Which steels are magnetic?

When it comes to magnetism, steel presents a complicated picture. Since all steel contains iron, the element that makes a material ferromagnetic, it seems logical that all steel should be magnetic. However, the alloying elements and the crystal structure within the steel dramatically affect its magnetic properties.

The key to magnetism in steel comes down to its internal structure. Only steels that maintain a specific crystalline structure called ferrite or martensite will be magnetic. Steels that adopt the austenite structure are generally non-magnetic.

The Magnetic Steels (Ferromagnetic)

The vast majority of commercially produced steels are magnetic. These fall into the following main categories:

1. Carbon Steel and Low-Alloy Steel

These are the most common and magnetically strongest steels. They consist primarily of iron and a small amount of carbon, which creates the ferrite or martensite crystalline structure at room temperature.

Carbon Steel: From low-carbon (mild steel) used in construction to high-carbon steel used in tools, virtually all plain carbon steels are strongly attracted to a magnet steel and can often be permanently magnetized.

Low-Alloy Steel: Steels that contain small additions of alloying elements (like manganese or silicon) for improved strength or hardness remain strongly magnetic because their base crystal structure remains ferritic.

2. Ferritic and Martensitic Stainless Steels

While many people assume stainless steel is non-magnetic, two common families of stainless steel are definitely magnetic:

Ferritic Stainless Steels (e.g., 430 grade): These steels are primarily alloyed with chromium (10.5% to 18%) and contain very little carbon or nickel. They maintain the magnetic ferrite structure at all temperatures and are strongly magnetic.

Martensitic Stainless Steels (e.g., 410 grade): These steels are formulated to be hardenable (like cutlery steel). Their heat-treated structure, called martensite, is highly magnetic.

These magnetic stainless steels are often used where corrosion resistance is needed alongside magnetic response, such as in certain pumps, valves, and precision tools.

The Non-Magnetic Steel (Non-Ferromagnetic)

The most notable exception to the rule is a specific type of stainless steel known for its anti-magnetic properties:

Austenitic Stainless Steel (The Non-Magnetic Exception)

This family of steel, which includes the most common grades like 304 (kitchen sinks) and 316 (marine grade), is generally non-magnetic.

High Nickel Content: The key ingredient here is a high percentage of nickel (usually 8% or more) and chromium. This specific combination of alloys stabilizes the steel's crystalline structure into an austenite phase, even at room temperature.

Austenite Structure: The austenite crystal structure is face-centered cubic and is non-magnetic. Therefore, standard 300 series stainless steel is often used in applications where a non-magnetic material is required, such as in MRI machines or certain electronic enclosures.

An Important Note: Even austenitic stainless steel can become slightly magnetic if it is mechanically worked (like being bent, drilled, or cold-rolled). This stress can cause the austenite structure to partially transform into the magnetic martensite structure near the surface, resulting in a slight magnetic attraction.

In summary, if you can stick a magnet to it, it’s likely carbon steel, low-alloy steel, or a ferritic/martensitic stainless steel. If your steel object (like a high-quality kitchen utensil) shows no attraction, it’s most likely an austenitic stainless steel.