"If a magnet sticks, it's not real stainless steel" — this statement causes more returns and disputes on the purchasing floor than any other misunderstanding. It is only half right: non-magnetic does not necessarily mean 304, and magnetic does not necessarily mean fake.
One-sentence judgement
Austenitic stainless steel (304, 316L, 321) is non-magnetic in the solution-annealed and annealed states; once it undergoes cold working (cold rolling, stretching, bending, stamping), part of the austenite transforms into deformation-induced martensite, and it becomes magnetic. So cold-rolled sheet, deep-drawn parts, screws and bent parts being magnetic is normal.
Why cold working causes magnetism
The austenitic structure itself is non-magnetic. Cold deformation inputs deformation energy, causing part of the austenite to transform into martensite (a magnetic structure); the greater the deformation, the more transformation, and the stronger the magnetism. Typical examples:
- From the same coil of 304, the middle of an annealed sheet barely attracts a magnet, while the sheared narrow-strip edges after cold rolling attract it noticeably;
- In deep-drawn pot bodies and drawn parts, the magnetism is strongest at the bottom where deformation is greatest;
- In cold-headed and thread-rolled bolts, the threaded section is more magnetic than the shank;
- Because 316L has higher nickel content and more stable austenite, its magnetism at the same deformation is usually weaker than 304, but it is by no means "completely non-magnetic".
This magnetism can be eliminated by solution treatment (heating to about 1050℃ and then rapid cooling) — but most finished parts will not be heat-treated again just to "demagnetise", because that would change strength and dimensions.
So how do you judge authenticity
Using a magnet to judge grade is fundamentally the wrong method. What determines grade is these three things:
- Chemical composition analysis: a handheld spectrometer (PMI) on site shows the chromium, nickel, molybdenum and manganese content at a glance. This is the most direct means; 201 passed off as 304 is exposed immediately (201 has only 3.5–5.5% nickel and 5.5–7.5% manganese, while 304 has 8–10.5% nickel).
- Material test certificate (MTC): heat number, measured chemical composition values, mechanical properties and applicable standard must all be present, and the heat number must match the stamp/label.
- Third-party re-inspection: when there is a dispute, send it for testing; this is more effective than any on-site "folk method".
Magnetism can actually tell you one thing
If the other party claims it is annealed, cold-rolled 304 sheet with almost no cold deformation, yet it is strongly magnetic, that is indeed suspicious (it could be 430 ferritic stainless steel — which is itself magnetic). That is to say: magnetism can serve as a "hint", but not as a "conclusion". 430, 410 and similar ferritic/martensitic stainless steels are inherently magnetic, and their price difference from 304 is large — that is what you need to guard against.
A practical on-site sequence
- First check whether the heat number, standard number and grade on the stamp and label are complete and consistent;
- Then check whether the measured chemical composition values on the material test certificate fall within the standard range (focus on Ni, Cr, Mo, C);
- When in doubt, use a spectrometer; do not draw conclusions with a magnet;
- Where written evidence is required, carry out third-party re-inspection.
Common pitfalls
- Accepting on the basis of "does it attract", and as a result rejecting qualified hot-rolled annealed sheet as fake;
- Accepting 430 (magnetic, cheap) as 304, and the magnet actually "passes" it;
- Only looking at the appearance of the goods without checking the heat number — the certificate is genuine, but the goods are not from that heat;
- Using strong magnets for lifting or holding workpieces, bringing iron filings into subsequent processes and causing surface rust spots.
If you need to judge grade on site, tell us the grade, batch and form of the incoming goods. We can provide a material test certificate with each batch, and can also cooperate with third-party re-inspection at your request — speaking with chemical composition is more reliable than using a magnet.
