JP3226071B2 - Magnetic flaw detection - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic flaw detection method suitable for inspecting a steel pipe, particularly a curved pipe portion, for defects such as corrosion thinning.

[0002]

2. Description of the Related Art Conventionally, a magnetic flaw detection method as described below has been widely used as a method for inspecting defects such as corrosion thinning of a steel pipe. That is, the magnetic flaw detection method uses the magnetization head 1 shown in FIG.
Is to detect. The magnetization head 1 has a pair of permanent magnets 4a and 4b and a wire brush 5 provided on the permanent magnets 4a and 4b.
The magnetic sensor 6 is arranged at an intermediate position between the two. The magnetized head 1 is brought into contact with the steel pipe 2 via the wire brush 5. Then, the steel pipe 2 is magnetized, and lines of magnetic force are formed from one permanent magnet 4 a side to the other magnet 4 b so as to pass through the steel pipe 2 via the wire brush 5. At this time, the magnetic line of force partially leaks at the thinned portion 3 of the steel pipe 2 to be inspected, and the amount of thinned wall can be measured by detecting the leaked magnetic line of force with the magnetic sensor 6.

[0003]

However, the magnetic flaw detection method can be applied only to a straight steel pipe 2, and in a curved tubular portion as shown in FIG. Because the profiling state of the tube wall changes,
The magnetization level of the steel pipe 2 varied, and stable measurement was difficult. In other words, the steel pipe 2
This is because the magnetic flux density in the medium and the stray magnetic flux density of the stray magnetic field formed in the space between the permanent magnets 4a and 4b fluctuate, and the detection value of the leakage magnetic flux by the magnetic sensor 6 fluctuates. The present invention has been made in order to improve such inconvenience, and measures a stray magnetic field that fluctuates depending on a copying state with a steel pipe to be inspected, and based on the measured value, measures a magnetic flux density of a leakage magnetic flux line. It is an object of the present invention to provide a magnetic flaw detection method that enables stable measurement by compensating for flaws.

[0004]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention is directed to a steel pipe by magnetizing a steel pipe with a magnetizing head having a pair of magnets and detecting the magnetic flux density of lines of magnetic force leaking from the steel pipe. In a magnetic flaw detection method to detect defects such as corrosion thinning of steel, leakage occurs at the thinned part of the steel pipe.
While detecting the magnetic flux lines formed in the space between the pair of magnets, detecting the magnetic flux density in the steel pipe from the detection signal applied to the floating magnetic flux density, and detecting the magnetic flux density in the steel pipe.
From the flux density, find the correction coefficient of the detection signal applied to the leakage magnetic field line, and correct it by taking into account the detection signal applied to the leakage magnetic field line
Then, the corrected detected value is taken out .

[0005]

[Function] Detects the stray magnetic flux density, finds the magnetic flux density in the steel pipe from the detection signal concerning the stray magnetic flux density,
From the magnetic flux density, the correction coefficient of the detection signal applied to the leakage magnetic field line is calculated and compensated by taking into account the detection signal applied to the leakage magnetic field line.
Since the corrected and detected value is taken out , stable measurement is possible regardless of the fluctuation of the stray magnetic flux density.

[0006]

Next, a magnetic flaw detection method according to the present invention will be described.
An embodiment will be described in detail below. FIG. 1 shows a steel pipe 11 to be inspected for a magnetized head 10 for performing magnetic flaw detection.
The place where it contacted is shown. The magnetizing head 10 includes a pair of permanent magnets 12a and 12b and permanent magnets 12a and 12b.
b, and a wire brush 13 provided at a position between the pair of permanent magnets 12a and 12b.
In addition, a magnetization level correction sensor 15 is arranged at a position spaced apart from the magnetic sensor 14 by a suitable distance in the figure. With such a magnetized head 10, a steel pipe 11
The line of magnetic force leaking at the thinned portion 16 of the magnetic sensor 1
4. The magnetization level correction sensor 15 measures a floating magnetic flux density formed in a space between the permanent magnets 12a and 12b. In addition, such a magnetization level correction sensor 15 is, as shown in FIG.
You may make it arrange | position in the space part of the position close to the permanent magnet 12a.

