A steel washer may or may not be magnetic, depending entirely on the specific type of steel used in its manufacture. Steel alloys are engineered for various properties, such as corrosion resistance or strength, which directly impact their response to a magnetic field. The magnetic nature depends on the steel’s precise chemical composition and the internal arrangement of its atoms.
The Mechanism of Ferromagnetism in Iron
The magnetic properties of steel originate from its primary component, iron (Fe), a ferromagnetic material. Ferromagnetism is a phenomenon where a material is strongly attracted to a magnetic field due to the alignment of electron spins within its atomic structure. Iron atoms have unpaired electrons that act like tiny magnets, or magnetic moments.
In unmagnetized steel, these magnetic moments are randomly oriented in microscopic regions called domains, resulting in no net magnetic field. When an external magnetic field is applied, the domains align themselves parallel to the field, creating a strong magnetic attraction. The material’s crystal structure must be suitable for this domain alignment to occur. Alloying iron with other elements changes this crystal structure, which can either maintain or disrupt the magnetic alignment.
Carbon Steel Versus Stainless Steel Washers
The most common types of steel washers are carbon steel and stainless steel, and their magnetic properties differ significantly. Standard carbon steel washers are highly magnetic because they are composed primarily of iron and maintain a crystal structure known as ferrite. This ferritic structure is highly conducive to the alignment of magnetic domains, resulting in a strong attraction to magnets. Carbon steel is often used for low-cost, general-purpose fasteners.
Stainless steel is an alloy that includes chromium for corrosion resistance and often elements like nickel. The addition of nickel in common grades, such as the 300 series, changes the crystal structure to austenite. This austenitic structure prevents the stable alignment of the magnetic domains, making the material non-magnetic in its standard state. Most stainless steel washers encountered in general applications will therefore show little to no magnetic attraction.
Why Some Stainless Steel Alloys Are Magnetic
Despite the general rule that austenitic stainless steel is non-magnetic, not all stainless steel washers behave the same way. Certain families of stainless steel, specifically the ferritic and martensitic types, are strongly magnetic. These alloys are designed with a high chromium content but contain little to no nickel, allowing them to retain the magnetic ferritic or martensitic crystal structure. This makes them magnetic, similar to carbon steel, even though they possess the corrosion resistance characteristic of stainless steel.
The magnetic nature of stainless steel can also be unintentionally induced through mechanical processes. Non-magnetic austenitic stainless steel can become slightly magnetic when heavily deformed, such as during the manufacturing process of shaping a washer. This process, called cold working, can cause some austenite to transform into a magnetic phase called martensite. The induced magnetism is typically weak but can be noticeable, especially at the edges.
Practical Material Identification
The easiest way to determine the material of a steel washer is to perform a simple magnetic test. If a small magnet strongly attracts the washer, the material is likely a highly magnetic type of steel, such as carbon steel, ferritic stainless steel, or martensitic stainless steel. A strong pull indicates that the material possesses a crystal structure that supports ferromagnetism.
If the magnet does not stick or only shows a very weak interaction, the washer is almost certainly made of non-magnetic austenitic stainless steel. Visual inspection can also provide clues. Carbon steel washers are commonly coated with a finish like zinc or cadmium for corrosion protection, giving them a silver or yellowish appearance. Conversely, many stainless steel washers have a bright, unfinished appearance, or may sometimes be marked with codes that indicate their specific alloy type.