What is demagnetization?
Magnet manufacturers sometimes demagnetize magnetic materials, so what is demagnetization? What is the principle of demagnetization? What methods can be used for demagnetization? Below by NdFeB strong magnet manufacturers net small ribbon with you to understand.
1. Demagnetization definition:
Demagnetization is also called magnetic cleaning and degaussing. Demagnetization refers to the process that the magnet returns to the magnetic neutral state.
2. Principle of demagnetization:
Demagnetization is to put the workpiece in the alternating magnetic field, and produce the hysteresis loop. When the amplitude of the alternating magnetic field gradually decreases, the track of the hysteresis loop becomes smaller and smaller. When the magnetic field intensity decreases to zero, the residual magnetic br in the workpiece is close to zero. During demagnetization, the direction and magnitude of current and magnetic field must be changed simultaneously.
The difficulty of demagnetization depends on the coercivity of the material, but the high coercivity is not necessarily related to the high coercivity. Therefore, the magnetization intensity can not reflect the difficulty of demagnetization. The general method of demagnetization is to apply a magnetic field equal to or greater than the magnetization to the workpiece, and then continuously change the direction of the magnetic field and gradually reduce the magnetic field intensity to zero.
3. Introduction of demagnetization methods:
Demagnetization of mining industry: the application of general magnetic material industry is not much, so it is not repeated.
- (1) Magnetic cleaning is the process of dealing with the unstable components (interference factors) of rock magnetism by means of external magnetic field (constant or alternating) in paleomagnetic research, which is called magnetic cleaning.
- (2) Thermal magnetic cleaning is a process in which unstable components (interference factors) in rock magnetism are treated by heating or low temperature in paleomagnetic research, which is called thermal magnetic cleaning.
- (3) Chemical magnetic cleaning (CMC) is a process of chemical treatment of unstable components (interference factors) in the paleomagnetic study, which is called CMC.
Industrial demagnetization: the demagnetization of magnetic materials generally belongs to industrial demagnetization, and the industrial demagnetization method is also commonly used, which is mainly explained below.
- (1) Static demagnetization: add a magnetic field opposite to the original magnetization direction of the magnetic body. The strength of the counter magnetic field should ensure that when it is removed, the magnetic induction of the magnetic body will become zero. The resulting magnetic neutral state is called static magnetic neutral state.
- (2) Dynamic demagnetization: a sufficiently strong alternating magnetic field is applied to the magnetic body, and then the amplitude of the alternating magnetic field is gradually reduced to zero. The resulting magnetic neutral state is called dynamic magnetic neutral state.
The principle is to put the workpiece in alternating magnetic field and demagnetize by decreasing hysteresis loop. With the amplitude of alternating magnetic field gradually decay, the track of hysteresis loop is also smaller and smaller. When the magnetic field gradually decreases to zero, the residual magnetic field in the workpiece will be close to zero. The demagnetization principle is shown in the following figure. It can be seen that the direction and size of the current and magnetic field must be reversed and decayed simultaneously.
Demagnetization of AC:
A. Alternating current demagnetization pass method
For batch demagnetization of small and medium-sized workpieces, it is better to put the workpiece on demagnetizer with track and trawler. When demagnetization, place the workpiece on the trawler 30cm before the coil. When the coil is powered on, the workpiece will slowly pass through the coil along the track and be at least 1m away from the coil.
For heavy or large workpieces that cannot be demagnetized on demagnetizer, the coil can also be sheathed on the workpiece. When power is on, the coil can be slowly passed and away from the workpiece, and the power is cut off at least 1m away.
B. Attenuation method
Because of the continuous commutation of alternating current, the demagnetization can be performed by automatically decaying demagnetizer or voltage regulator to gradually reduce the current to zero, such as placing the workpiece in the coil, between the two magnetizing clamps clamped in the flaw detector, or by reducing the current to zero after contacting the workpiece with the contact of the rod contact
For the welding seams of large pressure equipment, the AC electromagnetic yoke can also be used for demagnetization. Connect the two poles of the yoke on both sides of the weld, turn on the power supply, and let the yoke move slowly along the weld. When the power is cut off 1m away from the weld, demagnetization is carried out.
The DC electromagnetic work piece is demagnetized by DC, and the DC commutation attenuation or ultra-low frequency current can be used to demagnetize automatically.
A. DC commutation attenuation demagnetization
By changing the direction of direct current (including three-phase full wave rectifier) and decreasing the current through the workpiece to zero, demagnetization is carried out. The times of current attenuation should be as many as possible (generally more than 30 times), and the amplitude of current attenuation should be as small as possible. If the amplitude of current attenuation is too large, demagnetization can not be achieved.
B. Automatic demagnetization of ultra low frequency current
The frequency of ultra-low frequency is usually 0.5 ~ 10Hz, which can be used to demagnetize the three-phase full wave rectification electromagnetic workpiece.
C. Demagnetization of heated workpiece
It is the most effective demagnetization method to raise the temperature of workpiece above Curie point by heating, but this method is neither economical nor practical.
(3) Thermal demagnetization: the magnetic body is heated above Curie temperature, and then cooled without external magnetic field. The resulting magnetic neutral state is called thermally induced magnetic neutral state. The sintered NdFeB can be roasted at 350 ℃ for 30 minutes to 1 hour by thermal demagnetization. Within the working temperature, the magnetic force of the magnet will decrease with the increase of temperature, but most of the magnetic force will recover after cooling. If the temperature reaches Curie temperature, the molecules in the magnet will move violently and demagnetize, which is irreversible.
No matter which of the above three ways is used to demagnetize the magnet, the internal structure of the magnet will be permanently changed. After demagnetization, the magnetic properties of the magnet can not be restored to the previous level.
4. Post demagnetization test:
The effect of demagnetization needs to be checked. Some simple methods can be used, such as pin, paper clip, compass, iron powder, collision detector, etc