The question usually arrives as a complaint: a magnet sticks to a part that was supposed to be stainless, so someone thinks the material is wrong. Sometimes it is. Often it is a perfectly good 430 or a work-hardened 304, and the material is behaving exactly as it should. Grade, and the amount of cold work in the part, decide the answer.
Is stainless steel magnetic?
It depends on the family. The austenitic grades - 304, 316, 321 - are non-magnetic when annealed, with a relative permeability close to 1.0, so a magnet has almost no effect on them. The ferritic and martensitic grades - 430, 430F, 410, 420 - are ferromagnetic and a magnet sticks to them firmly. Duplex grades sit in between. There is a second effect worth knowing: cold working converts some of the austenite in 304 to martensite, so a deep drawn or heavily formed 304 part can show a weak magnetic pull while the flat sheet it was made from does not. That is why a magnet is a quick screening test, not a material certificate.
Magnetic behaviour by grade family
| Grade family | Example | Magnetic behaviour | Notes |
|---|---|---|---|
| Austenitic | 304, 304L | Effectively non-magnetic when annealed | Becomes weakly magnetic where cold worked or drawn |
| Austenitic with Mo | 316, 316L | Effectively non-magnetic | Same cold-work effect, usually less pronounced |
| Ferritic | 430 | Magnetic | Used for sensor and exhaust fittings where magnetism is acceptable |
| Free-machining ferritic | 430F | Magnetic | The usual grade for turned sensor hex nuts: machines cleanly, holds a thread |
| Martensitic | 410, 420, 440C | Magnetic | Hardened grades for wear parts |
| Duplex | 2205 | Partially magnetic | Ferrite content makes it attract a magnet weakly to moderately |
| Precipitation hardening | 17-4PH | Magnetic | Martensitic structure after ageing |
Why a drawn 304 part can be magnetic
Strain-induced martensite forms where the material is deformed most - the drawn wall, the bend, the thread root. A deep drawn 304 housing often pulls a magnet at the wall while the flange, which saw less strain, does not. Post-forming annealing removes most of this and returns the part to its non-magnetic state, which is why a magnetic specification on a heavily formed part has to be agreed as either "after anneal" or with a stated permeability limit.
When the magnetic answer matters
- Sensor and instrument assemblies - a ferromagnetic part next to a magnetic sensing element can bias the reading. Where that matters the drawing should specify a non-magnetic grade, and the small parts - the fitting, the screw, the spring - are the ones that get missed.
- Incoming inspection - a magnet sorts 430F from 304 in a second, which is why mixed deliveries of fittings are usually checked this way before they reach assembly.
- Materials handling - magnetic conveyors and chutes move 430 parts and ignore 304, which changes how a line is designed.
- Welding and fixtures - magnetic fixturing works on ferritic grades; austenitic parts need clamps.
Testing it properly
A magnet tells you yes or no; it does not tell you the grade or the condition. If the magnetism matters to the function, specify the grade and, where a limit is needed, the relative permeability and where it is measured. Mill certificates and a chemistry check are the only way to confirm what arrived. For the corrosion side of the same decision, see whether stainless steel rusts; for the wider question across materials, see which metals are magnetic.
Key point
304 and 316 are non-magnetic when annealed but pick up a weak pull where they have been cold worked; 430 and 430F are magnetic and always will be. Decide from the function, write the grade and the condition on the drawing, and use the magnet for sorting rather than for specification.
Frequently asked questions
Is 304 stainless steel magnetic?
Annealed 304 is effectively non-magnetic. Cold working - drawing, bending, thread rolling - can form strain-induced martensite, so a heavily formed part may attract a magnet weakly.
Which stainless steel is magnetic?
The ferritic and martensitic families: 430, 430F, 410, 420 and the precipitation-hardening grades such as 17-4PH. They are strongly attracted to a magnet.
Does cold working make stainless steel magnetic?
It can. Deformation converts part of the austenite in 304 and 316 to martensite, which is magnetic, so the magnetic response rises where the material was worked hardest.
How do I get a non-magnetic stainless part?
Specify an austenitic grade such as 304 or 316 and, where the part is heavily formed, either anneal after forming or agree a permeability limit and measuring method on the drawing.