Magnetic Behavior of Stainless Steels

Magnetic Behavior of Stainless Steels

The magnetic properties of stainless steels vary considerably, ranging from paramagnetic, non-magnetic,  in fully austenitic grades to hard or permanent magnetic behavior in the hardened martensitic grades.

Austenitic (non-magnetic) Stainless Steels

All austenitic stainless steels are paramagnetic, non-magnetic, in the fully austenitic condition as occurs in well-annealed alloys. The DC magnetic permeabilities range from 1.003 to 1.005 when measured with magnetizing forces of 200 Oersteds (16 kA/m). The permeability increases with cold work due to deformation-induced martensite, a ferromagnetic phase. For certain grades such as Types 302 and 304, the increase in magnetic permeability can be appreciable, resulting in these grades being weakly ferromagnetic in the heavily cold-worked state. For example, Type 302 and 304 increase in permeability by ten fold when the cold reduction is 80%.

The magnetic permeabilities achievable in austenitic stainless steels are very low as compared to conventional magnetic materials. Consequentialy, their non-magnetic behavior is of more concern. In the event that the magnetic property of an austenitic stainless steel is of particular concern, it can be measured by relatively simple means as described in ASTM Standard Method A342.

Austenitic stainless steels are not hardenable by heat treatment.  If the application requires that the steel be hardenable, consider using Martensitic and precipitation hardenable stainless steels (see below); however, note that these are ferromagnetic.  It is important to consider the effects of using ferromagnetic materials in magnetic circuits if the circuit was originally designed using paramagnetic steels.

Examples of austenitic stainless steels are Type 302, 303, 304, 316 and 316L.

Ferritic Stainless Steels

Ferritic stainless steels are ferromagnetic and have been used as soft magnetic components such as solenoid cores, pole pieces and return paths. Although their magnetic properties are not generally as good as conventional soft magnetic alloys, they are successfully used for magnetic components that must withstand corrosive environments. As such, they offer a cost-effective alternative to plated iron and silicon-iron components. Additionally, the relatively high electrical resistivity of Ferritic stainless steels has resulted in excellent AC performance.

These stainless steels have soft magnetic properties: high magnetic permeability, low coercive force, Hc, and low residual induction Br, which depend on alloy chemistry. In particular, impurities such as carbon, sulfur, and non-metallic inclusions, and stresses due to cold working. Magnetic permeability decreases and the coercive force increases; the behavior is less magnetically soft with increasing amounts of impurities and stress.  Hence, optimum magnetic performance is obtained with well-annealed, high-purity alloys. Carpenter 430F and 430FR (Solenoid Quality) is an excellent choice for soft magnetic alloy applications.  Note that if the material has been cold worked, its coercivity, Hc, will increase, and when exposed to magnetic fields, it will retain some magnetic effects, hence acting as a weak permanent magnet. 

Examples of Ferritic stainless steel are Type 430F Solenoid Quality, Type 430FR Solenoid Quality, and Type 446

Martensitic and Precipitation Hardenable Stainless Steels

All Martensitic and most precipitation Hardenable stainless steels are ferromagnetic.  Due to the stresses induced by the hardening transformation, these grades exhibit permanent magnetic properties if magnetized in the hardened condition.  For a given grade, the coercive force, Hc, tends to increase with increasing hardening, rendering these alloys more difficult to demagnetize.

Examples of martensitic stainless steels are Type 410, 416, 420, 440B and 17-4.