Do Magnets Stick to Stainless Steel?

A common conception about magnets is that they stick to metallic substances, objects containing a measure of iron. This means that steel, which also contains iron, is considered magnetic and capable of holding magnets. However, this is not often true for all types of steel; stainless steel, which, though containing iron, comprises other steel alloys of different compositions, which could alter its magnetism. Therefore, the age-long question about the ability of magnets to stick to steel is related to the composition of the stainless steel and the strength of the magnets used. Read on to learn more about the magnetism of stainless steel.

What Makes Steel Stainless?

Typical steel comprises a large percentage of iron, alloyed with a little portion of carbon to improve its properties. Due to the dominant presence of iron in steel, it easily combines with atmospheric oxygen to form compounds that cause rust on the surface of the steel. Stainless steel was invented to enhance the properties of steel and alleviate its disadvantages.

Stainless steel is an improved form of steel in which chromium (Cr) and other metallic elements like Nickel (Ni), Manganese (Mn), and Molybdenum (Mo) are alloyed with steel to make it resistant to corrosion and oxidation. The chromium in stainless steel reacts with atmospheric oxygen to form thin, hard layers across the surface of the steel, providing a barrier to the contact of oxygen with the iron.

The adjective ‘stainless,’ used to describe this kind of steel, fits its nature in that it cannot be stained by rust. Its surface has been fortified from the impact of rust. This stainless ability is achieved by the presence of more than 10.5% chromium in the stainless steel; if below this quota, it is not considered to be stainless. Though not as strong as mild steel, stainless steel is mostly useful in producing some industrial equipment and kitchen utensils that are in constant contact with water and air. Even after long use, stainless steel could still maintain a glistening, ‘stainless’ appearance due to the presence of chromium.

What Makes Stainless Steel Magnetic?

Since most people are unsure about the magnetic properties of stainless steel, some have assumed that it is magnetic because it contains iron. But, stainless steel is only magnetic if the crystal structure of its alloy, which contains iron, is arranged in the structure of a martensitic or ferritic structure. Stainless steels with austenitic crystal structures are not magnetic due to the presence of a Nickel compound in the structure, which alters the physical properties of the stainless steel and tones down its magnetic properties.

The variation in the composition of iron in most stainless steels influences their magnetism. The difference in this composition gives rise to several grades and types of stainless steel.

Stainless Steel Grades and Types.

Stainless steel can be categorized into three broad types, depending on the crystalline structure of the iron in their alloy. They are austenitic, ferritic, and martensitic stainless steel.

Austenitic Stainless Steel

This type of stainless steel is achieved by adding austenite stabilizing agents like nickel (6% – 22%) to chromium steel alloy (16% – 26%) and iron to produce a non-magnetic substance that cannot be hardened by heat treatment. Through special thermal treatment and work-hardening, they can be made partially magnetic. They make up the 300 series of stainless steel.

Ferritic Stainless Steels

This is considered straight chromium, a non-hardenable steel alloy containing 10.5% to 30% chromium and low carbon content of less than 0.20%. Its molecular structure comprises microstructures known as ferrites, which provide a body-centered-cubic grain structure responsible for its magnetic nature. Its corrosion resistance is achieved by forming chromium-oxide films that protect the steel surface from the impact of atmospheric oxygen.

Martensitic Stainless Steel

It is a chromium-iron alloy steel type with a relatively high carbon content of 1% and 11.5% to 18% of chromium content. Due to its chemical composition, it can be hardened and strengthened through heat treatment; this gives it an advantage over others. It makes up most kitchen utensils, surgical equipment, and bearings. Martensitic steels can be formed into different shapes without breaking with controlled carbon content. They form better shapes with increased carbon content. Martensitic steels are magnetic and comprise many 400 series of stainless steel.

Other types of stainless steel include duplex and precipitation-hardening stainless steel.

The grade of stainless steel is a long list that this article cannot cover. A few prevalent grades are the 300 series, the 304 stainless steel and 316 stainless steel being the most common, and the 400 series, the 409 stainless steel and 430 stainless steel are more common.

300 Series Stainless Steel

The stainless steels in this series have a large amount of chromium content(about 18 to 30%) and nickel content (about 6 to 20%), providing impressive corrosion resistance, strength, and versatile properties. The 300 series are usually classified as austenitic; they are solids that consist of face-centered cubic crystals specifically used in construction, aerospace, and automotive industries.

304 Stainless Steel

It is an austenitic stainless steel type that contains 18% of chromium and 8% nickel. It makes up about 50% of the stainless steel that exists in the world. It is appreciated for its aesthetic properties and the ease at which it can be molded into different shapes. It is resistant to pitting corrosion in water and is considered to be very sensitive at room temperature. It is commonly used in the food industry.

316 Stainless Steel

It also exists in another form, 316L, with a lower carbon content than 316—nothing greater than 0.03% carbon. It is the second most common type of stainless steel, containing a primary alloying constituent of chromium (16-18%), nickel (10-12%), and other supporting components, which enhances its properties. The presence of molybdenum (2-3%) in 316 and 316L stainless steel increases their resistance to corrosion. Depending on their use, both can be used based on their unique properties.

400 Series Stainless Steel

The 400 series contains 11% chromium and more carbon content than the 300 series. It is more ferromagnetic and has a martensitic crystal structure, which is usually less resistant to corrosion than the austenitic types. It is usually very strong and durable. It contains a body-centered-tetragonal microstructure, having a base composition of iron, chromium, and carbon.

409 Stainless Steel

This contains approximately 11% chromium, 0.03% carbon, 0.5% Nickel, and other supporting components. It has a ferritic crystal structure that provides heat and high corrosion resistance compared to carbon steel. When stabilized with titanium, it forms another grade of stainless steel suitable for producing automotive exhaust systems and can also be used in other applications.

430 Stainless Steel

This type of steel contains a ferritic crystal structure comprising 16 -18% chromium, <0.12% carbon, <1% manganese, and a few other components, excluding nickel and molybdenum. Being a low-carbon and non-hardenable steel, it does not respond to heat treatment. It is resistant to stress corrosion and acid attacks. It is often used in automotive industries, industrial roofing buildings, heat resistance, and more.

Can One Test for the Magnetism of Stainless Steel?

It is possible to know if the stainless steel is magnetic or non-magnetic by a simple, physical test with a magnet. When a magnet is placed on stainless steel and falls off, instead of sticking firmly to it, it is considered non-magnetic. This type of stainless steel contains an austenitic crystal structure and forms the 300 series, which comprises the bulk of stainless steel we have today. The presence of chromium in the alloy annuls the magnetic properties of iron and nickel, making it non-magnetic.

Martensitic and ferritic stainless steels do not contain any amount of austenite and exhibit magnetic properties. They attract magnets and are classified as ferromagnetic. The 400 series of stainless steels fall into this category. Duplex stainless steel, a mixture of austenite and ferrite, is also magnetic and exhibits higher mechanical strength than the rest.

It is important to test the magnetism of the stainless steel before application to know its strength of performance and how to use them appropriately.

Conclusion

The alloying of steel with other elements to enhance its effectiveness is one of the innovations the world will be grateful to have. The use of stainless steel across several industrial processes confirms that these steels are vital to industrialization. No one buys a kitchen utensil or surgical equipment and worries about it rusting, on exposure, over a while. With stainless steel, this concern is alleviated. With time, there will be more improvements in stainless steel production as manufacturers and scientists will continue to discover more ways to alloy elements for better products.