"If a magnet sticks, it's not real stainless steel" — this statement has caused 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 judgment
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 basically does not attract a magnet, while the edge of a narrow strip sheared after cold rolling clearly does;
- In deep-drawn pot bodies and drawn parts, the bottom where deformation is greatest has the strongest magnetism;
- In cold-headed and thread-rolled bolts, the threaded section is more magnetic than the shank;
- 316L, because of its higher nickel content and more stable austenite, is usually less magnetic than 304 under the same deformation, but it is absolutely not "completely non-magnetic".
This magnetism can be eliminated by solution treatment (heating to about 1050℃ then rapid cooling) — but most finished parts will not be heat-treated again just to "demagnetize", because that would change strength and dimensions.
So how do you judge authenticity
Using a magnet to judge material is fundamentally the wrong method. What determines the material is these three things:
- Chemical composition analysis: a handheld spectrometer (PMI) on site reveals the content of chromium, nickel, molybdenum, and manganese at a glance. This is the most direct means; 201 posing 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, actual measured chemical composition values, mechanical properties, and execution standard must be complete, and the heat number must match the steel stamp/label.
- Third-party re-inspection: in case of dispute, send for testing; it is more effective than any on-site "folk method".
Magnetism can actually indicate one thing
If the other party claims it is annealed, cold-rolled 304 sheet with almost no cold deformation, yet it is strongly magnetic, then it is indeed worth suspecting (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 other ferritic/martensitic stainless steels are inherently magnetic, and their price difference from 304 is large — this is what you need to guard against.
A practical on-site sequence
- First check whether the heat number, standard number, and grade on the steel stamp and label are complete and consistent;
- Then check whether the actual 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 use a magnet to draw conclusions;
- Where written evidence is required, conduct third-party re-inspection.
Common pitfalls
- Accepting based on "whether it attracts", resulting in 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 and holding workpieces, bringing iron filings into subsequent processes and causing surface rust spots.
If you need on-site material judgment, tell us the grade, batch, and arrival form. We can provide a material test certificate with each batch, and can also cooperate with third-party re-inspection as you require — speaking with chemical composition is more reliable than using a magnet.
