Why is Stainless Steel Not Magnetic?

Many of us have an immediate association when we think about stainless steel — anything from kitchen appliances, medical instruments, components for renewable energy technologies, and building construction may come to mind. When you really think about it, chances are you’ll find something made from stainless steel nearly everywhere you look. 

While stainless steel is known for being corrosion resistant, strong, and aesthetically pleasing, one quality that oftentimes comes into question is its magnetism.  

Why Is Stainless Steel Not Magnetic?

You could also ask, is stainless steel magnetic? The truth of the matter is, some stainless steels are magnetic, while others are not. You see, stainless steel is conventionally thought of as a single type of material, but within metallurgy, stainless steel actually accounts for a group of metals with varying qualities and chemical compositions. In fact, it might be helpful to think of stainless steel as a kind of generic term based on the chemical composition of steel.

How does a steel become classified as stainless? 

Steel alloys composed with a minimum of 10.5% chromium fall into the stainless category. 

The chromium content attributes particular qualities to the steel, including exceptional corrosion resistance. This quality is what gives stainless its rust-free status. It also allows steel to repair itself after being scratched or damaged - unlike plated steels which often become scratched, with those scratches leading to the eventual corrosion of the steel. 

What Makes Something Magnetic? 

But back to magnetism. In the case of steel, whether or not it is magnetic comes down to the microstructure of the steel. Basic stainless steels have what’s known as a “ferritic” structure, which enables them to be magnetic. Remember the chromium content? It’s the addition of chromium that leads to the ferritic structure. This, plus the addition of carbon, hardens the steel and qualifies it as a martensitic steel. Stainless steel knives are typically martensitic.

Martensitic steel differs from the most common stainless steels, which are referred to as austenitic. In austenitic steel, there is a higher percentage off chromium, and nickel is also present. In terms of magnetism, it is the addition of nickel that renders the steel non-magnetic. 

Is Stainless Steel Magnetic or Not? 

As I’ve covered above, this is not an all-or-nothing answer. Some stainless steels are magnetic, and others are not. The defining factor of magnetism comes down to the the steel’s microstructure. Martensitic stainless steels (which have a ferritic microstructure) are magnetic. Austenitic stainless steels contain nickel and are non-magnetic.

It’s worth noting that during processing the permeability of austenitic steels can change. From the British Stainless Steel Association: 

For example, cold work and welding are liable to increase the amount of martensite and ferrite respectively in the steel. A familiar example is in a stainless steel sink where the flat drainer has little magnetic response whereas the pressed bowl has a higher response due to the formation of martensite particularly in the corners.

In practical terms, austenitic stainless steels are used for “non-magnetic” applications, for example magnetic resonance imaging (MRI). In these cases it is often necessary to agree to maximum magnetic permeability between customer and supplier. It can be as low as 1.004.

--BSSA 

Mead Metals Stocks Non-Magnetic Stainless Steel

Here at Mead Metals, we stock 300 series stainless steels, which are austenitic. 304 stainless, while non-magnetic when freshly formed, typically becomes magnetic after cold working. If a material’s magnetism is an important quality for your upcoming project, please connect with one of our reps who will help you determine the best material for your application.