JPH0843358A - Eddy-current flaw detecting method for steel pipe - Google Patents

Abstract

PURPOSE:To increase a sensing force for a flaw on the inner surface by determining the DC magnetic suturation level to such a degree that the magnetic permeability mu maximizes on a model material which is equipped with an inner surface flaw to a degree as to be sensed, subjecting a test specimen to a DC magnetization to this level determined, and thereby maximizing the impedance change of a test coil. CONSTITUTION:The impedance change of a test coil is sensed in the process that the DC magnetic suturation level of a model material is varied by a DC magnetic saturating coil, and it is judged that the magnetic permeability muhas maximized when the impedance change is maximum. If for example the saturation level where the permeability mu of a test specimen is approx. one is assumed as 100%, setting to the saturation level 10% causes the permeability muaround an inner surface flaw of the test specimen to maximize, This widens the part which generates mu>100 around the flaw and the amount of leaking lines of magnetic flux in this part increases, which heightens the sensing force of the test coil. That is, increase in the leaking magnetic flux around the flaw causes the impedance change of the test coil to maximize around the defect, and it is possible to detect flaw with a high S/N ratio.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a penetration type eddy current flaw detection method for steel pipes.

[0002]

2. Description of the Related Art Conventionally, as a penetration type eddy current flaw detection method for detecting defects in a steel pipe, there is one disclosed in JP-A-3-188373. In this penetration type eddy flaw detection method, the DC magnetic saturation coil is used to magnetize the test material to a DC magnetic saturation level such that the magnetic permeability (μ) of the test material is about 1 (approx. The eddy current is generated in the test material by means of the eddy current, and the disturbance of the eddy current caused by the presence of the defect in the test material is detected by the detection function of the test coil to detect the defect in the test material.

[0003]

At this time, since the test frequency f of the test coil is usually 2 to 10 KHz, the penetration depth δ of the eddy current into the test material is 5 to 6 due to the skin effect of the alternating current.
It is about mm.

[Equation 1] However, σ is the electrical conductivity of the test material.

Therefore, the power of detecting inner surface defects of a steel pipe by the penetration type eddy current flaw detection method is significantly lower than the power of detecting outer surface defects. For example, in a steel pipe with an outer diameter of 60.3 mm and a wall thickness of 12 mm,
The detectable depth of inner surface defects is limited to 50% or more of the wall thickness.

The present invention provides a method for eddy current flaw detection of a steel pipe,
The purpose is to improve the detectability of inner surface defects.

[0006]

DISCLOSURE OF THE INVENTION The present invention is directed to eddy current flaw detection of a steel pipe for detecting defects in a test material by detecting a change in impedance of the test coil by subjecting the test material to DC saturation by magnetically saturating the test material with a DC magnetic saturation coil. In the method, the sensitivity is set in advance to obtain the DC magnetic saturation level at which the magnetic permeability (μ) is maximized for the model material with the inner surface defects to be detected, and the DC magnetic saturation level of the test material is set to the above sensitivity setting. By DC magnetizing the test material with a DC magnetic saturation coil so that it is close to the degree obtained in
The impedance change of the test coil is maximized to detect the inner surface defect of the test material.

[0007]

[Action]

(1) A DC magnetic saturation coil 11 and a pair of test coils 12 are arranged around the through passage of the test material 1 (steel pipe) (FIG. 1). The pair of test coils 12 is a bridge circuit (not shown)
Connected to the bridge circuit, the bridge circuit outputs a defect signal from the imbalance caused by the output deviation of each coil 12.

The DC magnetic saturation coil 11 DC magnetizes the test material 1 to magnetically saturate it. The test coil 12 (a) generates an eddy current in the test material 1, (b) detects eddy current disturbance caused by the presence of defects in the test material 1, (c) the test material 1
To detect the leakage flux due to the presence of defects inside. The test coil 12 includes an induction coil for performing (a), (b),
It may be composed of a detection coil for performing (c).

(2) With respect to the model material having the inner surface defects to be detected, the sensitivity is set in advance to obtain the DC magnetic saturation level at which the magnetic permeability (μ) is maximized (FIG. 2). In FIG. 2, Ho is the magnetic field, μ is the magnetic permeability, and B is the magnetic flux density.

This sensitivity setting detects the impedance change of the test coil 12 in the process of changing the DC magnetic saturation level by the DC magnetic saturation coil 11 in the model material, and when this impedance change is the maximum, the magnetic permeability μ Is the maximum. This is the test coil 1
Based on the following relationship between the impedance z of 2 and the magnetic permeability μ of the test material 1.

[Equation 2] However, R is the resistance of the test coil 12, and L is the test coil 12.
The reactance of is ω = 2π. L = n · μ · H · S / i (3) where n is the number of turns of the test coil 12, H is the magnetic field, S is the cross-sectional area of the magnetic path through which the magnetic flux passes, and i is the alternating current flowing through the test coil 12. is there.

That is, when the direct current magnetic saturation level for setting the magnetic permeability μ of the test material 1 to about 1 is 100% and the direct current magnetic saturation level is set to, for example, 10%, the inner surface defects μ of the test material 1 around The magnetic permeability becomes maximum as shown by μmax in FIG.

When the DC magnetic saturation level of the test material 1 is set to 10%, for example, as shown in FIG. 3 (A), a portion where μ> 100 spreads around the inner surface defect 2 and leakage at this portion occurs. As the amount of magnetic flux lines increases, the detection power of the test coil 12 increases. That is, when the amount of leakage magnetic flux lines around the inner surface defect 2 also increases, the impedance change of the test coil 12 increases and the S of the inner surface defect 2 increases. The / N ratio is high, and the detection power of the test coil 12 is improved.

On the other hand, when the DC magnetic saturation level of the test material 1 is set to, for example, 50 to 100%, as shown in FIG.
There are few areas where μ> 100 around the inner surface defect 2 and μ <
The area 10 is widened, and the amount of leakage magnetic flux lines at this area is reduced. As a result, the detection power of the test coil 12 is reduced. The relationship between the detection power of the test coil 12 and the DC magnetic saturation level of the test material 1 is as shown in FIG.

(3) Based on the above (2), the DC magnetic saturation coil 11 is used so that the DC magnetic saturation level of the test material 1 is close to the degree obtained by the above sensitivity setting (for example, DC magnetic saturation level 10%). The test material 1 is DC magnetized. As a result, as described above, the area where μ> 100 spreads around the inner surface defect 2, and the amount of leakage flux lines in this area increases, so that the impedance change of the test coil 12 becomes maximum around the inner surface defect 2. Therefore, the inner surface defect 2 can be detected with a high S / N ratio.

[0015]

EXAMPLE The method of the present invention was carried out on a steel pipe having an outer diameter of 60.3 mm and a wall thickness of 12 mm, and the detection results shown in FIG. 5 were obtained. The DC magnetic saturation level by the DC magnetic saturation coil 11 was set to 10% (the conventional DC magnetic saturation level was 50% or more). Inner surface defects existing in steel pipe 2A (12.5% of the wall thickness depth, width 10mm, length 20mm), inner surface defects 2B (25% of the wall thickness)
The depth, width of 10 mm, and length of 20 mm) can be detected with high accuracy.

[0016]

As described above, according to the present invention, in the eddy current flaw detection method for a steel pipe, it is possible to improve the detectability of the inner surface defect.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a through-type eddy current flaw detection method.

FIG. 2 is a diagram showing BH and μ-H curves of a steel pipe.

FIG. 3 is a schematic diagram showing the relationship between the DC magnetic saturation level of a steel pipe and the occurrence of magnetic permeability.

FIG. 4 is a diagram showing the relationship between the detection power of the test coil and the DC magnetic saturation level.

FIG. 5 is a diagram showing a detection result by the method of the present invention.

[Explanation of symbols]

 1 Test Material 2 Inner Surface Defect 11 DC Magnetic Saturation Coil 12 Test Coil

Claims (1)

[Claims] 1. A eddy current flaw detection method for a steel pipe, which detects a defect of a test material by detecting a change in the impedance of the test coil by magnetizing the test material by direct current magnetization with a direct current magnetic saturation coil to magnetically saturate the inner surface to a degree to be detected. For the model material with defects, set the sensitivity to obtain the DC magnetic saturation level that maximizes the magnetic permeability (μ) in advance so that the DC magnetic saturation level of the test material is close to the value obtained by the above sensitivity setting. An eddy current flaw detection method for a steel pipe, which comprises maximizing impedance change of a test coil and detecting inner surface defects of the test material by direct-current magnetizing the test material with a DC magnetic saturation coil.