Is stainless steel magnetic? The grades that are, and the ones that are not
Some stainless steel is magnetic and some is not, and both answers are correct. Ferritic and martensitic stainless, most of the 400 series, hold a magnet firmly. Austenitic grades such as 304 and 316, the ones in sinks, cutlery and food plant, are practically non magnetic, although they can pick up a little magnetism after being bent or pressed.
Stainless steel and magnets: key facts
| Austenitic (304, 316, 18/8, 18/10) | Practically non magnetic as made. The bulk of what you will meet: sinks, cutlery, catering equipment, food and drink plant |
|---|---|
| Ferritic (430, 409, 439) | Magnetic. Appliance fronts, car exhausts, cheaper cookware and trim |
| Martensitic (410, 420, 440) | Magnetic. Knife blades, turbine parts, shafts |
| Duplex | Magnetic. Pipework and tanks in oil, gas and water treatment |
| Cold worked 304 | Can become slightly magnetic after bending, pressing or machining, so a formed sink bowl may hold a weak magnet while flat sheet does not |
| What a magnet cannot do | Tell 304 from 316, which is the difference that matters most to the price |
Why some grades stick and others do not
Stainless steel is iron with at least 10.5 per cent chromium, and iron is magnetic, so the surprising thing is not that some stainless is magnetic but that some is not. The answer is the arrangement of the atoms. In ferritic and martensitic grades the iron keeps the body centred cubic structure it has in ordinary steel, and that structure is ferromagnetic. The austenitic grades carry eight per cent or more nickel, and the nickel holds the iron in a different, face centred cubic arrangement called austenite, which is not ferromagnetic. Same chromium, same shine, opposite answer to the magnet.
Austenite is not perfectly stable. Bend, press, deep draw or machine a 304 part and some of the austenite transforms into martensite, which is magnetic. That is why a kitchen sink can hold a weak magnet in the pressed corners and not on its flat base, and why a bracket can read differently from the sheet it was cut from. The effect is small, and a 316 part, with its higher nickel and added molybdenum, is more stable and shows it less.
What the magnet test tells a scrap yard
A magnet divides stainless into two piles, and the piles are priced differently. Stainless that grips a magnet is almost certainly a 400 series grade, with little or no nickel, and it is bought at a lower rate, sometimes close to ordinary steel. Stainless that does not hold a magnet, or holds it only weakly, is a 300 series grade, and the nickel in it is what the yard is really paying for. Our stainless steel scrap price page shows this month's figures, and the gap between the two piles is usually wide.
The magnet cannot take you further than that. 304 and 316 behave the same way under a magnet, and 316 is worth more because of its molybdenum, so yards settle the question with a handheld analyser that reads the alloy in seconds. Our guide to 304 versus 316 stainless covers how to tell them apart from markings and where each is used.
Which grade is in which product
Kitchen sinks, saucepans, cutlery marked 18/8 or 18/10, catering counters, brewery and dairy tanks and food processing plant are almost always 304, with 316 where salt or chemicals are involved: marine fittings, coastal handrails, pharmaceutical and chemical plant. Car exhaust systems are usually 409 or 439, which is why a magnet sticks to them even though they are stainless. Appliance doors, cooker hoods and decorative trim are often 430, cheaper and magnetic. Knife blades are martensitic because they need to harden. If you are sorting a mixed pile, the magnet will separate the exhaust pipes and appliance panels from the sinks and tanks in a minute, and that is the sort that moves the price.
Selling stainless for the best price
- Magnet first. Sticks firmly: 400 series pile. Does not stick, or only weakly at bends: 300 series pile.
- Clean it. Plastic handles, rubber seals, glass and steel fixings all pull the price down, and a tank with its steel frame still on is bought as a mixed item.
- Do not mix it with steel. A load of stainless with mild steel in it is paid as steel.
- Let the yard tell 304 from 316. Ask them to analyse it rather than guessing, and ask for the grade and the rate on the ticket.
Stainless rarely comes in car boot quantities from a business, so scrap metal collection is usually the way it moves, and the scrap metal prices page puts stainless alongside the other metals so you can see where it sits.
Frequently asked questions
Is stainless steel magnetic?
It depends on the grade. Ferritic and martensitic stainless, most of the 400 series, are magnetic. Austenitic grades such as 304 and 316 are practically non magnetic, although they can become slightly magnetic where they have been bent or pressed.
Is 304 stainless steel magnetic?
Not as made. 304 is austenitic and will not hold a magnet in its annealed state. Cold working, such as pressing a sink bowl or bending a bracket, turns some of the austenite into magnetic martensite, so a formed 304 part can hold a weak magnet.
Is 316 stainless steel magnetic?
No, and it stays that way better than 304 does. Its higher nickel content and its molybdenum make the austenite more stable, so even formed 316 parts show very little magnetism.
Why does a magnet stick to my stainless steel exhaust?
Because exhaust systems are usually made from ferritic grades such as 409 or 439, which are magnetic. They are still stainless steel, just a different family from the 304 in a kitchen sink.
Can a magnet tell 304 from 316 stainless?
No. Both are austenitic and practically non magnetic. The difference is molybdenum, which 316 contains and 304 does not, and a scrap yard identifies it with a handheld analyser.
Is magnetic stainless steel lower quality?
No, it is a different family for a different job. Ferritic grades resist heat and are cheaper, which suits exhausts and appliance panels. Austenitic grades resist corrosion better and are worth more as scrap because of their nickel.