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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. The underlying reason is the crystal structure rather than the alloy content. Iron in its body-centred cubic form, which is what exists in ordinary carbon steel and in ferritic or martensitic stainless grades, aligns readily with an applied field and behaves as a soft magnetic material. The face-centred cubic austenitic structure has no easy alignment path, so 304 and 316 are very weakly magnetic in the annealed condition, with relative permeability close to 1.00 to 1.05. That is a fundamental property of the crystal structure and cannot be changed by cleaning, polishing or demagnetising a part. It follows that a strongly magnetic stainless screw is almost certainly not austenitic in the condition supplied, either because it is a different grade, or because it has been heavily cold worked, or because it has been heat treated in a way that formed martensite. A second and often overlooked source of confusion is the difference between magnetism and corrosion resistance. A magnet is not a corrosion test, and corrosion resistance is not a magnetism test: a grade 410 screw is strongly magnetic and moderately corrosion resistant, while a 316 screw is non-magnetic and highly corrosion resistant. Buyers occasionally reject a perfectly good 316 batch because a magnet is weakly attracted, which is a misunderstanding of what the test measures. The reverse mistake is accepting a magnetic part as stainless because it does not rust in a dry warehouse, when it may simply be a plated carbon steel screw that has not yet been exposed to moisture.

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 1.05 maximum) with measured data from the supplier. The practical scale of the effect is worth knowing. A solution-annealed 304 screw typically shows a relative permeability near 1.01 to 1.02 and a magnet will not noticeably attract it. The same screw after cold heading and thread rolling may rise to 1.2 or higher at the head-shank fillet and near the thread roots, where deformation is most severe, and a small strong magnet may then exert a faint pull. A subsequent solution anneal at about 1040 degC followed by rapid cooling restores the austenitic structure and removes most of that magnetism, but it also risks scale and distortion on a finished threaded part and is rarely done on standard fasteners. Where a low permeability is genuinely required, the order has to say so before manufacture, because the forming route and the heat treatment are decided at that stage and cannot be retrofitted economically. Among the alternatives, 316L is not measurably better than 304 in this respect since the deformation mechanism is the same, though the higher nickel content gives it slightly more austenite stability and marginally lower permeability after forming. Titanium is effectively non-magnetic and is the solution where both low permeability and low weight matter, at a substantial cost premium and with different strength and galling characteristics that must be designed for. Certain high-manganese or nitrogen-bearing austenitic grades are also used where non-magnetic properties and higher strength are both needed, though availability in Hong Kong is limited and lead times are longer.

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 maximum' on the order and sample-check before delivery. It also helps to know how the measurement is actually done, because a permeability figure without a method is not comparable. ASTM A342 covers several test methods, from a simple permeability indicator through to a vibrating sample magnetometer, and they do not all give the same number on the same part; a specification should therefore state the method as well as the limit. The measurement position matters too: because cold work is uneven, permeability is highest near a formed feature and lowest in the straight shank, so the report should identify where the reading was taken. Where a customer needs to screen incoming goods without laboratory equipment, a calibrated permeability meter is a reasonable middle ground, but the acceptance limit and the pass-fail procedure should be agreed in advance so that a marginal part is not accepted one week and rejected the next. A worked comparison of common approaches: a pocket magnet gives a yes or no sort and is useful only for separating carbon steel from stainless, at negligible cost but with no traceable figure; a permeability meter gives a numerical reading on the surface of a part, suitable for goods-in screening with a documented limit; a magnetometer report to ASTM A342 gives a traceable measurement with the method and position stated, and is what an aerospace or medical customer's quality department will expect; and destructive metallographic examination confirms the microstructure and can identify the deformation martensite directly, which is the appropriate route when a dispute has already arisen. For most industrial applications, the certificate plus a permeability meter check on a sample is proportionate. For medical imaging, sensor and avionics work, only the traceable report is adequate, and the requirement should be written into the purchase order rather than agreed verbally.

Common pitfalls and acceptance points in non-magnetic supply

Pitfall one is treating the magnet test as a material identification test. It cannot distinguish a low-nickel substitute from genuine 304, and it will wrongly condemn a heavily cold-worked but perfectly sound part. Use certificates to establish grade and a magnet only to screen. Pitfall two is specifying non-magnetic requirements after the parts have been made. Permeability is set by the forming route and heat treatment, so the requirement has to appear on the drawing and the purchase order before production. Pitfall three is assuming that 316 is automatically more non-magnetic than 304. Both are austenitic and both gain weak magnetism from cold work; the difference between them is corrosion resistance, not permeability. Pitfall four is accepting a permeability figure without the method or the measurement position. A number on its own is not evidence. Pitfall five is overlooking the other components in the assembly. A non-magnetic stainless screw fitted with a carbon steel spring washer, a steel retaining clip or a plated nut introduces a magnetic component into the assembly, and in a sensor or MRI application that single small part can defeat the purpose of specifying the fastener. Include every component in the non-magnetic requirement. At acceptance, the practical checks are: verify the material certificate against the ordered grade and standard; confirm the certificate covers the batch supplied, not a generic grade statement; take a permeability reading on a sample with a calibrated instrument at an agreed position and compare it with the limit; check that nuts, washers and any retaining hardware are also supplied to the non-magnetic requirement; and where the duty is safety or image-quality critical, retain a sample with its measurement record for traceability. Where a supplier cannot state a permeability figure or a measurement method, that in itself is useful information about how well the requirement will be understood in production. Need stainless or non-magnetic fasteners? Contact JIALI Industrial (164.hk) with your specification and permeability target for a quick match.

Related Reading

• Fastener purchase contracts and drawing tolerances: getting the quality requirements right the first time
• Stainless bolts still rust? Passivation, corrosion testing and procurement essentials
• JIALI launches Stainless Steel zone with full 304 / 316 range

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