The ultrasonic detection signal of the chamfered weld is easy to be confused with the lack of penetration and lack of fusion. How to distinguish it?

When pipe butt welding, it is common to chamfer the inner wall of the pipe. When performing ultrasonic testing on this type of weld, the root reflection often shows a "double peak" waveform. To determine this waveform, it is necessary to comprehensively determine the basic information and detection methods such as the pipeline thickness measurement data, the weld groove structure, the corresponding depth when the two reflection peaks are the largest, the horizontal positioning of the reflection position, and the different detection positions, etc., to avoid errors. Sentenced.

Pipe butt welds often have two pipes with different thicknesses being welded together. In order to ensure the same thickness on both sides of the weld, it is necessary to chamfer the inner and outer walls of the pipe on the thicker side.

Due to the presence of chamfers, the signal characteristics of ultrasonic testing are sometimes similar to those of incomplete penetration and non-fusion defects, which may easily cause misjudgment.

So, what is so special about the ultrasonic signal with chamfered welds that allows us to make better judgments?

★Structural characteristics of the groove of the pipe butt joint★

According to DL/T 869-2012 "Welding Technical Regulations for Thermal Power Plants", when the inner wall is not equal but the outer wall is required to be flush, the groove shall be processed according to the two structures shown in the figure below (δ1 in the figure is the thickness of the weldment on the thinner side) , Δ2 is the thickness of the weldment on the thicker side).

Schematic diagram of the processing of the butt groove of the weldment of different thickness

When processing the groove in accordance with the regulations, the distance from the obtuse edge on the thicker side to the inflection point of the chamfered bevel edge (the horizontal section distance) is not less than δ1. When the ultrasonic testing method is used to detect this kind of grooved pipe, the reflected signal of the root defect and the reflected signal of the normal structure are relatively easy to distinguish.

The processing of the groove structure of the butt weld of the pipeline of the thermal power plant is not carried out in accordance with the regulations. The actual machining will be chamfered directly at the starting point of the blunt edge.

As shown in the figure above, this structure has no horizontal sections, only chamfered edges; and due to the processing of the pipe itself, the thickness distribution along the pipe circumference is not uniform; in order to ensure the same depth of the molten pool during butt welding, the circumference direction The chamfer depth L2 on the upper side will not be the same, resulting in the chamfering hypotenuse distance L1 is also not the same. When the distance L1 is small and closer to the root of the weld, it will bring difficulties to ultrasonic inspection; if the ultrasonic inspectors are not experienced enough, this structure wave will be misjudged as a defect wave.

★Ultrasonic signal characteristics of pipe welds with internal chamfers★

Oblique probes are often used in ultrasonic inspection of pipe butt welds. Since the oblique probe uses the longitudinal wave oblique incidence, the longitudinal wave is obliquely incident into the second medium to generate transverse waves. The sound beam and the sound beam axis are not symmetrical, but there are two upper and lower half-diffusion angles, of which the upper half-diffusion angle θ is greater than the lower one. Under the half-diffusion angle θ.

When performing ultrasonic inspection on butt welds with chamfered pipes, according to the difficulty of identifying the reflected echo, it can be divided into two situations:

One case is the structure shown in the figure above. The distance between points B and C in the figure is ΔL. When point B is on the left side of point C, it is recorded as ΔL>0;

Another situation is that point B is on the right side of point C, which is not marked in the figure above, and is marked as ΔL<0. The signal characteristics of these two situations will be discussed separately below.

When ΔL>0

The position of point A and point B are relatively close. At this time, the thickness measured by the straight probe at the edge of the weld is H2, and the actual root reflection is at point A. This situation is more complicated and easy to misjudge.

When the probe moves from position 1 to position 2 during detection, when the sound path S1 reaches the root position B for the first time, the reflected wave will be displayed on the screen of the ultrasonic instrument.

When the instrument is adjusted according to the depth 1:1, the display depth of the reflected wave at this point is S1/cosθ. Since S1>S2, S1/cosθ>S2/cosθ=H2, that is, the depth of the reflected wave displayed on the screen is greater than H2.

