technical-knowledge/faqs/faq-why-are-my-300-series-austenitic-stainless-steel-welds-magnetic
- Outokumpu — “Understanding stainless steel: types, grades & applications.” https://www.outokumpu.com/en/products/stainless-steel-types
- Bossard — “ISO-designated steel groups (A2-70, etc.).” https://www.bossard.com/global-en/assembly-technology-expert/technical-information-and-tools/technical-information/stainless-steel-fasteners/iso-designated-steel-groups/
- YouTube (MIT OpenCourseWare) — “Magnetic Materials: Dia-, Para- & Ferromagnetism (8.02x, Lecture 21).” https://www.youtube.com/watch?v=OwDUzKl-u3E
Appendix
Acid spot test
A simple field check using dilute nitric acid on a cleaned area; lack of reaction suggests stainless, while darkening suggests carbon steel. Use proper safety measures.
Austenitic stainless
Stainless family (e.g., 304/316) with a face-centred cubic structure, typically non-magnetic and widely used in fasteners and fittings.
Cold work
Mechanical deformation (bending, threading, forming) that can transform small regions of austenitic stainless into martensite, adding slight magnetic response.
Corrosion resistance
Protection driven mainly by chromium forming a thin passive oxide layer; it is independent of magnetic behaviour.
Ferritic stainless
Stainless family with a body-centred cubic structure; usually magnetic and used in appliances, cladding, and some automotive parts.
Fastener markings (A2-70)
Code stamped on screws and nuts indicating stainless type and property class; a primary identifier for material and strength.
Magnet test
A fast, non-destructive screen for likely material family. Useful in the field but not definitive on its own.
Martensitic stainless
Hardenable stainless family with a body-centred tetragonal structure; typically magnetic and used where wear resistance is key.
My Diary - Leonardo Cardillo 