Short answer: A magnet is the first test, because 430 and other 400 series are magnetic whereas 304 and 316 are usually non-magnetic or only slightly magnetic after working, but a magnet cannot separate 304 from 316, which needs a chemical test kit for molybdenum or an analyser at the buyer, so for most sellers the practical rule is magnet for 400 series and ask a buyer to confirm the 300 series.
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List Free →The magnet test
Strongly magnetic stainless is almost certainly 400 series, and non-magnetic stainless is probably 300 series.
Worked metal can mislead
Cold working can make some 300 series slightly magnetic, so a weak pull does not rule it out.
Confirmation
A test kit or analyser is needed to separate 304 from 316, which is why buyers test.
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