JPS6248191B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for detecting flaws in cast iron pipes. When inspecting cracks in cast iron pipes, the surface scale that had formed on the surface of the cast iron pipe during annealing was first removed by shot blasting, etc., and then the cracks were visually inspected. It took a lot of effort. Therefore, when using an eddy current flaw detection head that generates an eddy current on the surface of a cast iron pipe and detects cracks by detecting the induced current generated in the induction coil by this eddy current, the induction coil of the eddy current flaw detection head detects cracks in addition to the induced current caused by the eddy current. An erroneous detection occurs when an induced current is detected due to a change in the magnetic field lines on the magnetization scale (curve B in Figure 2). The present invention proposes a flaw detection method that magnetically saturates the scale with a minute direct current magnetic field that does not reach magnetic saturation of the entire cast iron pipe, avoiding the adverse effects of changes in the magnetic lines of force of the magnetized scale. An embodiment of the present invention will be described below based on the drawings. In Fig. 1, reference numeral 1 denotes a device main body that is movable in the axial direction along the cast iron pipe 2 to be inspected. will be guided. Reference numeral 4 denotes an eddy current flaw detection head provided in the main body 1 of the apparatus, which, although not shown, has a drive coil for generating an eddy current on the surface of the cast iron pipe 2 and an induction coil for detecting an induced current due to the eddy current. ing. 5 and 6 are magnetic pole plates that sandwich the eddy current flaw detection head 4. For example 5,6
are arranged in the device main body 1 at opposite positions in the front and rear with a predetermined gap between them and the surface of the cast iron pipe 2, and an excitation coil 7 provided across the upper ends of the magnetic pole plates 5 and 6. The generated DC magnetic field forms a magnetic path and magnetizes the scale on the surface of the cast iron pipe 2 between the magnetic pole plates 5 and 6. Therefore, when a predetermined magnetizing current is passed through the excitation coil 7, the scale can be magnetically saturated.
At this time, no induced current is generated in the induction coil due to changes in the lines of magnetic force on the magnetization scale, and erroneous detection due to this is avoided. The magnitude of the magnetizing current is preferably so small that the DC magnetic field generated thereby does not magnetically saturate the entire cast iron pipe 2, but magnetically saturates the scale. Generally, the magnetization characteristics of cast iron and scale (iron oxide) are as shown in curves A and B shown in Figure 2.
Magnetization characteristic curve B of the scale shows that magnetic saturation occurs with a very small DC magnetic field. Specifically, cast iron becomes magnetically saturated at 8,000 to 10,000 Gauss, while scale becomes magnetically saturated at 1,000 to 2,500 Gauss. Therefore, only a small amount of magnetizing current is required to be passed through the excitation coil 7. Moreover, as in the above embodiment, the magnetic pole plate 5,
If coils 6 are placed opposite each other in the axial direction of the cast iron pipe 2, the DC magnetic field generated by the excitation coil 7 will pass through the surface of the cast iron pipe 2 in the shortest possible time, and it will be a minute DC magnetic field. It is also possible to effectively magnetically saturate the scale immediately below the eddy current flaw detection head 4. Furthermore, if the magnetic pole plates 5 and 6 are configured to span a half circumference of the cast iron pipe 2, and a plurality of eddy current flaw detection heads 4 are arranged over the half circumference, the device main body 1 can be configured to span a half circumference of the cast iron pipe 2. By simply moving the cast iron pipe 2 back and forth in the axial direction and rotating the cast iron pipe 2 by half a turn on the return trip, the entire circumference of the cast iron pipe 2 can be detected for flaws, and the flaw detection efficiency can be improved. Next, the effect of scale magnetic saturation in flaw detection of a 100φ×4M pipe will be explained using the following example. Two 50 mm wide eddy current flaw detection heads were used, and the effective detection width was approximately 85 mm. DC0~ for excitation coil
A magnetizing current of 5 A was applied, and the operating frequency of the eddy current flaw detection head was set to 2 KHz. As can be seen from Table 1,
In the non-defective part of the scaled tube, the magnetizing current from the induction coil changes depending on the magnetizing current of the excitation coil.
It reaches its minimum around 1A. This is because the scale is magnetically saturated due to a small amount of magnetization, suppressing noise signals from the scale, but if excessive excitation is performed, noise generated by the detection coil moving in the magnetic field due to vibration. The signal increases. Therefore, it can be understood that the risk of false detection can be reduced by applying an appropriate magnetic field necessary to magnetically saturate only the scale layer during flaw detection.

[Table] Figure 3a shows the detection output diagram of the measurement results when the excitation current is 0, and Figure 3b shows the detection output diagram of the measurement results when the excitation current is 3A.
A detection output diagram of the measurement results is shown. Here, c is a crack detection signal. Further, FIG. 4a shows a detection output diagram of the measurement results of the scaled tube, and FIG. 4b shows a detection output diagram of the measurement results after shot blasting this tube. Here, c is a crack detection signal as in FIG. These can also be used to determine the degree of interference that the scale is having on flaw detection. As described above, according to the present invention, cracks can be reliably detected without the risk of being disturbed by induced currents caused by changes in the magnetic lines of force of the magnetization scale other than the induced currents caused by the eddy currents generated by the eddy current flaw detection head.

[Brief explanation of the drawing]

Fig. 1 is a perspective view showing one embodiment of the flaw detection device used to carry out the method of the present invention, Fig. 2 is a comparison diagram of the magnetization characteristics of cast iron and scale, Fig. 3 a, b
4A and 4B are detection output diagrams showing the measurement results when the magnetizing current was set to 0 and 3A, and FIGS. 4a and 4b are detection output diagrams of the measurement results of a scaled tube and this tube after shot blasting. 4... Eddy current flaw detection head, 5, 6... Magnetic pole plate, 7
...excitation coil.

Claims (1)

[Claims] 1 A flaw detection method for cast iron pipes using an eddy current flaw detection head, in which the scale on the surface of the cast iron pipe facing the eddy current flaw detection head is detected while magnetically saturated with a minute DC magnetic field that does not reach magnetic saturation of the entire cast iron pipe. A cast iron pipe flaw detection method characterized by: