How much effect does the application of fluorescent magnetic particle flaw detection in bolts

How much effect does the application of fluorescent magnetic particle flaw detection in bolts

The quality of bolt products directly affects the assembly quality or service life of mechanical equipment and vehicles. The most important link to strictly control the quality of bolts is to detect whether there are defects on the surface, especially whether there are cracks after heat treatment. Although the newly developed new equipment-optical vision (non-contact) automatic detection equipment and new screening machines focus on bolt image screening machines and eddy current optical screening machines are put on the market, due to the price, at present, the fastest way to test bolt defects is non-destructive testing.

Non-destructive testing methods include: ultrasonic testing, magnetic particle testing, penetration testing, eddy current testing, radiographic testing, etc. Magnetic particle inspection is an effective method for detecting surface and near surface defects of ferromagnetic materials. Because of the advantages of intuitive defect, high sensitivity, fast detection speed and low cost, it is widely used in the surface flaw detection of bolts. The following is a summary of the practice in this area for peer reference.

1 The Concept

① Magnetic particle inspection. Refers to ferromagnetic materials. If there are defects (cracks) on the surface or near the surface, the magnetic field in the bolt body will be locally distorted in the area where the defect is located after being magnetized. A part of the magnetic lines of force will leak on the surface of the bolt to form a leakage magnetic field. At the same time, a corresponding magnetic leakage pole will be formed on the surface where the defect is located. If the magnetic powder or magnetic suspension is added to the surface of the bolt, the magnetic leakage pole will attract the magnetic powder to form fluorescent magnetic powder, thus showing defects, which is the principle of magnetic particle inspection.

② Fluorescent magnetic powder. Refers to the magnetic particles shown by the magnetic marks observed under ultraviolet light. It is based on magnetic iron oxide powder, industrial pure iron powder or carbonyl iron powder as the core, in the iron powder outside with resin adhesion layer made of fluorescent dye.

③ Wet method. Refers to the method of magnetic particle testing by suspending magnetic particles in a carrier liquid.

2. Advantages of fluorescent magnetic particle flaw detection

In addition to good magnetic properties and suitable particle size and shape, fluorescent magnetic powder should also have suitable fluorescence intensity. The fluorescent substance in the fluorescent magnetic powder particles emits yellow-green fluorescence under the irradiation of the ultraviolet lamp. In magnetic particle inspection, if there are defects such as cracks and pores on the surface or near the surface of the bolt when the bolt to be inspected is magnetized, the leakage magnetic field at the defect will attract the applied magnetic powder and form magnetic marks. This magnetic mark with fluorescent substance excites yellow-green visible light that is very sensitive to human eyes under the irradiation of ultraviolet lamp, so as to achieve the purpose of flaw detection. Fluorescent magnetic particle inspection has the advantages of high contrast, high sensitivity, and suitable for magnetic particle inspection in dark places and narrow working spaces.

3. Precautions for magnetic particle inspection

3.1 choose the right magnetic powder

There are many types of magnetic powder, according to the magnetic mark observation is divided into fluorescent magnetic powder and non fluorescent magnetic powder; according to the application method is divided into wet magnetic powder and dry magnetic powder. According to the material and performance of the bolt, the general choice of fluorescent magnetic powder inspection is more appropriate, and fluorescent magnetic powder is generally only suitable for wet inspection.

3.2 selection of the appropriate magnetization method

According to the geometric shape of the bolt, the size and the direction of the defect to be found, the appropriate magnetic field direction should be selected, which can be divided into circumferential magnetization, longitudinal magnetization and compound magnetization.

The circumferential magnetization of bars and bolts is directly energized, and the current passes along the axial direction of the bolt to generate a circumferential magnetic field on the bolt. If there is a crack or near-surface defect intersecting with the magnetic line of force, two magnetic poles N and S will be immediately generated on both sides of the crack, and part of the magnetic powder enters the nearby space to form a leakage magnetic field capable of absorbing the magnetic powder, thus detecting the longitudinal crack on the bolt. For the sleeve workpiece, the nut is magnetized by magnetic conductivity method, and a circumferential magnetic field is generated on the nut. The coil method is used to magnetize the nut longitudinally to generate an axial magnetic field. Similarly, the circumferential crack on the bolt can be detected. At the same time, the circumferential and longitudinal magnetization currents are applied, I .e. compound magnetization, to form an alternating rotating vector magnetic field on the surface and near surface of the bolt. During the continuous flaw detection inspection, cracks and defects in all directions on the surface and near surface of the bolt and nut can be detected.

