Steel is an alloy primarily composed of iron, a ferromagnetic material. The process of magnetizing steel involves aligning the internal structure of the metal to create a net magnetic field. Applying specific methods can turn a piece of steel into a temporary or even a permanent magnet. This allows for practical applications, from magnetizing a screwdriver tip to creating specialized electromagnetic tools.
How Steel Becomes Magnetic
The magnetism in steel originates from tiny regions called magnetic domains, which are microscopic areas where the magnetic fields of iron atoms are aligned. In unmagnetized steel, these domains are oriented randomly, causing their magnetic effects to cancel each other out.
When exposed to an external magnetic field, the domain walls shift, and the domains align, effectively aligning the overall magnetic structure. The composition of the steel determines its magnetic behavior, particularly its ability to retain this alignment. Hard steel alloys, like certain tool steels, have high coercivity, meaning they strongly resist demagnetization and are used to make permanent magnets. Softer steel has low coercivity and loses its magnetism quickly, making it suitable for temporary magnets like electromagnets.
Magnetizing Steel Through Contact
The simplest method for magnetizing steel is the “stroking” method, which uses an existing permanent magnet to physically align the internal domains. This process requires a strong magnet, such as a rare-earth magnet, and the steel object.
Stroke the steel object repeatedly with one pole of the magnet, moving only in a single direction from one end to the other. Lift the magnet high above the steel before beginning the next stroke, ensuring the magnetic field is applied consistently. This action provides the energy for the magnetic domains to align with the external field.
This method creates a weak, temporary magnet, effective for magnetizing a needle or a screwdriver tip. The strength of the resulting magnet relates directly to the number of strokes performed and the strength of the original magnet used. The end of the steel where the stroking finishes will take on the magnetic pole opposite to the one used for rubbing.
Creating Stronger Magnets with Electricity
A stronger magnet can be created using the solenoid technique, which utilizes an electrical current to generate a powerful magnetic field. This process involves wrapping insulated copper wire tightly around the steel object to form a coil. Running a direct current (DC) through the coil creates a uniform and intense magnetic field that penetrates the steel core.
The strength of the magnetic field depends on two main factors: the magnitude of the current and the number of turns in the wire coil. Increasing the current or adding more wraps of wire directly increases the resulting magnetic output. This controlled electrical field achieves a greater degree of domain alignment compared to simple contact.
The electrical method allows for the creation of either temporary or permanent magnets, depending on the type of steel used. Since steel has high retentivity, it will become a strong permanent magnet once the current is switched off.
Retaining and Removing Magnetism
Once steel has been magnetized, its ability to retain that charge depends on its hardness and composition. Hard steel alloys have high coercivity, meaning they strongly resist being randomized by external forces. Even permanent magnets can lose strength over time or if subjected to certain conditions.
The most common causes of demagnetization are physical impact and heat. Dropping or sharply striking the steel can physically disrupt the alignment of the magnetic domains, causing them to randomize. Heating the steel above its Curie point—approximately 770°C (1420°F) for iron—will completely destroy the magnetic properties as intense thermal energy overpowers the magnetic forces holding the domains in place.
A less destructive method for demagnetization involves using an alternating current (AC) field. By placing the magnetized steel within a coil connected to an AC source and slowly moving the steel away from the coil, the constantly reversing magnetic field gradually randomizes the magnetic domains. This technique is highly effective for returning the material to a nearly unmagnetized state without changing its physical structure.