How does stainless steel become non-magnetic?

Austenitic stainless steels (such as 304 and 316) are generally considered non-magnetic in their annealed condition, i.e. they are not significantly attracted to magnets. However, if they are cold worked, they will be attracted to permanent magnets.

This change occurs because cold work deformation induces a transformation of the microstructure from austenite to martensite. This effect is less pronounced in alloys containing high concentrations of austenite stabilizers such as nickel, nitrogen and carbon.

Once martensite is formed, it may also be sufficiently magnetized to pick up lightweight objects such as paper clips. The magnetic effect is most common in heavily cold-worked products, such as wire or the dished ends of pressure vessels. The magnetic effect can be eliminated by solution annealing and water quenching, but this will also reduce the tensile properties and may increase the relationship with austenitic In contrast to bulk alloys, ferritic stainless steels (such as 409 or 3Cr12/5Cr12) and martensitic stainless steels (such as 420) are strongly attracted to magnets, even in the annealed state. Duplex and superduplex stainless steels will also be strongly attracted because their microstructure contains approximately 50% ferrite. Electrical demagnetization is quite effective for soft magnetic materials such as ferrites when permanent magnetic fields must be avoided.

However, the strain-induced martensite in austenitic stainless steel furniture and the normal martensite structure of 420 are quite magnetic. Once magnetized, it is difficult to remove the permanent magnetic effect with electricity. Equivalent wrought alloy. Austenitic alloys usually have a few percent ferrite and are hardly attracted to magnets, i.e. they are ferromagnetic. The magnetic properties of ferritic, martensitic and duplex cast alloys are similar to those of wrought alloys.