The location, continuity, time range and orientation of welded tube defects are various, and the height and amplitude of the reflected wave of ultrasonic waves are also different, which is related to the production process and testing equipment.  The A-type pulse reflection ultrasonic detection can only provide the time and amplitude information of the defect echo. According to the shape, size, density and other defect characteristics of the defect echo and the bottom wave situation, the relative evaluation and evaluation of defect characteristics such as planar defects, point defects, dense defects and strip defects can be carried out.

1. Planar defects

The detection is carried out in two different directions, vertical and horizontal, on both sides of the weld. The height of the defect echo is significantly different and changes irregularly, and the height of the bottom wave has no obvious change. When the defect echo is strong and the bottom wave disappears, it can be considered as a large-area defect. The detection in the direction perpendicular to the defect shows a single sawtooth echo, the defect echo is relatively high, and the waveform is obviously sharp and steep. When the probe moves, the echo amplitude fluctuates randomly (amplitude difference>±6dB). In the detection direction parallel to the defect, the defect echo is low, or even no defect echo. Detection oblique to the defect direction shows a bell-shaped pulse envelope in which there is a series of continuous signals. Usually manifested as intense multi-spikes with variable (but not large) positions, and many small peaks, when the probe is moved. Each small wave peak moves in the pulse envelope, the amplitude gradually rises from zero to the maximum value, and then drops to zero again, and the signal amplitude fluctuates randomly (>±6dB). According to the different heights of the defect echoes in the longitudinal and transverse directions, and the characteristics of irregular changes, it can be evaluated as a planar defect.

Common planar defects include cracks, planar incomplete fusion, planar incomplete penetration and other defects. Such defects have length and obvious self-height, and the surface is both smooth and rough.

2. Point defects

The detection is carried out in two different directions, vertical and horizontal, on both sides of the weld, and the echo equivalent of the defect is small and not necessarily high. Defect indication length ΔL≤t (t is wall thickness), no obvious change in height, and no obvious change in bottom wave height. When defect wave and bottom wave coexist, it can be considered as point defect or other defects with small size. The defect echo shows a light circular wave (sharp echo), which is the waveform characteristic of a point defect smaller than the diameter of the sound field, and with the movement of the pipe body, the fluctuation of the defect echo changes greatly and rapidly. Keeping the sound path distance constant, it can be judged as a point defect based on the fact that there is no obvious change in the height of the defect echo in the longitudinal and transverse directions, and it shows a feature of a light circular wave.

The common point defects include pores, small slag inclusions and other small defects. Most of these defects are spherical volume defects, and there are also irregular shapes, which belong to small volume defects and can appear in different positions in the weld.

3 .dense defects

The inspection is carried out in two different directions, vertical and horizontal, on both sides of the weld. The defect echoes appear in different positions, and the display order is irregular. Each individual echo signal shows a single sharp echo, while the bottom wave disappears or its height decreases. When the probe moves to detect at different positions, the echo signal shows a group of dense defect echoes. The defect waves are densely connected to each other, with different heights, and the reflected signals come and go, fluctuating high and low. If they can be distinguished, each individual echo signal shows the characteristics of point defects. According to the position and display sequence of the defect echoes in the longitudinal and transverse directions are irregular, and the characteristics of the continuous disappearance of the back wave or the reduction of the back wave amplitude to less than 50% can be evaluated as dense defects.

Common dense defects include dense pores, reheat cracks and other defects. This kind of defect is a collection of a group of defects. Each small defect is very close to each other, and it is impossible to individually locate and quantify each small defect.

4 strip defects

The detection is carried out in two different directions, vertical and horizontal, on both sides of the weld. The defect echo amplitude is usually high and the shape is regular and single, the height is roughly the same and there is no obvious change. In a large range, defect echoes appear continuously. And at the same position, there is no obvious change in the height of the bottom wave, as long as the signal is not obviously disconnected by a large distance, the defect is basically continuous, and the indicated length of the defect can be measured ΔL. The peak value of the defect echo rises steadily from zero to the peak value, and maintains a straight section, and then smoothly decreases from the peak value to zero, which can be detected from both sides of the weld. According to the fact that the echo heights of defects in the vertical and transverse directions are approximately the same without significant changes, and the echo peaks rise and fall steadily, it can be judged as a strip defect.

Common strip-shaped defects include strip-shaped slag inclusions, incomplete penetration, incomplete fusion and other defects. The indicated length of such defects can be measured, but the cross-sectional size (height and width) is not easy to measure. However, it may also be intermittent in the length direction, such as chain slag inclusions, intermittent incomplete penetration, intermittent incomplete fusion, etc.