Industry News
Why are screws magnetic? Non-magnetic 304 stainless screw basics
2026-09-07
Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.
Where screw magnetism comes from
Whether a magnet sticks to a screw is decided by its material — not by age or use. Carbon and alloy steels such as 35#, 45#, 40Cr and SCM435 are ferromagnetic: their iron-based structure gives obvious magnetic attraction, so most zinc-plated carbon steel screws will be picked up by a magnet. In contrast, 304 / 316 austenitic stainless steels have a face-centred cubic austenitic structure that is essentially non-magnetic — which is exactly why stainless screws are specified in medical, electronics and precision instruments. If a buyer asks about screw magnetism, checking the material grade answers the question faster than repeated magnet tests.
Why 304 stainless screws are sometimes slightly magnetic
Customers occasionally find a 304 stainless screw 'slightly attracted' by a strong magnet. The main cause is cold-work transformation: cold heading and thread rolling convert part of the austenite into martensite, adding weak magnetism. A low-nickel batch or a little residual ferrite can also raise the attraction slightly. Such weak magnetism does not affect ordinary outdoor corrosion resistance, but for strictly non-magnetic applications (MRI medical equipment, precision sensors, avionics modules) the requirement must be specified: choose solution-treated or non-magnetic 304 / 316, or switch to titanium, and note the permeability target on the drawing (for example μ ≤ 1.05) with measured data from the supplier.
How to verify magnetic performance when buying
A pocket magnet is only a quick screening tool and should never be the sole test for judging stainless steel — a lightly attracted cold-worked 304 part does not mean the material is defective. The proper approach: (1) ask the supplier for material certificates per GB/T 20878 grades and ISO 3506 A2 / A4; (2) for strict requirements, measure permeability with a magnetometer and issue a report per ASTM A342; (3) when non-magnetic parts are needed, state 'non-magnetic, permeability ≤ 1.05' on the order and sample-check before delivery. Need stainless or non-magnetic fasteners? Contact JIALI Industrial (164.hk) with your specification and permeability target for a quick match.
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