Steel, an alloy of iron and carbon, is often associated with strong magnetic attraction. While a magnet readily sticks to many common steel items, the simple answer that magnets work on steel is not universally correct. The magnetic response depends highly on the steel’s precise chemical makeup and internal atomic structure. The behavior is a complex interplay between its main ingredient, iron, and the other elements added to the mix. These compositional changes can transform a strongly magnetic material into one that is nearly non-magnetic, a distinction with wide-ranging industrial importance.
The Scientific Reason Steel Can Be Magnetic
The fundamental reason steel can be magnetic lies in its primary component: iron. Iron is one of the few elements categorized as ferromagnetic, meaning it exhibits strong magnetic effects. This powerful attraction is due to iron’s atomic structure, where unpaired electrons act like tiny, intrinsic magnets that spontaneously align with one another.
Within unmagnetized steel, these aligned atoms form microscopic regions called magnetic domains. The magnetization direction within each domain is uniform, but the domains are randomly oriented, canceling out any net external magnetic field. When an external magnet is brought near, the domains reorient themselves to align with the external magnetic field. This alignment strengthens the overall magnetic field and results in visible attraction.
This induced magnetization is temporary in “soft” magnetic steels, as the domains return to their random state when the external field is removed. Other types of steel, known as “hard” magnetic steels, resist this randomization, allowing them to retain a permanent magnetic field. The ability of steel to be strongly attracted to a magnet is directly tied to the presence and mobility of these iron-based magnetic domains.
How Alloying Elements Change Magnetic Response
The variable magnetic response in different types of steel is primarily due to the inclusion of specific alloying elements. Adding other elements changes the crystalline structure of the iron lattice, which dictates whether ferromagnetism can occur. Iron naturally exists in a magnetic structure called ferrite, which has a body-centered cubic (BCC) atomic arrangement that allows electron spins to align easily.
However, elements like nickel and manganese are known as “austenite stabilizers” that fundamentally alter the crystal structure. When added in sufficient concentration, they force the steel into the austenite phase, which is a face-centered cubic (FCC) arrangement. This different atomic spacing and symmetry prevents the electron spins from aligning in a way that supports strong magnetic attraction.
Austenitic stainless steels, such as the common 300-series grades (e.g., 304 and 316), contain enough nickel and chromium to stabilize this non-magnetic structure at room temperature. Because their internal structure inhibits the formation of magnetic domains, these steels are considered non-magnetic. Consequently, while all steel contains iron, only the steel with a crystal structure that allows for electron spin alignment will be noticeably attracted to a magnet.
Magnetic Differences in Common Steel Types
The practical magnetic response of a steel is categorized based on its crystal structure. Carbon steel, which has high iron content and minimal alloying additions, retains the magnetic ferrite structure and is strongly magnetic. Similarly, ferritic stainless steels, such as Type 430, are also highly magnetic because they maintain the body-centered cubic structure, despite containing significant amounts of chromium.
Martensitic stainless steels, including grades like Type 410, are strongly magnetic due to their body-centered tetragonal crystal structure. These steels are often used where both high strength and magnetic attraction are necessary, such as in certain tools and instruments. In contrast, austenitic stainless steels (e.g., 304 and 316) are typically non-magnetic in their annealed condition because of their face-centered cubic structure.
Induced Magnetism in Austenitic Steel
Even these non-magnetic austenitic grades can develop some degree of magnetic attraction through mechanical processing. Severe plastic deformation, such as cold working or deep drawing, can induce a localized transformation of the non-magnetic austenite into the magnetic martensite phase. This structural change, particularly noticeable in heavily worked areas like wire or fasteners, can cause an otherwise non-magnetic piece of steel to become slightly responsive to a magnet.