Can neodymium magnets attract stainless steel?

In daily life, we often hear that stainless steel is a material that cannot be adsorbed by magnets. However, is this statement completely correct?

The first thing we need to understand is the basic properties of stainless steel and neodymium magnets and their interactions. Stainless steel is an alloy typically composed of elements such as iron, chromium, nickel, molybdenum, and titanium. Neodymium magnets, the strongest synthetic magnets currently available, have a strong adsorption force for ferromagnetic metals such as iron, nickel, and cobalt. However, this does not mean that all types of stainless steel can be attracted by magnets. In fact, whether stainless steel can be attracted by magnets depends mainly on its composition and internal structure.

Main types of stainless steel

Stainless steel is mainly divided into three categories. These different types of stainless steel exhibit different magnetic properties and physical characteristics due to their internal element content and crystal structure.

stainless steel

1. Ferritic stainless steel: This type of stainless steel has a chromium content between 10% and 30%, no nickel, and a carbon content of less than 0.12%. Some ferritic grades also contain small amounts of molybdenum, titanium, or niobium.

Ferritic stainless steel has a body-centered cubic crystal structure, with eight iron atoms at the corners of the cube and one atom at the center of the cube.  The eight iron atoms at the corners are closely aligned with the central atom. The interaction between iron atoms can lead to an ordered arrangement of electron spins.

This arrangement allows the magnetic moments between iron atoms to interact and form magnetic domains. (Magnetic domains are small regions within a material with the same magnetization direction, and their existence enables the material as a whole to exhibit ferromagnetism.) Under the action of an external magnetic field, the magnetic moments of these magnetic domains align along the direction of the magnetic field, causing the ferritic stainless steel to be adsorbed on the magnet.

Ordinary ferritic stainless steel has the disadvantages of high brittleness at low and room temperatures, high tendency to intergranular corrosion, and poor welding performance. This has resulted in limited application and low production. It is usually used in applications where corrosion resistance is not a high priority, such as building materials and automotive decoration.

These deficiencies of ordinary ferritic stainless steel are related to the purity of steel, especially the high content of interstitial elements such as carbon and nitrogen in steel. After the 1970s, due to the development of smelting technology, especially vacuum metallurgy and secondary refining processes, the performance of ferritic stainless steel has been significantly improved.

2.Austenitic stainless steel: Austenitic stainless steel refers to stainless steel with austenite structure at room temperature. When the steel contains about 18% chromium (Cr), 8% to 25% nickel (Ni), and 0.1% carbon (C), it has a stable austenite structure.

Austenitic stainless steel is the main material of food-grade 304 and 316 stainless steel, and it is the type that we come into contact with most in our daily lives. Due to its excellent corrosion resistance, good plasticity, and excellent welding performance, austenitic stainless steel is widely used in various fields, such as tableware, kitchenware, medical equipment, and chemical equipment.

Ferrite transforms into austenite at 912°C to 1394°C, changing from a body-centered cubic structure to a face-centered cubic structure. This structure enables it to exhibit excellent corrosion resistance and mechanical properties.

However, at the same time, the face-centered cubic structure makes the magnetic moment interaction between iron atoms weaker, making it difficult to form magnetic domains. Therefore, austenitic stainless steels usually exhibit paramagnetism or non-magnetism. (Magnets will have a weak attraction on paramagnetic materials, but this attraction is only one hundred thousandth of that of ferromagnetic materials and can only be measured with precision instruments.) The interaction between iron atoms in austenitic stainless steels and other alloy element atoms also inhibits the generation of magnetic properties.

However, when the temperature drops, austenite will turn back into ferrite. We can add a sufficient amount of chemical elements that can stabilize and expand the austenite phase region, such as nickel (Ni) and manganese (Mn), to the steel, thus achieving a stable austenite structure after cooling. And with the increase of nickel content, the residual ferrite will gradually disappear.

In addition, this characteristic makes austenitic stainless steel advantageous in applications that require avoidance of magnetic interference. For example, in some medical equipment and aerospace equipment, the sensitivity to magnetic fields is very high. Using austenitic stainless steel can reduce problems caused by magnetic interference and ensure the normal operation of the equipment.

3.Martensitic stainless steel: Martensitic stainless steel contains 12-18% chromium.  This type of stainless steel has high strength, but its plasticity and weldability are relatively low. Similar to ferritic stainless steel, it may also be added with some other elements to improve mechanical properties and corrosion resistance.

In high temperature conditions, steel forms austenite, which has a face-centered cubic structure. Then, through a rapid cooling process called quenching, the austenite structure gradually transforms into a body-centered cubic structure called martensite, which increases the hardness and strength of the steel. This transformation process is influenced by various factors such as quenching speed, chemical composition, and shape of the steel.

Because martensitic stainless steel also has a body-centered cubic structure, it also has ferromagnetism and can be adsorbed by a magnet.

Martensitic stainless steel has good corrosion resistance in oxidizing media, but its corrosion resistance in sulfuric acid and hydrochloric acid is very low. It is often used to make components that require high strength but do not require high corrosion resistance, such as aircraft skins.

Summary

To sum up, whether stainless steel can be attracted by a magnet mainly depends on its type. Ferritic stainless steel and martensitic stainless steel can be attracted by a magnet due to their ferromagnetic internal structure. However, austenitic stainless steel usually has no magnetic properties and cannot be attracted by a magnet.

For other types of stainless steel, such as austenitic-ferritic duplex stainless steel and precipitation-hardened stainless steel, their magnetic properties may vary depending on the specific composition and processing techniques.

Austenitic 304 stainless steel, which is commonly used as a pipe material for home decoration, is generally considered to be non-magnetic. However, weak magnetic properties may also occur due to composition fluctuations caused by smelting or different processing conditions, which cannot be considered as counterfeit or unqualified.

In addition, after cold processing, the structure of austenitic 304 stainless steel will also transform into martensite. The greater the degree of deformation during cold processing, the more martensite transformation, and the greater the magnetic properties of the steel. On the other hand, lower-quality 200 series stainless steel is likely to be non-magnetic.

Contact us:Contact | HQ Magnet (hq-magnet.com)

Learn more:Magnet Knowledge | Can neodymium magnets attract stainless steel? (youtube.com)