22nd of December ,2016

How does stainless steel become non magnetic?

Austenitic stainless steels, such as 304 and 316, are generally regarded as non-magnetic in the annealed condition, i.e. they are not attracted significantly by a magnet. However, if they are cold worked they will be attracted to a permanent magnet.


The change occurs because the cold work deformation induces a transformation of the microstructure from austenite to martensite. The effect is less marked in alloys with high concentrations of austenite stabilisers such as nickel, nitrogen and carbon.

Once the martensite is formed, it may also become magnetised sufficiently to pick-up light objects such as paper clips. Magnetic attraction effects are most often noticed in heavily cold worked fabrications such as wire or the dished end of a pressure vessel.It is possible to remove the magnetic effects by solution annealing and water quenching but this will also reduce the tensile properties and may give rise to distortion.In contrast to the austenitic alloys, ferritic stainless steels such as 409 or 3Cr12/5Cr12 and martensitic stainless steels such as 420, are strongly attracted to a magnet even in the annealed state.

The duplex and super duplex stainless steels will also be strongly attracted because they contain about 50% ferrite in their microstructure. The electrical demagnetising treatments applied when it is necessary to avoid permanent magnetic fields are fairly effective with magnetically soft materials such as ferrite.


However, the strain induced martensite in an austenitic stainless steel furniture and the normal martensitic structure of a 420, are reasonably magnetically hard and, once they are magnetised, it is difficult to electrically remove the permanent magnetic effect.Castings have subtly different compositions than the equivalent wrought alloys. The austenitic alloys typically have a few percent ferrite and are weakly attracted to a magnet, ie they are ferromagnetic. Ferritic, martensitic and duplex casting alloys have similar magnetic properties to their wrought counterparts.

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