304 is an austenitic stainless steel. Its chromium and nickel balance helps stabilize the face-centered cubic austenitic structure at room temperature. Austenite has very low magnetic permeability compared with ferritic or martensitic stainless steels, which is why properly solution-annealed 304 normally shows little attraction to a hand magnet.
| Cause | What changes | Where it is often noticed |
|---|---|---|
| Cold rolling or drawing | Some austenite can transform to strain-induced martensite | Strip, wire, springs and heavily reduced products |
| Bending, stamping or machining | Local deformation increases magnetic response | Corners, formed areas and machined surfaces |
| Welding | Weld metal may contain controlled ferrite; the heat-affected area may respond differently | Weld bead and adjacent zone |
| Mixed or contaminated material | A different alloy or embedded carbon-steel particles may attract a magnet | Cut edges, ground surfaces and mixed stock |
No. A weak or localized attraction can be normal after cold work or welding. Conversely, a low magnetic response does not prove that a sample is 304 because several stainless and non-ferrous alloys can also be weakly magnetic or non-magnetic. The magnet test is a screening observation, not a grade certificate.
If low magnetic permeability is a functional requirement, specify a measurable permeability limit and the test method instead of simply writing “non-magnetic.”