A Magnet Sticks to My Stainless Screw. Is It Defective?

A magnet sticking to the screw does not, on its own, make it defective. It is a common material characteristic of cold-formed STS304, and ISO 3506-1 has no pass/fail criterion for magnetism.

Why it happens

STS304 in the solution-annealed condition is austenitic and usually only very weakly magnetic; wire that has been cold drawn may already show slight magnetism. Screws are then made by cold forging and thread rolling. That plastic deformation can convert part of the austenite into strain-induced martensite, which is magnetic.

The effect tends to be more pronounced 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.

High-strength classes such as A2-70 and A2-80 are heavily work hardened and are often more magnetic. But stronger magnetism does not prove good processing: a material with more stable austenite can be fully cold worked to its property class and still be only weakly magnetic. Magnetism after cold working can be a normal material characteristic, but its presence or absence alone cannot be used to judge process quality or acceptance.

Why the same part varies between deliveries

"The last delivery was not magnetic, this one is." Even within STS304, magnetism can vary from lot to lot. Wire at the lower end of the nickel range (about 8–10.5%) transforms more readily, and other alloying elements, the wire's heat treatment and drawing history, and the actual amount of forging and rolling also play a part.

A difference in magnetism between deliveries therefore does not necessarily mean the material was changed. Confirm the material with the mill certificate and, if needed, a chemical analysis, not with a magnet.

Does corrosion resistance suffer?

Corrosion resistance rests on the chromium oxide (passive) film, and cold working does not change the Cr and Ni composition. Magnetism alone is therefore not evidence of poor corrosion resistance.

That said, the structural changes, residual stress and surface condition that come with cold working can affect local corrosion resistance — pitting and crevice corrosion — in certain environments such as chlorides. Corrosion performance should be judged on the surface condition and actual service environment as well as the steel grade. For demanding environments, agree the steel group (such as A4) and surface treatment at the quotation stage.

What the standard says

For austenitic stainless fasteners, ISO 3506-1 specifies chemical composition, tensile strength, 0.2% proof stress and elongation (breaking torque for small screws). There is no pass/fail criterion for magnetism; the standard simply describes austenitic grades as usually having low or no magnetism. Under ISO 3506-1 alone, magnetism is not an acceptance criterion. If the customer has specified a separate magnetism requirement on the drawing or contract, that requirement applies.

Quantifying magnetism — relative permeability

Whether a magnet sticks depends on the magnet and the size of the part, so it is only a qualitative check. Where magnetism matters, as in instrumentation and medical or semiconductor equipment, it is often controlled by relative magnetic permeability (μ_r). The closer μ_r is to 1, the less the part responds to a magnetic field.

The key is to agree the maximum permissible relative permeability and the test method in advance. Test methods for feebly magnetic materials include ASTM A342/A342M. Results can vary with the test method, part geometry and measuring position, and because of specimen size and shape limits such methods are sometimes easier to apply to material before forming than to small finished parts. If magnetism must be guaranteed on the finished part, agree the measuring conditions and applicable test method in advance.