JPH04259853A - Water immersion type ultrasonic flaw detection - Google Patents

Abstract

PURPOSE:To detect a micro defect present in the vicinity of the surface of a steel material with high sensitivity by setting the water distance of a probe of a specific frequency and a specific focal length so that ultrasonic beams are converged in a specific range including the surface of the material. CONSTITUTION:The water distance (distance from a probe to the surface of a material to be inspected) for a probe having a frequency of not smaller than 15MHz and not larger than 50MHz and a focal length of not smaller than 12.7mm and not larger than 38.1mm is set so that ultrasonic beams are converged in the range of 2mm below the surface of water including the surface of the material. When this kinds of point convergent ultrasonic waves is brought into the material from the surface perpendicularly within + or -5 degrees, a multi- reflecting echo generated from a micro defect within 2mm from the surface can be detected. Therefore, a defect below the surface can be detected by utilizing the multi-reflecting echo. If a wide-band probe having a narrow pulse width is used, the insensitivity band of the surface echo can be reduced more.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrasonic flaw detection method for detecting minute defects existing near the surface (hereinafter referred to as "subcutaneous") of steel materials such as plates, pipes, and bars.

0002

[Prior Art] In recent years, line pipes for sour gas, etc.
As the number of products that require strict usage conditions increases, there is a growing demand for guaranteed quality uniformity. In particular, hydrogen-induced cracking (
Among the nonmetallic inclusions that cause problems (hereinafter referred to as HLC), sulfide-based inclusions (M
Microscopic inclusions (such as nS) are often present in the center of the wall thickness. On the other hand, oxides with a size of about 5 to 500 μm (
Microscopic inclusions (such as Al2O3) often exist within 1 mm directly below the surface.

[0003] As a general ultrasonic flaw detection method, the perpendicular method in which ultrasonic waves are incident perpendicularly from the surface of the test material is described in the paper published by the Japan Society for the Promotion of Science and Nikkan Kogyo Shimbun (December 20, 1978).
17 in the document entitled "Ultrasonic flaw detection method (revised new edition)"
As shown on pages 9 to 180, defects in the endoplasmic part at the center of the wall thickness can be sufficiently detected, but
Subcutaneous defects within m are disadvantageous in that a so-called dead zone, which cannot be detected by transmitted pulses (in the case of water immersion method, surface echo), occurs, leaving areas that cannot be detected.

[0004] As a means for detecting defects existing on the surface of steel materials and under the skin, there is an ultrasonic flaw detection method that uses surface waves to detect flaws only in the surface layer, as discussed in the above-mentioned literature. The surface wave method is used when the frequency is within twice the wavelength (for example, the frequency is 10M).
Although it is possible to detect defects existing under the skin (approximately 0.6 mm in the case of Hz), it cannot reliably compensate for the dead zone of the general vertical flaw detection method. By lowering the frequency, the penetration depth of the surface waves becomes deeper, but since the detection limit size of defects is proportional to the wavelength, the detection characteristics conversely deteriorate, making it difficult to detect minute defects. Therefore, it is desired to establish a high-precision flaw detection method that reliably detects minute defects in the endoplasmic part existing under the skin.

[0005]

[Problems to be Solved by the Invention] The present invention was made in order to eliminate the drawbacks of such conventional methods. The purpose is to detect micro defects with high sensitivity by detecting multiple reflection echoes generated from micro defects existing in an area of 2 mm or less.

[0006]

[Means for solving the problem] A probe with a frequency of 15 MHz or more and 50 MHz or less and a focal length of 12.7 mm or more and 38.1 mm or less is placed under the skin 2, including the surface of the specimen.
The water distance is set so that the ultrasonic beam is focused in the range of mm, and the point-focused ultrasonic wave is perpendicularly incident on the surface of the material to be inspected, and is generated from minute defects existing within an area of 2 mm from the surface. This is a water immersion ultrasonic flaw detection method characterized by detecting multiple reflected echoes.

FIG. 1 shows a conventional vertical ultrasonic flaw detection method using a water immersion method. As shown in FIG. 1, the cathode ray tube figure 6 of the ultrasonic flaw detector 5 when the ultrasonic beam 3 is perpendicularly incident into the test material 4 from the probe 1 through the water 2 is the ultrasonic beam 3. A surface echo S1 appears at the incident point S, and a bottom echo B1 appears at a point B on the back side. in this case,
The area within 2 mm near the surface is the dead zone of surface echo S1 (Fig. 1
(shaded area), making it impossible to detect flaws.

