Magnetic properties of austenitic stainless steels

Introduction

Austenitic stainless steels are usually described as 'non-magnetic' since their response to a hand-held magnet is negligible. However, 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.

Relative magnetic permeability

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 austenitic stainless steels is quite different from ferritic, martensitic and duplex stainless steels. Austenitic steels can be classed as 'paramagnetic' with relative permeabilities approaching 1.0 (generally in the range of 1.003 to 1.05 in the fully annealed condition). These low permeabilities enable austenitic steels to be used where 'non-magnetic' materials are required. Such applications include casings for medical equipment such as body scanners, and concrete reinforcing bars for radar installations.

Realative permeabilities above 1.0 are associated with the amount of either ferrite or martensite phases present in the 'austenitic' steel and so are dependent on:-

  • chemical composition

  • cold work and heat treatment conditions.

 

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