Next, a procedure for executing magnetic flaw detection by the above-described magnetization head 10 will be described with reference to a block diagram shown in FIG. As shown in FIG. 3, a detection signal of the magnetic sensor 14 and a detection signal of the magnetization level correction sensor 15 are taken into the calculation means 16 and a signal relating to a predetermined calculation result is output. That is, the floating magnetic flux density that varies by copying the state of the wire brush 13 and the steel pipe 11 is detected by the magnetization level correction sensor 15, the arithmetic unit 16, the magnetization level correction sensor 1
The magnetic flux density in the steel pipe 11 is obtained from the value related to the stray magnetic flux density according to 5, and the correction coefficient of the magnetic sensor 14 is further obtained from the magnetic flux density in the steel pipe 11 and corrected in consideration of the detection signal of the magnetic sensor 14. , And takes out the corrected detection value. Here, FIGS. 4 and 5 are graphs each showing an example of the relationship between the stray magnetic flux density—the magnetic flux density in the steel pipe and the magnetic flux density in the steel pipe—the correction coefficient of the magnetic sensor. According to FIG. 4, between the stray magnetic flux density and the magnetic flux density in the steel pipe, the magnetic flux density in the steel pipe decreases as the stray magnetic flux density increases, and according to FIG. 5, the magnetic flux density in the steel pipe and the correction coefficient of the magnetic sensor. There is a relationship that the correction coefficient of the magnetic sensor decreases due to an increase in the magnetic flux density in the steel pipe. In other words, the relationship between the stray magnetic flux density and the correction coefficient of the magnetic sensor is such that as the stray magnetic flux density increases, the correction coefficient of the magnetic sensor increases. That is, the calculating means 1
Reference numeral 6 denotes a configuration for taking out a corrected output value of the magnetic sensor 14 based on the relationship between the floating magnetic flux density—the magnetic flux density in the steel pipe, and the magnetic flux density in the steel pipe—the correction coefficient of the magnetic sensor. Means are appropriate.

According to the magnetic flaw detection method described above, even if the lift-off changes and the stray magnetic flux density fluctuates, the corrected output of the magnetic sensor 14 is taken out according to the stray magnetic flux density. Thus, stable measurement can be performed regardless of the lift-off, that is, the fluctuation of the stray magnetic flux density. Accordingly, corrosion thinning and the like can be detected without any problem not only in the straight portion of the steel pipe 11 but also in a curved portion.

[0009]

As described above, according to the present invention, the measured value of the magnetic flux density of the leakage magnetic force lines is corrected based on the measurement of the stray magnetic field which fluctuates according to the state of following the steel pipe to be inspected. It is possible to correct the variation of the measured value of the magnetic flux density of the leakage magnetic force line for each yoke caused by the change of the copying state of the wire brush, which occurs in the curved pipe portion of the steel pipe to be inspected, thereby enabling stable measurement. Become.

[0010]

[Brief description of the drawings]

FIG. 1 is a schematic side view illustrating an example of a magnetization head for performing a magnetic flaw detection method according to the present invention.

FIG. 2 is a schematic side view showing another example of the magnetization head for performing the magnetic flaw detection according to the present invention.

FIG. 3 is a block diagram showing a procedure for correcting an output signal of a magnetic sensor for performing a magnetic flaw detection method according to the present invention.

FIG. 4 is a graph showing a relationship between a stray magnetic flux density and a magnetic flux density in a steel pipe.

FIG. 5 is a graph showing a relationship between a magnetic flux density in a steel pipe and a correction coefficient of a magnetic sensor.

FIG. 6 is a schematic side view showing an example of a magnetized head for performing a conventional magnetic flaw detection method.

FIG. 7 is a schematic explanatory view of a principal part when detecting a defective portion in a bent portion of a steel pipe to be detected by the magnetization head shown in FIG. 6;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 Magnetization head 11 Steel pipe 12a, 12b Permanent magnet 13 Wire brush 14 Magnetic sensor 15 Magnetization level correction sensor 16 Calculation means

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G01N 27/72-27/90

Claims (1)

(57) [Claims] In a magnetic flaw detection method for detecting defects such as corrosion thinning of a steel pipe by magnetizing the steel pipe with a magnetizing head having a pair of magnets and detecting a magnetic flux density of lines of magnetic force leaking from the steel pipe, Leaks at thinning points
While detecting the lines of magnetic force, the stray magnetic flux density formed in the space between the pair of magnets is detected, and the magnetic flux density in the steel pipe is obtained from the detection signal applied to the stray magnetic flux density.
From the density, the correction coefficient of the detection signal applied to the leakage magnetic field line is obtained, and corrected in consideration of the detection signal applied to the leakage magnetic field line,
A magnetic flaw detection method wherein a corrected detection value is taken out .