Can I tell 304 and 316L apart by appearance or a magnet?
No. Both are austenitic and non-magnetic in the annealed condition, and after the same finish they are visually identical. Grade is established by documentation or by testing, and any supplier who offers a visual or magnetic check is offering a guess.
Accepted verification methods
An EN 10204 3.1 mill certificate traceable to the heat number, or positive material identification with an XRF analyser or spectrometer on the delivered sheets.
Why a magnet misleads
Cold work makes 304 slightly magnetic, and bending or laser cutting can make 316L read magnetic too. Magnetism reflects forming history, not grade.
What to record
Heat number, certificate, and the panels it covers. On a phased project, keep the record so later batches can be compared against the same reference.
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