If a magnet sticks, then it’s not stainless steel. Truth or myth to be debunked? Let’s dive deeper into this topic and clarify whether the “magnet test” can actually be considered reliable.

The broader question we should ask is whether stainless steel is a magnetic material or not. The short answer is: it depends. First, we need to distinguish between different types of stainless steel and make a few considerations.

Types of stainless steel and magnetism

Magnetism is a natural phenomenon through which certain materials can exert an attractive force on other bodies made of specific metals, such as iron, nickel, or cobalt. This force works without direct contact, manifesting when the materials are close enough to each other. The attraction typically occurs at the ends—known as magnetic poles. The classic example is a magnet attracting a paperclip or nail. But what happens when the metal in question is stainless steel?

Stainless steels are primarily divided into three categories, each with distinct magnetic properties based on their chemical composition and crystal structure.

  • Ferritic stainless steels (400 Series, e.g., AISI 430):
    these steels contain a high percentage of chromium and have a body-centered cubic (BCC) crystal structure, which makes them inherently magnetic. They are often used in applications where magnetism is beneficial, such as in household appliances and architectural cladding.
  • Martensitic stainless steels (e.g., AISI 410, 420, 440, 431): these steels contain more carbon than the other families to increase hardness. Like ferritic steels, they also have a BCC crystal structure that makes them magnetic.
  • Austenitic stainless steels (300 Series, e.g., AISI 304 and 316): known for their excellent corrosion resistance, these steels contain high levels of chromium and nickel, which stabilize the austenitic phase even at room temperature. In the annealed (non-cold worked) state, they have a face-centered cubic (FCC) structure, which generally makes them non-magnetic. This structure does not allow for the alignment of electron spins needed to generate a magnetic field.

AISI 304 and AISI 316: are they truly non-magnetic?

It is commonly believed that stainless steels from the 300 series—especially the popular AISI 304 and AISI 316—are non-magnetic and therefore not attracted to a magnet. As we’ve seen, in their annealed state (i.e., not cold worked), they have a structure that makes them non-magnetic. However, using the magnet test to confirm whether a steel is austenitic can be misleading.

If stainless steel 304 or 316 undergoes cold working processes such as rolling, drawing, or forming, it can develop slight magnetism and respond to the magnet test. Cold deformation from mechanical processing leads to the formation of areas with martensitic or ferritic structure, which increases the material’s magnetic permeability.

It’s important to highlight that this is a surface-level phenomenon that does not alter the metal’s properties, especially its corrosion resistance. In summary, even high-quality austenitic steel may become magnetic due to mechanical processing. Therefore, we can conclude that the magnet test cannot be considered a reliable method for determining the quality or composition of stainless steel.

Mondeo purchases and uses stainless steel bars, tubes, and sheets only with material quality certificates (3.1 according to EN 10204) from its suppliers.

Also read: AISI 304 and AISI 316: how are they different from other stainless steel types