The details of the present invention will be explained below with reference to the drawings. As a result of various experiments and analysis of ultrasonic waveforms, the present inventors found that by optimizing the flaw detection conditions, multiple reflected echoes could be obtained from defects existing within 2 mm from the surface. Therefore, we were confident that subcutaneous defects could be detected by using this multiple reflection echo. FIG. 2 is a diagram showing how multiple reflected echoes are generated from a minute defect located 0.3 mm from the surface. As shown in this figure, by making a high-frequency, focused ultrasonic beam perpendicularly incident on the specimen, the dead zone caused by surface echoes can be reduced, and multiple reflection echoes F can be obtained from minute defects existing near the surface. It was confirmed that subcutaneous defects, which were hidden by surface echoes and difficult to detect using conventional vertical ultrasonic flaw detection methods, could be reliably detected. Note that the broken line in FIG. 2 indicates the surface echo area of the conventional vertical ultrasonic flaw detection method. The occurrence of multiple reflection echoes is greatly influenced by frequency and focal length. Table 1(a) shows the results of flaw detection of a 0.5 mmφ flat bottom drill hole located 0.1 mm from the surface under various flaw detection conditions. Table 1(b) shows the results of flaw detection of a 0.5 mmφ flat bottom drill hole located 2.0 mm from the surface under various flaw detection conditions. The circle mark in the table indicates that a 0.5 mmφ flat bottom drill hole can be detected, and the cross mark indicates that it cannot be detected.

[0009]

[Table 1]

0010

[Table 2]

As shown in Table 1, the frequency is 15 MHz or more and 50 MHz or less, and the focal length is 12.7 mm or more.
．． Subcutaneously 0.1 to 2
．． It was observed that multiple reflection echoes were obtained from minute defects in the range of 0 mm. In order to obtain multiple reflected echoes, it is necessary to set the water distance so that the ultrasonic beam is focused within a range of 2 mm below the skin, including the surface, and to set the incident angle for vertical flaw detection within ±5 degrees. FIG. 3 shows an example of how multiple reflection echoes occur when the distance from the surface to the defect is varied. The flaw detection conditions were to use a point-focusing broadband type probe with a frequency of 25 MHz, a transducer size of 10 mm, and a focal length of 25.4 mm at a depth of 1 mm from the surface of the material being tested, and at a water distance ( The distance from the probe to the surface of the material being tested was set. The defect is 0.5 from the back side of the board.
A flat bottom drill hole of mmφ was machined. In addition, 0 from the surface side
．． Since it is difficult to machine a flat-bottomed drill hole with a depth of less than 1 mm, it was not possible to confirm the detection characteristics. As shown in this figure, multiple reflected echoes can be obtained within a range of 0.1 to 2.0 mm from the surface, making it possible to reliably detect subcutaneous defects. Note that by using a broadband probe with a narrow pulse width as the probe, it is possible to further reduce the dead zone of the surface echo.

0012

[Example] Frequency 25MHz, vibrator size 10mmφ
, the water distance was set to focus the ultrasonic beam using a point-focusing broadband probe with a focal length of 25.4 mm at a depth of 1 mm from the surface of the material to be inspected, and the flaw detection gate was set at the rising edge of the surface echo. Set in the range of 0.5 to 2.5 mm from
Flaws were detected by scanning rectangularly at a pitch of 0.1 mm using a water immersion ultrasonic testing device. The material to be tested is a flat plate (width 2
0 mm, length 100 mm, and plate thickness 10 mm). FIG. 4 shows an example of the ultrasonic flaw detection results and cutting test results of the HIC test piece. FIG. 4(a) shows the C scope figure (plan view) after flaw detection of the test material, and FIG. 4(b) shows the result of a cutting test on the XX' cross section of the C scope figure. As shown in this figure, it was confirmed from the cutting test that cracks existing at a position 0.1 mm from just below the surface could be reliably detected.

[0013]

[Effect of the invention] According to the present invention, it is possible to improve the accuracy by 2.0 m from the surface, which was a dead zone for surface echoes in the conventional water immersion vertical flaw detection method.
It is possible to reliably detect micro-defects in the internal parts that exist within m, and it is possible to significantly improve quality assurance accuracy near the surface of steel materials.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a vertical ultrasonic flaw detection method using a conventional water immersion method.

FIG. 2 is a diagram showing how multiple reflected echoes are generated from a minute defect located 0.3 mm from the surface.

FIG. 3 is a diagram showing an example of how multiple reflection echoes occur when the distance from the surface to the defect is varied.

FIG. 4 is a diagram showing an example of ultrasonic flaw detection results and cutting test results of a HIC test piece.

[Explanation of symbols]

1. Probe 2 Water 3 Ultrasonic beam 4 Test material 5 Ultrasonic flaw detector 6 CRT diagram S1 Surface echo B1 Bottom echo F Multiple reflection echo

Claims (1)

[Claims] [Claim 1] Frequency is 15MHz or more and 50MHz
or less, and the focal length is 12.7 mm or more and 38.1 mm
Set the water distance so that the ultrasonic beam is focused within a range of 2 mm subcutaneously, including the surface of the test material, using the following probe, and apply the point-focused ultrasound to the test material within ±5 degrees from the surface. A water immersion ultrasonic flaw detection method characterized by detecting multiple reflected echoes generated from minute defects existing within 2 mm from the surface by vertically incident on the surface.