304 stainless steel is usually weakly magnetic or effectively non-magnetic in the solution-annealed condition, but it can attract a magnet after cold working. A magnetic response at a bend, drawn surface or weld therefore does not, by itself, prove that the material is counterfeit or the wrong grade. The useful next step is to check the product condition and where the attraction occurs.
This distinction matters when receiving bright bar, wire, formed sheet or welded components. A buyer may compare a soft sheet sample with a heavily drawn wire and see different behaviour even though both meet their respective 304 requirements. Treat magnetism as an observation to investigate, not a complete acceptance test.
| Where you notice attraction | Possible explanation | What to verify |
|---|---|---|
| Cold-drawn wire or bright bar | Deformation can create magnetic martensite from austenite | Actual manufacturing condition, heat identity and any agreed permeability requirement. |
| Bend, pressed corner or deep-drawn area | Local strain can differ greatly from the flat parent material | Compare worked and unworked areas under the same test conditions. |
| Weld metal | Ferrite can be present in an austenitic stainless weld | Filler, welding procedure and required weld acceptance; attraction is not a defect diagnosis. |
| Unexpected patches or particles | Contamination or local processing may contribute | Inspect the surface and processing history; do not assume this explains all bulk attraction. |
| Strong response throughout an unfamiliar part | Grade, condition or processing history may differ from expectations | Hold for identification and documented review rather than deciding by magnet alone. |
304 is an austenitic chromium-nickel stainless steel, commonly identified as UNS S30400. “18/8” is a familiar description, not a complete chemistry specification. Composition within the allowed range, deformation severity and processing temperature influence the amount of strain-induced martensite. There is no universal hand-magnet strength that all compliant 304 products must show.
Record the heat or lot number, dimensions, surface and ordered condition. Ask whether the material was solution annealed, cold drawn, cold rolled or formed after annealing. Photograph the location of attraction and retain a labelled sample if a dispute is possible. Avoid mixing test results from different heats or from unrelated reference pieces.
Review the material certificate against the purchase specification. If identification remains uncertain, arrange appropriate chemical analysis. XRF can help identify major alloying elements, but conventional handheld XRF does not establish carbon content and cannot by itself distinguish standard-carbon from low-carbon variants. It also does not certify heat treatment, mechanical properties or freedom from contamination.
If the order contains a permeability limit, use the agreed method and inspect the stated locations. If the problem is a retained field affecting sensors, use an appropriate residual-field measurement. A gaussmeter reading is not interchangeable with a relative-permeability result. Record measurement conditions so the supplier and customer can reproduce the assessment.
Not on its own. Corrosion behaviour must be assessed using the grade, environment, surface condition and fabrication history. A magnetic 304 bend is not automatically rusty material; equally, a non-attracting sample is not proof of resistance to chlorides or a particular cleaning chemical. Weld heat tint, embedded iron and trapped deposits require their own assessment.
Do not use the “magnet sticks” observation to approve or reject food, marine or medical suitability. Those uses involve separate design and material requirements. Where pits, cracking or recurring staining are present, investigate the damage and exposure rather than treating magnetism as the root cause.
A qualified solution treatment can reverse strain-induced martensite, but may also change strength, dimensions and surface finish. It is not simply a stress-relief operation or a harmless last-minute adjustment. For a cold-worked component whose strength depends on drawing, that trade-off may be unacceptable. Any treatment must be compatible with the drawing and validated fabrication route.
Degaussing addresses retained magnetisation; it does not remove the microstructure responsible for attraction. Likewise, switching automatically to 316 is not a guaranteed solution. For a strict limit, compare the supplied and finished conditions using an agreed test before approving a production route.
For 304 stainless steel round bar, include the delivery condition and machining plan. For stainless steel wire, include diameter, strength or condition and subsequent forming. Describe any magnetic limit explicitly rather than adding only “non-magnetic” to the grade name.
Technical reference: ASSDA Technical FAQ 3: Magnetic Effects of Stainless Steels. Reference information is not a test certificate for a supplied batch.
Send the 304 product form, dimensions, quantity, ordered condition and the observed magnetic response. If the application is sensitive, include the acceptance limit and test method for review.