Ferritic, martensitic and duplex stainless steels normally respond to a magnet; annealed austenitic grades such as 304 and 316 usually respond very little. Most martensitic precipitation-hardening grades, including 17-4PH, are also magnetic. However, the grade name alone does not define the magnetic performance of a finished component: cold forming, welding and the supplied condition can change the result.
For material selection, first decide whether magnetic attraction is required, permitted or restricted. Then check corrosion exposure, strength and fabrication. A stainless grade that attracts a magnet is not necessarily inferior, and a grade that does not attract one is not automatically suitable for a magnetically sensitive assembly.
| Family and examples | Expected response | Selection trade-off |
|---|---|---|
| Ferritic: 409, 430, 439 | Normally magnetic, including in annealed material | Consider corrosion exposure, forming and welding requirements; magnetic response alone does not establish suitability. |
| Martensitic: 410, 420, 440C | Normally magnetic | Useful where hardening and wear resistance matter; toughness and corrosion performance depend on grade and heat treatment. |
| Duplex: 2205, 2507 | Normally magnetic because ferrite is present | Consider strength and chloride resistance together with fabrication and phase-balance requirements. |
| Austenitic: 304, 316, 310, 321 | Usually low attraction when properly solution annealed | Cold work or magnetic phases can increase response; specify final-condition limits if important. |
| PH: 17-4PH, 15-5PH | These martensitic PH grades are magnetic | Age-hardening condition controls mechanical properties; the PH family is not a universal non-magnetic option. |
These descriptions concern normal engineering conditions, not measured permeability values for every heat or product. The table is a starting point for shortlisting, not a substitution chart. A magnet cannot distinguish 430 from 410, prove a duplex phase balance, or confirm that a PH bar has received the specified aging treatment.
A hand magnet provides a quick qualitative observation. Record where it attracts, whether the response is uniform and whether the same result occurs on unworked material. Magnet size, gap, component thickness and geometry affect the observation. Comparing a thick bar with a thin strip by hand is not a controlled material test.
Relative magnetic permeability describes field response. A low-permeability requirement needs an agreed limit, measurement method, field conditions and test locations. It should also state whether acceptance applies before or after bending, machining and welding. “Non-magnetic stainless” without this information can mean different things to the designer, buyer and inspector.
Residual magnetism is a separate issue. A part can retain magnetisation after handling or processing. Measuring a residual field with a suitable instrument is not the same as measuring permeability. Degaussing may reduce a retained field; it does not turn ferritic steel into austenitic steel or remove deformation-induced martensite.
Medical and scanner applications require separate qualification. A low-permeability material description or a hand-magnet result is not evidence that a component is MRI safe or approved for medical use.
Keep the grade identification and magnetic acceptance requirements separate. The material certificate and appropriate chemical analysis address composition; dimensional and mechanical tests address their respective requirements. Additional magnetic testing must match the design question. If carbon content is essential to identifying the grade, do not assume every handheld XRF instrument can resolve it.
For shafts and machined parts, review the stainless steel bar range and state the delivered condition. For corrosion-resistant high-strength designs, the duplex stainless steel range should be assessed against the complete service specification, not magnetic attraction alone.
Technical reference: BSSA guidance on magnetic properties of stainless steels. Reference information is not a test certificate for a supplied batch.
Send the grade or candidate family, product form, dimensions, quantity, service environment and the required magnetic test. Include downstream fabrication so acceptance can be defined at the correct production stage.