Washers are thin, disc-shaped plates used in mechanical assemblies to distribute the load of a threaded fastener. The question of whether a washer is magnetic depends entirely on the specific metallic composition of the material used to manufacture it. This determination is rooted in material science and the atomic structure of the alloy, making the answer variable across different types of hardware.
Material Science Determines Magnetism
Washers fall into two broad categories defined by their magnetic response: highly magnetic or non-magnetic. Carbon steel and low-grade steel washers, common for general-purpose applications, are strongly magnetic because they are rich in iron and feature a ferritic microstructure. The high concentration of iron atoms allows the material to exhibit a powerful attraction, classifying it as ferromagnetic.
Conversely, many non-ferrous washers, such as those made from copper, brass, or aluminum, are non-magnetic. These materials lack the necessary iron content and atomic structure to respond to a magnetic field. The most common types of stainless steel washers, specifically the 300 series like grades 304 and 316, are considered non-magnetic when manufactured in their typical annealed state.
Stainless steel introduces complexity, as certain grades and manufacturing processes can alter the magnetic outcome. The 400 series stainless steels, such as grade 430, contain a different crystal structure that is inherently magnetic. Additionally, cold-working or deforming typically non-magnetic 300-series stainless steel can induce slight magnetism. This mechanical stress causes a transformation in the metal’s internal structure, converting some of the non-magnetic austenite phase into the magnetic martensite phase.
The Physics of Magnetic Attraction
Magnetism is dictated by a material’s atomic arrangement and the behavior of its electrons. Ferromagnetism, the strong attraction seen in iron and carbon steel, occurs because the atoms have unpaired electrons whose spins align within microscopic regions called magnetic domains. When an external magnetic field is applied, these domains rotate and align, resulting in a powerful magnetic pull.
Non-magnetic materials operate under different physical principles. Austenitic stainless steels are non-magnetic because their face-centered cubic (FCC) crystal structure stabilizes the austenite phase, which prevents the magnetic domains from aligning even in the presence of a strong field. The addition of nickel in these alloys helps ensure this non-magnetic state.
Materials that are not ferromagnetic exhibit much weaker magnetic behaviors. Aluminum is classified as paramagnetic, meaning it is very weakly attracted to a strong magnetic field, though this is often imperceptible in daily use. Copper and brass are diamagnetic, meaning they are very weakly repelled by a magnetic field, a force only detectable with highly sensitive equipment.
Testing and Practical Applications
Determining the magnetic nature of an unknown washer is straightforward and can be done with a common permanent magnet. If the washer is carbon steel or 400-series stainless steel, it will be strongly attracted. If the washer is a non-ferrous metal like brass or a 300-series stainless steel, there will be little to no attraction.
This difference in magnetic response leads to specific uses across industries. Non-magnetic washers are necessary in environments like sensitive electronic equipment, medical devices such as MRI machines, or marine applications where corrosion resistance is the priority. Using non-magnetic materials prevents interference with electromagnetic fields and avoids the attraction of metallic debris.
Conversely, the magnetic nature of carbon steel washers is often an advantage in manufacturing and construction. It allows for easy sorting using magnetic separation equipment and simplifies installation when magnetic tools are used to hold the fastener. Common protective coatings applied to steel washers, such as zinc plating or galvanization, are extremely thin layers that do not change the core material’s magnetic properties. The magnetic response remains solely determined by the underlying steel base.