A Magnet Sticks to My Stainless Screw. Is It Defective?
Why STS304 parts become magnetic, and why that is not a quality problem.
No — it is not a defect. A magnet sticking to a stainless screw is normal and expected behaviour for cold-formed STS304 fasteners.
Why it happens
STS304 in the annealed condition is austenitic and essentially non-magnetic. Screws, however, are made by cold forging and thread rolling. That plastic deformation converts part of the austenite into strain-induced martensite, which is magnetic.
The effect concentrates where deformation is heaviest — the underside of the head and the rolled thread. The chemistry of the steel has not changed; only the crystal structure of the worked zones has.
Why the same part varies between deliveries
"The last delivery was not magnetic, this one is." This is the most common form of the question, and the usual cause is lot-to-lot variation in the wire.
The nickel content of STS304 is specified as a range, typically around 8–10.5%. Nickel stabilises austenite, so wire at the lower end of the range transforms more readily under the same forming conditions and comes out more magnetic. The material has not been substituted. Both lots are STS304 and both conform.
Does corrosion resistance suffer?
Corrosion resistance rests on the chromium oxide (passive) film. Cold working changes the crystal structure but does not change the Cr and Ni composition, so the ability to form a passive film is retained.
Severely deformed zones can see some local reduction in pitting resistance, so we finish parts by pickling to clean the surface.
What the standard says
ISO 3506-1 specifies chemical composition, tensile strength, proof stress and hardness for stainless fasteners. There is no requirement regarding magnetism. A part that is magnetic is fully conforming, and magnetism is not a valid basis for rejection unless it was agreed as a separate requirement.
Where magnetism must be minimised — instrumentation, sensor housings, some medical and semiconductor equipment — the practical route is STS316L (A4) material. Its higher nickel and molybdenum content makes the austenite more stable, so less martensite forms during cold work. Note that this is a material change, not a process adjustment, and it should be agreed at the specification stage. Tell us your requirement and we will review feasibility with you. Request a quote