Magnetic particle inspection

Introduction

Magnetic particle inspection uses the interaction between the leakage magnetic field at the workpiece defect and the magnetic powder. It uses the difference between the magnetic permeability of the surface and near-surface defects (such as cracks, slag inclusions, hairlines, etc.) of steel products and the permeability of steel. After magnetization, The magnetic field at the discontinuity of the material will be distorted, and a leakage magnetic field will be generated on the surface of the workpiece were part of the magnetic flux leaks, thereby attracting the magnetic powder to form the magnetic powder accumulation at the defect – magnetic trace, which shows the position and shape of the defect under appropriate lighting conditions. , to observe and interpret the accumulation of these magnetic particles, and realize magnetic particle inspection.

Technical principle

Magnetic particle inspection is a non-destructive testing method that detects defects on the surface or near the surface of ferromagnetic materials by the accumulation of magnetic particles in the leakage magnetic field near the defect. The workpiece made of magnetic materials such as steel is magnetized, and the magnetic leakage energy of the defected part is used to absorb the characteristics of magnetic powder, and the surface defects and near-surface defects of the detected object are displayed according to the distribution of magnetic powder. The flaw detection method is characterized by simplicity and intuitive display.

Magnetic particle inspection and inspection methods using Hall elements and magnetic-sensitive semiconductor elements, magnetic recording inspection methods using magnetic tapes, and coil-induced electromotive force inspection methods are both magnetic inspection methods.

Main classification

Types of magnetic particle inspection:

According to the magnetization direction of the workpiece, it can be divided into circumferential magnetization, longitudinal magnetization, compound magnetization, and rotational magnetization.
According to the difference in the magnetizing current, it can be divided into the DC magnetization method, half-wave DC magnetization method, and AC magnetization method.
According to the different preparation of magnetic powder used in flaw detection, can be divided into the dry powder method and wet powder method.
According to the time of applying magnetic powder on the workpiece, can be divided into a continuous method and residual magnetism method.

How to operate

The object to be measured is placed in a strong magnetic field or magnetized with a large current. If there are defects (cracks, folds, inclusions, etc.) on or near the surface of the object, because they are non-ferromagnetic, the resistance to the passage of magnetic lines of force is very large. , the magnetic field lines will generate magnetic flux leakage near these defects. When magnetic powder with good magnetic permeability (usually magnetic iron oxide powder) is applied to the object, the leakage magnetic field near the defect will attract the magnetic powder and accumulate to form visible magnetic powder traces, thereby showing the defect.

Step 1: Pre-cleaning

The surface of all materials and test pieces should be free of grease and other impurities that may affect the normal distribution of magnetic powder and affect the density, characteristics, and clarity of magnetic powder deposits.

Step 2: flaw detection

Magnetic particle inspection shall be carried out to ensure satisfactory detection of any detrimental defect. The magnetic field lines are made to traverse as far as practicable any defects that may be present in the test piece.

Step 3: the choice of a flaw detection method

Wet method: The magnetic suspension should be applied to the test piece by the hose pouring or dipping method so that the entire surface to be tested is completely covered, and the magnetizing current should be maintained for 1/5~1/2 second, after which the magnetizing current should be cut off, and the magnetic suspension is applied by hose pouring or dipping.

Dry method. The magnetic powder should be directly sprayed or sprinkled on the inspected area, the excess magnetic powder should be removed, and the test piece should be shaken gently to obtain a relatively uniform magnetic powder distribution. Care should be taken to avoid using excessive amounts of magnetic powder, which will affect the effective display of defects.

Detect near-surface defects. When detecting near-surface defects, the wet powder continuous method should be used. Because the leakage flux caused by non-metallic inclusions is the smallest, the dry powder continuous method can be used when detecting near-surface defects in large castings or welded parts.

Circumferential magnetization. The center conductor method should be used when inspecting the inner surface of any cylindrical specimen; there should be a gap between the specimen and the center conductor to avoid direct contact with each other. When current is passed directly through the test piece, care should be taken to prevent burns at the electrical contact surfaces, and all contact surfaces should be clean.

Longitudinal magnetization. When magnetizing a test piece with a solenoid, to obtain sufficient magnetization, the test piece should be placed in an appropriate position within the solenoid. The size of the solenoid shall be sufficient to accommodate the specimen.

Step 4: Demagnetization.

