Electromagnetic control method

MoMagnetic method

The magnetic method is a non-destructive testing method based on measuring the parameters of magnetic fields created in a controlled object by means of its magnetization. According to the method of obtaining primary information, the magnetic particle method is distinguished. It is based on the detection of stray magnetic fields above defects using a ferromagnetic powder or magnetic suspension as an indicator.

Magnetic control methods are based on the indication and analysis of stray magnetic fields that occur at the locations of defects or changes in the physical, mechanical and geometric characteristics of ferromagnetic products when exposed to a magnetic field. The magnetic flux, closing along a product placed in a magnetic field and having a defect, for example, in the form of a crack, is forced to go around an obstacle with reduced permeability. In this case, the lines of force go beyond the surface of the product (see fig.).

Where they go out and back into the product, magnetic poles appear. After removing the external magnetizing field, these poles establish their own magnetic field over the defect. In the practice of magnetic flaw detection, it is commonly called the stray flux field near the defect. There are several methods for recording stray fields above a defect.

The change in the magnetic field strength in the defective areas is recorded using a ferromagnetic powder (magnetic powder method of control), a magnetic tape (magnetographic method of control), introduced into the magnetic fields of the ferroprobe (ferroprobe control), etc. Electromagnetic (eddy current) non-destructive testing is based on registration changes in the interaction of the coil’s own electromagnetic field with the electromagnetic field of eddy currents induced by this coil in the controlled object.

In chemical engineering, magnetic and electromagnetic control methods are used for flaw detection, thickness measurement, structural and phase analysis of metals, determination of the presence and depth of stainless steel ICC.

Among the magnetic methods of control, the most widely used is magnetic particle flaw detection.

Magnetic particle control method.

This method makes it possible to detect fine surface and subsurface defects: hairlines, cracks, delaminations, flocks, folds, lack of penetration of butt welded joints, etc. Magnetic powders or suspensions serve as indicators of the stray field in the magnetic particle inspection method. The magnetic field generated by the defect is inhomogeneous and can be detected by ferromagnetic particles. A magnetic particle in an inhomogeneous magnetic field is affected by a force that tends to pull it into the places of the highest concentration of field lines and bring it closer to the defect.

The magnetic particle control method provides for the following technological operations: preparation of the product for control; product magnetization; applying a magnetic powder or suspension to the product; product inspection; sorting out; demagnetization

Electromagnetic control method

The electromagnetic control method is used to control parts made of electrically conductive materials. It allows you to determine the shape and size of the part, identify surface and deep cracks, voids, non-metallic inclusions, intercrystalline corrosion, etc. The essence of the method is to measure the degree of interaction of the electromagnetic field of eddy currents induced in the surface layers of the controlled part with the alternating electromagnetic field of the transducer coil . This method makes it possible to detect surface and subsurface defects with a depth of 0.1-0.2 mm and a length of more than 1 mm, located at a depth of up to 1 mm from the metal surface.
The laid-on electromagnetic converter is an excitation winding located in a housing with power supply through a cable. An inductive coil with a ferrite core is located at the bottom of the converter. To concentrate the magnetic flux in the control zone, to reduce wear when the transducer slides over the controlled surface, to fix a constant gap between the winding and the controlled surface when testing a part, a ferrite core is required.
Under the influence of the alternating electromagnetic field of the inductor, eddy currents are induced in the surface layer of the product, forming their own alternating electromagnetic field, which interacts with the excitation field. Since the material of the part and the distance of the transducer from the part are unchanged, some balance is established in the interaction of two electromagnetic fields.
When a crack or other defects is established in the controlled part, the intensity and nature of the distribution of the electromagnetic field of eddy currents are modified, which inevitably causes a change in the resulting electromagnetic field. The presence of a defect is recorded using the electrical circuit of the device. The indication may be:

  • turnout,
  • light,
  • sound,
  • digital,
  • on a cathode ray tube.

The following elements influence the occurrence of an electromagnetic field:

  • defect size,
  • the location of the defect
  • the nature of the defect
  • electrical conductivity and magnetic permeability of the material,
  • material structure,
  • frequency and current strength in the converter,
  • distance and relative position of the coil and the controlled part, etc.

This method has a number of advantages:

  • high resolution in detecting surface defects (especially fatigue cracks);
  • portability and autonomy of the equipment;
  • simplicity of converter design;
  • high productivity and simplicity of control methods;
  • the possibility of non-contact measurements through a layer of paint;
  • the possibility of automating control.

By appointment, electromagnetic converters are:

  • checkpoints,
  • overhead,
  • combined.

Flaw detectors with interchangeable transducers of various designs are used to inspect complex-shaped parts. It is necessary to take into account the shape of the controlled surface, the dimensions of the control zone, the radius of curvature, accessibility, etc. when choosing a transducer from among those included in the flaw detector kit. To control different zones, a very specific converter must be selected. The most typical control zones:

  • fillets,
  • flat surfaces,
  • surface areas of double curvature,
  • stiffening rib,
  • grooves,
  • areas around holes
  • cylindrical surfaces (holes, shafts, axles),
  • corner joints,
  • threaded connections, etc.

The diameter of the clamp-on transducer must meet the required sensitivity. The minimum length of a crack that can be registered by a flaw detector with a clamp-on transducer is equal to half the transducer diameter. To detect defects with a length less than the diameter of the transducer, the transducer should be moved in the direction of the defect with a step equal to half the diameter. To detect defects with a length greater than the diameter of the transducer, it should be moved perpendicular to the direction of the defect.
The travel speed should be no more than 20 mm/s. In the process of moving the invoice transducer is placed normal to the surface. The gap between the transducer and the surface of the part must be constant.

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