Analysis of limitations and advantages of portable magnetic particle flaw detector

First, the limitations of portable magnetic particle inspection machine:

1, can only detect the surface and near surface defects of the workpiece.

2, can only detect ferromagnetic materials.

3. When magnetized by electric method and contact method, it is easy to produce spark burns.

4, by the workpiece geometry will produce non-correlated display.

Advantages:

1, can intuitively show the location, size, shape and severity of the defect, and can roughly determine the nature of the defect.

2, can detect the surface of the ferromagnetic material workpiece and near the surface of the opening and non-opening defects.

3, has a high detection sensitivity, can detect the width of micron defects.

4. Check the repeatability of defects.

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5, the comprehensive use of a variety of magnetization methods, almost unaffected by the size and geometry of the workpiece, can detect the defects in all directions of the workpiece.

6. Magnetic particle flaw detection - rubber casting method can intermittently detect the early fatigue crack generation and expansion rate of the inner wall of the small hole.

7, a single workpiece detection speed, simple process, low cost, light pollution.

Improvement Measures:

1. For the raised high point generated in the process of wheelset pressing and unloading, the raised high point is eliminated as much as possible through manual grinding to reduce the occurrence of shaft end surface ignition. The inspection personnel of the magnetic particle inspection machine self-inspect the axle end face before the inspection. Once the raised high point is found, the grinding method is adopted to deal with it, so as to eliminate the phenomenon that the axle end face is not closely adhered to the clamping device of the inspection machine as far as possible.

2, for the axle center hole edge and shaft end of the three-hole edge of the metal accumulation during the processing process can be adopted manual grinding method to eliminate the high point as far as possible; In addition, the small amount of iron filings adsorbed by the axle during the production and processing of the axle should be wiped by the inspection personnel, and the inspection can be carried out after the axle is wiped clean; As far as possible, reduce the convex high point of the axle center hole edge and the three holes at the shaft end, and improve the contact tightness between the shaft end and the magnetic particle inspection machine clamping device.

It is beneficial to adjust the demagnetizing field of portable magnetic particle flaw detection machine

Source: | Release Date: 2021-04-22

In order to make the workpiece more easily magnetized, we require the demagnetizing field of the portable magnetic particle inspection machine to be small enough.

 So what determines the size of the demagnetizing field of the flaw detection machine?

1, the size of the applied magnetic field determines the size of the demagnetizing field:

The stronger the external magnetic field, the better the magnetization effect, the stronger the magnetic field generated by the equivalent N and S poles at both ends of the workpiece, the stronger the demagnetizing field outside the workpiece.

2, the size of the demagnetizing field is inversely proportional to the L/D value of the workpiece:

L refers to the total length of the workpiece, D refers to the maximum diameter of the section of the workpiece, and for a hollow cylindrical workpiece, D refers to its equivalent diameter, whose value is (square square of the outer diameter and the inner diameter); For hollow, non-cylindrical workpieces, the D value is (the difference between the outer overall cross-sectional area and the inner hollow cross-sectional area is divided by the square of π and multiplied by 2).

It can be seen from the formula of the second point that the demagnetizing field of magnetized hollow workpiece is smaller than that of solid workpiece under the same conditions of L and D. Under the same D condition, the demagnetizing field generated by the workpiece with longer magnetization length is smaller than that of the short workpiece.

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3. The demagnetizing field is related to the geometry of the workpiece:

The demagnetization field of the workpiece with different geometric shapes is different. Under the same magnetization intensity, the demagnetization factor is different. For the completely closed ring sample, N=0; For spherical parts, N=0.333;

4. The demagnetizing field is related to the form of magnetization current:

Magnetizing current using alternating current will produce skin effect, than direct current penetration depth shallow, N, S pole magnetism formed at both ends of the workpiece is poor, so when alternating current and direct current magnetize the same workpiece, alternating current produces a small demagnetizing field.

Attention points of flaw detection machine in power and weld:

1, power:

1) When using the continuous magnetization method, it is necessary to clarify the power time to ensure that the magnetic powder can be used in the case of power;

2) When using the remanent magnetization method, the magnetization time is generally 0.2-1s, and when using the impulse current, it should be more than 10ms and be powered on continuously for more than two times. If the magnetic particle flaw detector can have sufficient magnetic potential when detecting the flaw, it can not be restricted in accordance with this regulation.

2. Weld

1) One is that when heat treatment is applied to the weld after welding, it is necessary to implement flaw detection after the final hot processing.

2) For the crack detection of the gap and pressure vessel after hot processing, the magnetization method should not use the direct current method.

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