Demagnetization is the process of eliminating the residual magnetic field in a metal object, returning it to a magnetically neutral state. Ferromagnetic materials like steel and iron often become inadvertently magnetized through common industrial processes, such as machining, grinding, or handling by magnetic lifting equipment. This residual magnetism, sometimes called remanence, causes problems like attracting metal shavings that interfere with precision work or causing welding arcs to wander (“arc blow”). Neutralizing this unwanted magnetic attraction ensures smooth operation and product quality.
Understanding Magnetism in Metals
The magnetic properties of metals like iron and steel originate within microscopic structures called magnetic domains. A magnetic domain is a small region where the magnetic moments of the atoms are aligned parallel to each other. In a demagnetized metal, these domains are oriented randomly, causing their magnetic fields to cancel out, resulting in no net external magnetism.
When the metal is exposed to an external magnetic field, the domains shift and rotate to align themselves with that field. This alignment creates a bulk magnetic field, and some alignment remains even after the external field is removed, resulting in residual magnetism. Demagnetization works by disrupting this uniform alignment, forcing the domains back into a chaotic, random orientation where their fields neutralize one another.
Utilizing Alternating Current Fields for Demagnetization
The most effective and widely used method for demagnetizing tools and components involves subjecting the object to a rapidly reversing and gradually decreasing magnetic field. This field is created using alternating current (AC) and is often referred to as degaussing. The alternating field constantly flips the direction of the object’s magnetic domains, while the decreasing strength ensures they do not settle into a new, aligned state.
Equipment and Process
Specialized equipment, such as commercial demagnetizers, degaussing coils, or demagnetizing tunnels, generate this necessary field. The object is first placed in a strong AC field, powerful enough to magnetically saturate the material in both directions. The success of the process depends on the subsequent, slow reduction of the field’s strength while the AC current is still flowing.
Application Methods
For smaller tools, a portable demagnetizer or coil is used, and the object is slowly pulled away from the field source. Moving the object away naturally weakens the magnetic field, allowing the domains to randomize as the field decays. Removing the object too quickly can cause the domains to “freeze” in a partially aligned state, retaining residual magnetism. For larger parts, the field strength is often electronically reduced to zero while the object remains stationary within the coil.
Thermal and Mechanical Demagnetization Methods
Thermal Demagnetization
Demagnetization can be achieved through thermal means, which involves heating the metal above its Curie temperature. This is the point at which ferromagnetic materials lose their permanent magnetic properties, typically around 770°C (1420°F) for common industrial steel. Above this temperature, the vibration of the atomic structure completely randomizes the magnetic domains.
When the object is cooled slowly in a magnetic-field-free environment, it returns to a neutral state. While highly effective, this method is often impractical for finished components because the high heat can damage surface finishes, affect the material’s temper, or cause structural warping.
Mechanical Demagnetization
Mechanical demagnetization relies on physical shock to disrupt the alignment of the magnetic domains. Repeated impacts, such as striking the object with a hammer, provide the necessary force to overcome the internal resistance that keeps the domains aligned. This method is imprecise and is reserved for robust items, as impacts can easily damage precision tools or components. Furthermore, the resulting demagnetization is often temporary or incomplete compared to AC field methods.
Testing for Residual Magnetism and Prevention
Testing Methods
After demagnetization, verifying its success is the final step. The simplest test involves using a small ferrous object, like a paperclip, to check for attraction to the surface. A more accurate method uses a compass, which will deflect if brought near a magnetized object. Specialized field-measuring devices, such as a magnetometer or a residual field meter, provide a quantitative reading of the magnetic flux density, often measured in Gauss or Amperes per centimeter, to meet industry standards.
Prevention
Preventing accidental remagnetization involves minimizing exposure to strong magnetic fields and physical shock. Tools should be stored away from permanent magnets, electric motors, and high-current electrical cables, which generate magnetic fields. Avoiding dropping or striking metal objects also helps prevent the magnetic domains from realigning. For sensitive items, using specialized magnetic shielding can help maintain the neutral state.