Magnetic Permeability in SS

What is Magnetic Permeability?

Magnetic permeability describes how easily a material can become magnetised in the presence of a magnetic field. It defines how easily magnetic lines of force can pass through a substance when exposed to a magnetic field. In stainless steels, it indicates the degree of magnetic response, which is important for applications requiring controlled magnetic behaviour. And it is an essential parameter to consider for applications involving sensors, electrical components, and precision instruments. 

Magnetic Permeability of Stainless Steel

The magnetic permeability of stainless steel varies depending on its microstructure and chemical composition. Among the different types of stainless steels, ferritic and martensitic grades are magnetic due to their body-centred cubic (BCC) / body-centred tetragonal (BCT) crystal structure, respectively. In contrast, austenitic stainless steels, which have a face-centred cubic (FCC) structure, are generally non-magnetic or have a very low magnetic permeability in their annealed state. However, cold working or deformation can induce slight magnetism even in austenitic grades due to the formation of strain-induced martensite phase. 

Factors Affecting Magnetic Permeability in Stainless Steel

Several factors influence the magnetic permeability of stainless steel, including:

• Chemical composition:  Higher nickel content stabilises the austenitic (FCC) structure and reduces magnetic permeability. Ferritic stainless steels, which contain high chromium but very little nickel content, are strongly magnetic.
• Microstructure: Austenitic grades show low magnetic permeability, while ferritic & Martensitic grades are highly magnetic.
• Cold working: Mechanical deformation can increase magnetism in non-magnetic grades.
• Heat treatment: Proper annealing can restore the non-magnetic state in austenitic stainless steels.

How Magnetic Permeability is Measured

Magnetic permeability is measured using devices such as permeameters or magnetometers. The ratio of magnetic flux density to magnetic field strength gives the material’s relative permeability. For stainless steels, this value typically ranges from near 1.0 (non-magnetic) in austenitic grades to much higher in ferritic types. Measurements are often performed in accordance with ASTM A342 standards for accurate material classification.

Applications of Magnetic and Non-Magnetic Stainless Steels

• Magnetic stainless steels (ferritic or martensitic) are used in electric motors, transformer cores, solenoid valves, magnetic brackets, inductive components and fasteners where magnetic response is beneficial. 
• Non-magnetic stainless steels (austenitic) are preferred in medical devices, cryogenic equipment, and electronic housings, where magnetic interference must be avoided.