Preparation, Hazards and Protective Measures of 3.3 Magnetic Suspension

① Preparation of magnetic suspension. The suspension liquid formed by mixing fluorescent magnetic powder and water in a certain proportion is called magnetic suspension. The volume of magnetic particles precipitated per 100ml of magnetic suspension is called the concentration of magnetic suspension. The concentration of magnetic suspension has a great influence on the sensitivity of displaying defects. Different concentration, detection sensitivity is also different. If the concentration is too low, it will affect the adsorption of magnetic powder by the leakage magnetic field, and the unclear magnetic mark will cause the defect to be missed. If the concentration is too high, a lot of magnetic powder will be retained on the surface of the bolt, forming excessive background and even covering up the relevant display. The selection of magnetic suspension concentration is related to the type, particle size, application method and bolt surface state of magnetic powder. Brand products should be selected for fluorescent magnetic powder, and the concentration of magnetic suspension is generally controlled at 0.02~0.04ml/100ml. Before preparing the magnetic suspension, it is recommended to use a small container to completely dissolve the magnetic powder, and then pour it into the magnetic suspension working liquid tank to stir evenly, so that the prepared magnetic suspension has uniform concentration and less precipitation, and it is easy to wet the bolt surface during spraying, thus easily showing the defect position and direction. In addition, it should be noted that when there are various impurities in the contaminated magnetic suspension or the detection needs to be interrupted for a period of time, it is easy to cause the phenomenon of blocking the nozzle copper rod and the like, which brings a lot of trouble to the next inspection. In order to save time and labor, at the end of each inspection or when the magnetic suspension pollution needs to be replaced, the liquid tank and each nozzle must be thoroughly cleaned, and the magnetic suspension should be replaced by clean water for many times, and then a new magnetic suspension should be equipped as required.

② Harm of magnetic suspension to human body. The main components of fluorescent magnetic powder magnetic suspension are emulsifier diethanolamine, sodium nitrite, fluorescent magnetic powder and defoaming agent. 40% magnetic suspension can cause skin burns. Therefore, it is not suitable for direct contact with skin and mucous membrane in production. Another hazard factor of fluorescent magnetic particle flaw detection is ultraviolet rays, which can be absorbed by deep tissues and dermis, causing erythema, pigmentation, and even headache, dizziness, fatigue, elevated body temperature and other symptoms. Excessive irradiation can also cause acute keratitis (electro-optic eye disease).

③ Protective measures. The first is to wear ultraviolet protective glasses and special protective gloves; the second is to shorten the working time and not to work continuously for more than half an hour; the third is to configure a separate dark room with an area of more than 15m3, and to set up another dressing room and toilet; the fourth is to wash hands before work. Wash your hands frequently, do not smoke or eat snacks in the workshop, and develop good hygiene habits.

4. Testing of products

Turn on the power button switch, the device is powered on, and the power indicator light is on. Adjust the circumferential and longitudinal magnetization currents. Directly energize the bolts in the circumferential direction for circumferential magnetization and check the longitudinal cracks. Use the coil in the longitudinal direction for longitudinal magnetization and check the circumferential cracks. Simultaneous circumferential and longitudinal magnetization can check the circumferential and longitudinal cracks on the bolt surface in all directions. The flaw detection method uses a continuous method, that is, spraying and magnetization.
The operator checks under ultraviolet irradiation and finds defective bolts or marks the defects. At this time, the white light intensity of the darkroom shall not exceed 20 lux, the lighting system shall be a 36W fluorescent lamp, and the ultraviolet light irradiation intensity shall not be less than 1000 μW/cm2.

If there is a crack in the bolt, magnetic marks will form in the crack area. Magnetic particle inspection is the use of magnetic particle aggregation to form magnetic marks to show the discontinuity and defects on the bolt. We usually refer to the image formed by the aggregation of magnetic particles during magnetic particle inspection as magnetic marks. The width of the magnetic mark is several times the width of the discontinuity and the crack, that is, the magnetic mark has an amplification effect on the width of the crack. Therefore, the invisible crack can be displayed, and the detection sensitivity is very high.

After the magnetic mark analysis confirms that it is a defective magnetic mark, the sample should be reserved for treatment. Usually, the method of photographing can be used to retain the shape, size and direction of the crack magnetic mark. The advantage is intuitive, easy to save.

5. Conclusion

Magnetic particle flaw detection is a very demanding quality inspection work, in order to ensure the quality and reliability of magnetic particle flaw detection, many factors that affect the test results must be controlled one by one. Comprehensive control from the five aspects of man, machine, material, method and ring can ensure that the three variables of magnetic particle inspection (process variables, equipment variables and application variables) are well controlled.

Source: Golden Spider Fastener Web