When the workpiece is electromagnetized by DC, since the current value does not change over time, it is a constant value, and the current density is the same. Fig. 1 shows a direct current electromagnetization mechanism, and the B-H curve of Fig. 1 is an initial magnetization curve of the workpiece. When Directive I = I 0 , the magnetic field intensity corresponding to the workpiece is h 0. If continuous magnetization is used, the magnetic induction intensity at the near surface of the workpiece is B 0 . If the removal magnetization is used, the remaining magnetic induction strength at the near surface of the workpiece is B R.
The advantage of DC magnetization is that the penetration depth is larger than other current magnetization magnetic fields. For the skin of the cast steel, DC electromagnetization is used, and the detection depth is up to the dry test. At the same time, the workpiece after DC magnetization is difficult, and if you want to completely retreat, you need to use ultra-low frequency dentinity equipment.
Direct electricity is also a peak and valid value.
AC power peak, effective value
AC power is a size and direction with time periodic changes Current, and sinusoidal alternating current is an alternating current that makes a sinusoidal or cosine change, its mathematical expression is
Total flow and AC electrical respectively Through the same resistance, if the heat generated by the two, the amount of heat generated by the two is equal, the size of this direct current is defined as the effective value of the alternating current, and
, the period of the T-AC powerThe visible valid value is the top square of the average value of the AC instantaneous value square, Therefore, the valid value is also called the root value, and the formula (2) is substituted (1) to obtain
AC magnetization characteristics
When the electromagnetic chemical workpiece, since the size and direction of the current are changed over time, the magnetic properties in the workpiece are varied along the magnetic hysteresis (as shown in FIG. 2). The peak value of the alternating current is
When using a continuous method, the magnetic induction intensity of the workpiece surface is
visible, with AC electromagmatization and use removal method, there is an unstable phenomenon in the workpiece, so it is possible to cause a missed inspection, in order to overcome the unstability of the remanence, A power-down phase controller is equipped in an AC magnetic machine. Since the direction of the alternating current is constantly changing, the magnetic field direction it produces is constantly changed in a straight direction, which can agitate the magnetic powder to help the magnetic powder migration, thereby increasing the inspection sensitivity. In addition, an alternating current can be magnetized and composite magnetization, as both need to alternate magnetic fields.
has DC permeability and alternating pulsation with single-phase semi-wave rectification, which can detect defects under the surface of the workpiece, and Stir dry magnetic powder, which is conducive to the migration of magnetic powder. Therefore, it is often combined with dry methods to check the defects under the surface. In addition, the one-way half-wave rectifier is electromagmate, whenever the power is turned off, the residual magnet is not zero, and there is always a stable residual magnetic magnetic on the test piece.
The magnetization characteristics of single-phase full-wave rectification are similar to the half-wave rectification, and the DC component is doubled than half wave, and the lowest harmonic is a second harmonic.
For three-phase half-wave rectification, since the DC component is increased, the harmonic amplitude is reduced, so the penetration depth is increased, while the pulsation is reduced.
three-phase rectification, especially the three-phase full wave rectification is substantially similar to DC electrical, having a large penetration depth and a small pulpity. Therefore, it is most suitable for inspection of cast steel parts, ductile iron blanks, and welding members to find the air holes or inclusions under the surface layer. The three-phase full-wave rectifier has the disadvantage of demagnetization.
In order to correctly select magnetization current, the magnetic characteristic parameters caused by material components and tissue states, we have hundreds of magnetic special curves The map has been analyzed and summarized, and common steel materials and flawats are divided into four types of magnetization current.
The first class, the magnetic property is soft.
They include carbon steel containing less than 0.4% in the supply state, containing less than 0.3% of the amount of 0.3% carbon, high carbon steel (tissue as a ball-shaped pearlescent) under annealing state. body). Such steels are highly magnetic in the magnetic permeability and low elimination, and the loss is also subject to finding. The magnetization field intensity value is 1,600 ~ 3,200, and the current used is 5 to 10 times that of the workpiece diameter, which can show common defects.
Sequence, the magnetic is hard.
Magnetic material is a general quenching and performing a carbon steel, medium and low alloy steel, high-glutinous steel supply state, half martensite and martensite Fire and normal heating back heat tempering state and their drawing state. The magnetic properties of such materials are hard than that of the first two categories, so the magnetization field strength value is 4800 ~ 6400, and the circumferential magnetization current should be 15 to 20 times the weapon diameter. This type of steel magnetic energy is generally It is large, and the removal method can be used.
Third category, the magnetic resistance is hard.
including the alloy steel quenching post-tempered temperature below 300 ° C and the hardness of the tool steel is higher than that of HRC55 and martensite stainless steel hardness greater than HRC40. This type of workpiece is hard to magnetize, and the magnetization is difficult. It is usually selected from 6400 to 8,000 security / meters, and the circumferential magnetization current is typically selected from 20 to 25 times of the diameter of the workpiece. It is also possible to perform removal detection. Special attention should be particularly paying special attention to the deterneration.
fourth class, soft in magnetic properties.
They include the supply or the carbon steel, carbon tool steel and some high alloy steel, carbon tool steel and some high alloy steel (hardness value less than HRC23) under the supply or normal state. ) At the same time, it also includes a tempering that such steel is 450 ° after quenching. Such steel magnetic permeability is relatively low, and the coercive force has improved. Continuous method is usually used. Some magnetic energy can also use residual magnetic flaws. When the continuous method is impaired, it is selected from 3200 to 4800 m., and the current is selected from 10 to 15 times the workpiece diameter. The above magnetization current selection is a standard specification. Strict norms and relaxation specification can be taken to one level.
Selection of longitudinal magnetization can be referred to the circumference of the magnetization magnetic field. The magnetization field is not only related to the aforementioned factors, but also related to the shape of the workpiece and the magnetization coil.