Magnet sticks to 316L stainless: does it mean the steel is wrong?
Published
TL;DR: In the annealed condition 316L is essentially non-magnetic. But a magnet can still stick to a perfectly good 316L part (cold work, welds), and a magnet cannot prove a part IS 316L. Use a certificate, not a fridge magnet. The situation: you order 316L sheet or tube, a magnet clings to a bent edge or a weld, and the first thought is "I was sold the wrong steel." This comes up constantly, so here is what is actually going on. Why 316L normally is not magnetic 316L is an austenitic stainless steel. Austenite does not respond to a magnet the way carbon steel does, which is why people choose it near sensors, instruments and vacuum hardware. As supplied it is soft and ductile: our archive lists a density of 8.03 g/cm3, 170 MPa minimum yield, 485 MPa tensile and 40 % elongation (ATI plate, sheet and strip). Three reasons a magnet can still stick - Cold work. Bending, drawing or heavy machining can turn some austenite into martensite, which is magnetic. A tight bend can attract a magnet while the flat plate does not. - Welds. Weld metal often contains a little ferrite on purpose to resist cracking, and ferrite is magnetic. A magnet that grabs the bead but not the plate next to it is normal. - It is not 316L. A magnet cannot tell 316L from 304, and ferritic or martensitic stainless steels are strongly magnetic. A magnet test proves very little either way. What I would actually do If magnetic response matters (sensors, vacuum chambers, sensitive instruments), skip the argument about the grade name and ask for the relative permeability of the part as delivered and after forming and welding. Put the limit on the drawing. If the grade matters, get the mill certificate or a composition check. L or no L? The L means low carbon, which helps after welding, and it costs a little strength: our datasheets list 316 at 205 MPa minimum yield against 170 MPa for 316L. For most non-magnetic jobs that gap is not what decides it. Where I might be wrong: how magnetic a cold-worked part gets depends on composition and how hard you deform it, so I cannot give you a number for a given bend. Real measurements beat my generalisations. Have you had a 316L part surprise you with a magnet? Was it cold work, a weld, or a mislabelled bar?