Steel is a broad term for various iron alloys, so the question of whether it sticks to a magnet does not have a simple yes or no answer. While iron is highly magnetic, the addition of different elements and specific manufacturing processes profoundly changes the material’s behavior. Understanding the magnetic attraction of any steel requires looking closely at its internal composition and crystal structure.
The Short Answer: Steel’s Magnetic Behavior
Most types of steel, such as common carbon steel used in construction and tools, are strongly attracted to a magnet. This attraction occurs because steel is an alloy made primarily of iron, which has inherent magnetic properties. The high iron content in these common grades ensures they respond readily to an external magnetic field.
The magnetic response of steel is not universal; the specific grade and composition determine the strength of the attraction. Alloying steel with other metals allows engineers to fine-tune its physical properties for different applications. Sometimes, this alloying process deliberately eliminates the steel’s magnetic behavior, which is necessary for many specialized uses.
The Role of Iron and Ferromagnetism
The strong attraction between a magnet and certain materials is explained by ferromagnetism. Iron, cobalt, and nickel are among the few elements that exhibit this property. Ferromagnetism arises from the atomic structure of iron, where electrons align to create tiny, internal magnetic moments.
Within ferromagnetic steel, these magnetic moments are grouped into microscopic regions known as magnetic domains. Each domain acts like a miniature magnet with its own North and South pole. In an unmagnetized piece of steel, these domains are oriented randomly, canceling out the overall magnetic effect.
When an external magnet is brought near, its field causes the domains within the steel to rotate and align themselves. This alignment results in a net magnetic force strong enough to create a noticeable attraction. The steel effectively becomes an induced magnet, which is why it sticks to the source magnet.
Distinguishing Magnetic and Non-Magnetic Steels
The distinction between magnetic and non-magnetic steel lies in how alloying elements affect the iron’s crystal structure. Steel is iron mixed with carbon and other elements. The two primary crystal structures that determine magnetism are ferritic and austenitic.
Magnetic steels, such as carbon steel and 400-series stainless steels, possess a body-centered cubic crystal structure known as ferrite. This structure allows magnetic domains to easily align when exposed to a magnetic field, resulting in a strong attraction. Martensitic steels are another magnetic type that allows for this domain alignment.
In contrast, non-magnetic steels, most notably 300-series austenitic stainless steels (like Grade 304 used in kitchenware), contain significant amounts of nickel and chromium. Nickel stabilizes a different crystal structure known as the face-centered cubic structure, or austenite. This austenitic structure physically prevents the formation of stable magnetic domains, making the material non-ferromagnetic under normal conditions.
Even typically non-magnetic austenitic steels can develop a weak magnetic response if they undergo cold working, such as bending or stamping. Mechanical deformation can cause a localized transformation of the crystal structure from non-magnetic austenite into a slightly magnetic form called martensite. This explains why a magnet might stick to the corner of a stainless steel sink bowl but not the flat drainboard.
How Steel Can Be Temporarily Magnetized
Many magnetic steels can be turned into temporary magnets through induced magnetism. When steel is placed in a strong magnetic field, its internal domains align, and the steel becomes magnetized. If the steel has low coercivity (meaning it is magnetically soft), it will lose most of this induced magnetism almost immediately after the external field is removed.
However, steels with a higher carbon content are magnetically harder and can retain a portion of their magnetic alignment, known as residual magnetism. This allows the steel to act as a weak, temporary magnet even after the original magnet is taken away. This principle is used when a technician magnetizes a screwdriver tip by stroking it with a strong permanent magnet.
Steel can also be magnetized using electromagnetism, which involves wrapping a wire coil around the steel and running an electric current through it. This creates a powerful temporary magnet. When the current is turned off, the steel retains some degree of residual magnetism, the duration of which depends on the specific chemical composition and processing of the alloy.