Ultrasonic Testing Method for Longitudinal Welds of Curved Surfaces

1. Formation and common defects of straight seam pipes

Straight seam pipe weld is a typical curved longitudinal weld, which is made of steel plate coil. According to the wall thickness, there are two weld forms: I-shaped and X-shaped. Generally I-shaped welds for thin wall (less than 12mm) that are formed once with internal and external submerged arc weld, and X-shaped welds for thicker wall (greater than or equal to 12mm) that are formed through internal and external submerged arc weld multiple times. Figure 1 is a schematic diagram of the appearance of straight seam pipe and longitudinal weld.

The common defects of straight welded pipe welds are the same as other welds, mainly porosity, incomplete fusion, incomplete penetration, slag inclusion, cracks, therefore its ultrasonic testing principle is the same as other welds. The special case is the curved testing surface, and the influence of the surface shall be considered. As the surface with a radius of curvature greater than 250mm is close to the plane and the radius of curvature less than 50mm is rare, the followings are concerned. The method introduced is mainly suitable for longitudinal welds with curved radii between 50mm~250mm. ?

2. Probes for ultrasonic testing of curved longitudinal welds

2.1 Refractive angle (K value) recommended

The choice of transducer refractive angle (K value) for ultrasonic testing of curved longitudinal welds is affected by the pipe diameter and wall thickness, and the selection is based on ensuring that the sound beam can achieve full coverage of the entire tube wall weld and heat affected zone after the transducer is incident on the curved surface. The refractive angle (K value) of the probe derived from this basis is shown in Table 1

2.2 Recommended probe nominal frequency, testing surface and probe moving area width

Recommended probe nominal frequency, testing surface, and probe movement area width are shown in Table 2.

2.3 Grinding of probes

When detecting the longitudinal weld of the curved surface, in order to increase the contact area of the probe, improve the coupling conditions, and improve the coupling effect, in the case of the radius of curvature less than 250mm, it is necessary to sharpen the angled wedge of the probe to make it similar to the radius of curvature of the steel pipe to ensure that the contact surface of the probe and the steel pipe has good acoustic coupling, and the shape of the refinement probe is shown in Figure 2. After grinding, the probe contact surface and acoustic parameters change, so a special test block is required for calibration and DAC curve production.

3.Compare the test block

The model of the special comparison test block for detecting the longitudinal weld of the surface is RB-L, and the dimensions are shown in Table 3, the appearance shape

See Figures 3 and 4.

4. Probe refractive angle (K value) and flank determination

?After the probe for detecting the curved weld is sharpened to the angled wedge, its refractive angle (K value) and incidence point change accordingly, and it needs to be re-measured, as shown in Figure 5, that is, L0 and tanβ. The operation and calculation formula are as follows:

In addition, derive formulas (2) and (3) from the triangle AOF in Figure 5 to calculate the K value (refractive angle β) with equation (3).

5.Curve

The distance-amplitude curve is made on the RB-L test block, and its sensitivity is set according to Table 4.

6. Testing

When inspecting longitudinal defects in longitudinal welds on curved surfaces, the Angled Probe should be placed perpendicular to the centerline of the weld on the inspection surface for zigzag scanning, as shown in Figure 6. The range of the probe moving forward and backward should ensure that the entire weld section is scanned. While keeping the vertical weld of the probe head moving forward and backward, it should also rotate left and right of 10°~15° during scanning. In order to observe the dynamic waveform of the defect and distinguish the defect or pseudo-defect signal. To determine the location, direction, and shape of the defect, four basic scanning methods of the probe can be used, such as front and back, left and right, corner, and surround, as shown in Figure 7.

7.Defect positioning

Defect positioning is divided into two types of analysis: the discovery of defects before the primary wave and the discovery of defects before the secondary wave after the primary wave. ?

7.1 Positioning of defects found before the primary wave

Referring to Figure 8, assume that defect E is found in primary wave when the probe is at position A, and set the vertical depth indicated by the defect on the display H1 1, R is the radius, and K is the probe K value. Then the actual depth h of the defect is obtained by equation (4).?

7.2 Positioning of defects found after the primary wave and before the secondary wave

Referring to Figure 8, suppose that when the probe is in position B, the secondary wave finds defect F, set the vertical sound path indicated by the defect on the display H1 2. R is the radius, r is the inner radius, and β is the refractive angle of the probe. Then the actual depth h of the defect is obtained from the equation (6).

8.?Defect quantification

For defect echoes whose amplitude reaches or exceeds the rating line, the amplitude, position, and indicated length shall be determined.

Defect echo amplitude: The maximum reflected amplitude of the defect obtained by moving the probe is recorded as the defect amplitude;

Defect position: The defect position should be based on the position to obtain the maximum reflected amplitude of the defect;

Defect Indication Length: If the defect echo has only one high point and is located in or above Zone II, its indicated length can be measured using the -6dB method; If the peak of the defect echo fluctuates with multiple highs, and they are all located in or above Zone II, ?the indicated length should be measured using the tip -6dB method; If the maximum echo amplitude of the defect is in Zone I, move the probe left and right to lower the amplitude to the rating line, and the defect indicated length is measured with the absolute sensitivity of the rating line.

9.?Defect assessment

The echo exceeding the rating line should be noted whether it has cracks, incomplete fusion, incomplete penetration and other types of defect characteristics, if in doubt, methods such as changing the refractive angle (K value) of the probe, increasing the detection surface, and observing the dynamic waveform should be adopted in combination with the structural process characteristics for judgment; If the waveform cannot be judged, It should be supplemented with other detection methods for comprehensive determination.

If the distance between two defects adjacent to the length of the defect is less than the lesser defect length and the distance between the two defects perpendicular to the defect length is less than 5mm, it shall be treated as a defect, and the sum of the two defect lengths shall be used as its indicated length (spacing is counted). If the two defects are projected in the length direction overlapping, the distance between the left and right endpoints projected by the two defects in the length direction is used as its indicated length.