The part is placed in a DC electromagnetic field, continuously changing the direction of the current and gradually reducing the current to zero. Large parts can be demagnetized in zones with moving electromagnets or solenoid coils.

Step 5: Post-cleaning.

After inspection and demagnetization, all magnetic particles on the test piece should be cleaned; care should be taken to completely remove all blockages in holes and cavities.

Basic use

In industry, magnetic particle inspection can be used for the final finished product inspection to ensure that the workpiece does not produce harmful defects on the surface after various processing steps (such as welding, metal heat treatment, and grinding). It can also be used for the inspection of semi-finished products and raw materials such as bars, billets, forgings, castings, etc., to find existing surface defects. Railways, aviation and other transportation departments, smelting, chemical, power, and various machinery manufacturing plants, etc., often use magnetic particle inspection on important steel parts during regular maintenance of equipment to find defects such as fatigue cracks generated during use. Prevent catastrophic accidents during continuous use of the equipment.

Main feature

The advantages of magnetic particle inspection are: the inspection of defects such as steel materials or surface cracks of workpieces is very effective; the equipment and operation are relatively simple; the inspection speed is fast, which is convenient for inspection of large equipment and workpieces on site; the inspection cost is also low. The disadvantages are: only suitable for ferromagnetic materials; it can only show the length and shape of the defect, but it is difficult to determine its depth; some workpieces that have an impact on remanence need to be demagnetized and cleaned after magnetic particle inspection.

Magnetic particle inspection has high sensitivity and convenient operation. However, it cannot find parts within the bed casting and materials with poor magnetic permeability (such as austenitic steels), and it cannot find deep defects in the castings. The surface of castings and steel materials to be inspected is required to be smooth and can only be carried out after grinding.

Material selection

1. Magnetic powder medium

The function of the magnetic powder is as a display medium, and its types include:

a. Black magnetic powder – the composition is iron tetroxide (Fe3O4), which is black powder, suitable for workpieces with light or bright backgrounds.

b. Red magnetic powder – the composition is iron trioxide (Fe2O3), which is in the form of an iron red powder, suitable for workpieces with a dark background.

c. Fluorescent magnetic powder – the ferrous oxide magnetic powder particles are coated with fluorescent substances, which can emit yellow-green fluorescence under ultraviolet irradiation. It is suitable for workpieces with dark backgrounds, especially due to the color sensitivity of the human eye. Compared with other magnetic powders, fluorescent magnetic powder has higher sensitivity for magnetic particle inspection of the magnetic medium.

d. White magnetic powder – The ferrite magnetic powder is coated with a white substance, which is suitable for workpieces with a dark background.

To facilitate the use of on-site inspection, there are many types of commercial magnetic media, in addition to black, red, and white magnetic powder, fluorescent magnetic powder, spherical magnetic powder (hollow, colored, for dry powder method), and pre-configured magnetic paste, concentrated magnetic suspension, and magnetic suspension spray cans, etc., as well as surface brighteners (contrast enhancers), to improve the testability of workpieces with dark backgrounds or rough surfaces.

To ensure the reliability of the magnetic powder inspection results, the magnetic powder (including magnetism, particle size, and shape) and the concentration, uniformity, and suspension of the magnetic suspension must be verified before use, and also need to be used regularly during the use process. In addition, the white light intensity of the environment during the observation and evaluation, or the ultraviolet intensity of the ultraviolet lamp used in the fluorescent magnetic particle inspection, etc., are also verified items to ensure the inspection quality.

2. Sensitivity test piece

The sensitivity test piece can also be used to check the comprehensive performance of flaw detection equipment, magnetic powder, and magnetic suspension, and to check the distribution law of the magnetic field at each point on the surface of the workpiece. The method of using the sensitivity test piece is to stick the grooved side of the A-type standard test piece on the clean surface of the tested workpiece with tape paper and spray the magnetic suspension on the test piece while magnetizing the workpiece and the test piece. Observe the magnetic marks on the test piece. If the magnetizing current is appropriate, then there will be clear grooved magnetic marks on the unmarked groove surface of the test piece.

Flaw Detection Sensitivity When magnetizing complex workpieces, the magnetic field intensity distribution on the workpiece surface is very uneven, and the magnitude of the magnetizing current is difficult to estimate. Therefore, sensitivity test pieces are generally used to select the best magnetization specification.

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