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Ferritic, martensitic, duplex (and most precipitation hardening) stainless steels are usually classified as ‘magnetic’ since they exhibit a strong response (or pull) to a hand-held magnet. By comparison, austenitic stainless steels are often described as ‘non-magnetic’ since their response to a hand-held magnet is negligible.
For some applications, it is necessary to use a stainless steel which has very low or negligible response to electromagnetic fields. A limit is then usually specified on the relative magnetic permeability of the steel (more often described simply as ‘permeability’). The lowest relative magnetic permeability of a paramagnetic material is 1.0, i.e. the magnetic response of the material is the same as ‘free space’ or a complete vacuum.
The permeability of any given steel is dependent on the distribution of ferrite, martensite and austenite in its structure. Permeability values for ferromagnetic steels are usually high, at around 14. This is because the principal metallic phases in these steels at room temperature are ferrite and martensite, both of which have high permeability. Non-ferromagnetic steels have permeabilities of 1.0, or slightly more. Austenitic stainless steels fall into this category because their austenitic structure at room temperature has low permeability.
The ferritic and martensitic stainless grades (‘400’ series) are generally free from austenite, have high permeabilities and so will attract a magnet. They are classed as ferromagnetic. These steels can be magnetised in an electric field and so can be used for electric solenoid cores, where good corrosion resistance is required. An example of this type of steel is the “soft” ferritic 430 (1.4016). Some steels are certified with guaranteed magnetic properties for solenoids.
Duplex stainless steels with a balance of austenite & ferrite phases (around 50 / 50) are also ferromagnetic but would be classed as “harder” due to their higher mechanical strength.
The hardenable martensitic stainless steels can be classed as “hard” magnetic materials which, although ferromagnetic, will not magnetise and demagnetise as easily as the ferritic or duplex grades and so could be regarded as having magnetic properties closer to those of materials used for permanent magnets.
Typical values of some martensitic and ferritic stainless steels can be found here.