As the probe continues to move, when the central sound beam reaches point B, that is, when the sound path S2 reaches point B, the reflected wave amplitude displayed on the instrument screen is the highest, and the depth is displayed as H2.

Continue to move the probe so that when the sound path S3 reaches point B, the depth displayed on the instrument screen is S3/cosθ. Since S2>S3, S2/cosθ>S3/cosθ can be obtained, that is, the depth of the reflected wave displayed on the screen is less than H2.

When the probe moves from position 1 to position 2, the display characteristics of the reflected wave at point B on the screen of the instrument are: the depth range is from large to small, and the height of the reflected wave amplitude gradually increases, and then gradually decreases after increasing to the maximum. ; When the sound beam center position S2 reaches point B, the reflected wave amplitude is the highest, and the depth is displayed as H2, which is the largest, and then gradually decreases.

Similarly, when the sound paths S1, S2, and S3 reach point A at the root of the weld in sequence, the reflection characteristics are the same. When point A and point B are in the coverage of the sound beam at the same time, two reflection amplitudes will be displayed on the screen of the instrument at the same time, that is, the "double peak" structure.

According to the geometric principle, the sound path to point A is always greater than the sound path to point B. Therefore, the peak at the larger depth displayed on the screen is always the reflected wave from point A, and the peak at the smaller depth is the reflected wave from point B.

To determine the reflection waveform of this structure, it is necessary to find the depth corresponding to the maximum reflection amplitude of the two points A and B respectively, which are respectively recorded as HA and HB. The relationship between the two should satisfy HA＜HB, and the difference depends on the thickness of the chamfer. , Which is L2. Then move the probe to the opposite position 3. At this time, a reflected wave will be formed at point A, and a single reflected waveform will be displayed on the screen. The depth corresponding to the maximum reflected wave amplitude should be similar to that of HA.

And when the longitudinal sound beam coverage of the straight probe is AB slope, the straight probe cannot measure the specific value at the edge of the weld. If you continue to move the probe to the side away from the weld, the value H2 can be measured. Because point A and point B are far apart, the sound beam The effective range cannot cover two points at the same time, and the reflection waveform of the "double peak" structure will not appear, so it is easier to judge the reflection characteristics of this chamfered structure, so I won't repeat it here. When ΔL＜0

As shown in the figure above, when the probe is at position 1, and point A at the root of the weld and point B at the blunt edge are both within the coverage of the sound beam, a root reflection is formed at point A at the root of the weld, and point B forms an end angle reflection. The instrument screen The "bimodal" structure will also be displayed.

Move the probe to find the depths corresponding to the maximum reflection amplitudes at points A and B, respectively, and mark them as HA and HB, where HB is equal to the thickness of the workpiece H1, and the size of the two should be similar or HA slightly larger than HB.

Then move the probe to the opposite side, as shown in position 2 in the above figure, a reflected wave will be formed at point A at the root of the weld; at this time, the instrument screen will display a single reflected waveform, and the depth corresponding to the highest wave amplitude should be close to HA.

Similarly, comprehensive comparison and analysis based on the reflection waveform characteristics on both sides of the weld and the depth corresponding to the highest reflection amplitude can determine the unfused reflection characteristics of the pipe butt weld root.

As shown in the figure above, when the probe is at position 1, and point A and point B of the root of the weld are both in the coverage of the sound beam, points A and B will form a reflected signal, and the reflected signal characteristics are also "double peaks." structure.

Move the probe to find the depth corresponding to the maximum reflection amplitude of the two points A and B, respectively, and record them as HA and HB, where HB is equal to the thickness of the workpiece H1, and the relationship between the two should be HA<HB.

Then move the probe to the opposite side, as shown in position 2 in the figure above, the root A and the blunt edge C of the weld will form a reflected wave, and the reflected signal characteristic is also a "double peak" structure, and the maximum reflected wave amplitude at the two points corresponds to the depth The relationship is similar to that of the contralateral position 1.

The ultrasonic detection signal characteristics of the inner chamfering, root unfused and incomplete welding of the pipe belt have certain similarities and differences.

The same point is that they all have a "bimodal" structure

The difference is that the "double peak" corresponds to the highest reflection amplitude and depth, and the signal characteristics and depths corresponding to different detection positions are also different.