3.1 Weld microstructure
The microstructure near the fusion line, including base metal, HAZ and weld, was shown in Fig.5.The base metal had clearly isometric crystal with the grain size of 40-100μm. The HAZ includes coarse-grain zone with the grain size of 100-300μm and fine-grain zone with that of 40μm.Moreover, the weld metal had significantly anisotropic coarse crystal with the grain size up to 500um.
According to the dominant Rayleigh scattering mechanism[12-13], the wave would be sensitive to a minimum scatter size of λ/10, resulting in scattering of the grains and attenuating the ultrasonic energy. According to the Snell’s law[14], when the acoustic beam propagates between the grains with different orientations, it is easy to be reflected on the interface, forming grain nosie, at the same time, the refraction angle will change, resulting in positioning deviation.
3.2 Sound field of the probe
When the whole elements are excitated under the non-focusing state, the sound field of the LA probe is distributed along the natural angle of 58°of the wedge. The length of the near-field area of the LA probe is about 47.4mm, which is calculated by experience formula [15]


Fig.6 (a) and (b) show the sound field and pressure amplitude curve with natural refraction, respectively. The color scale represents the sound pressure amplitude in Fig.6 (a). There is a maximum sound pressure at a depth of about 27mm in the curve, corresponding to the focal area in the S-scanning.
When a focused depth was set to 30mm, the acoustic beam energy was concentrated in the range of the FD (Fig.7(a)). Fig.7(b) shows the variation of focal pressure with different FD. The maximum sound pressure of focus point decreases gradually with the increase of FD. When the FD increases to 40mm, the maximum sound pressure tends to be stable.
Due to the fluctuation of sound wave, there is an interference effect of sound wave near the focus. The focus of the focused acoustic beam is a cylindrical focusing region rather than a point. During the inspection, the location and the size of the focus influence the detective resolution and positional precision. The maximum sound pressure determines the location of focus, and the size of the focus, including the depth and width,are determined by half of sound pressure. Fig.8 shows the change law of the position through setting different FD. Within 20mm depth, the actual focused depth(actual FD)differs little from the FD to be set. As the FD increases, the actual FD deviation increases and is smaller than the FD. When the actual FD reaches 25mm, it tends to be stable with the increase of the FD. The turning point of 25mm corresponds to the depth of the near field of the sound field mentioned above. Once the FD exceeds the near field, it will lead to deviation in the quantification and location of defects. Fig.9 shows the change law of size of actual focus by setting different FD. As the FD increases, the length and width of the effective focusing region increase, which also indicates that the diffusion of the acoustic beam becomes larger and the directivity of the acoustic beam decreases. Furthermore, the near field and directivity of a probe are related to the structure of the probe itself, such as the number, spacing, size of channels and other designed parameters. These factors are not described in detail here.
To ensure the detection of defects and high efficiency, this paper adopted S-scanning projection focusing mode. According to the above simulation results of sound field distribution, the weld area was covered as much as possible by the sound field of the probe. The focus law parameters in butt weld are listed in Table 2. The horizontal projection distance was set to 20mm, and the scanning angles ranged from 30°to 85°.The horizontal displacement from weld center was set to -10mm. Fig.10 shows the distribution of sound field under the focus law. If one focus law can meet the requirements of weld inspection coverage, it can reduce the number of zone scanning and improve the inspection efficiency. As shown in Fig.10, the acoustic beam can cover the entire thickness of the weld. When the front end of the probe is close to the weld center, the upper surface can be inspected.
Table 2 Focus law parameters in butt weld
focusing distance
|
Scanning angle
range
|
Main beam angle
|
Depth coverage with main beam -6dB
|
Detecting depth
|
20mm
|
30°-85°
|
58°
|
10-34mm
|
0-46mm
|
3.2 Detection results
According to the above detection setting, the reference block was inspected with a perpendicular beam. The S-display was recorded when the amplitude ofφ3mm side transverse hole was up to 80%.At the same time, the SNR of theseφ3mm side transverse holes in different depths was recorded.
Fig.11(a-c) are the S-display of φ3mm side transverse holes with a depth of 11.5mm at different locations of weld, which visually shows the change of the background noise. The horizontal displacement from weld center was set to -10mm. When the amplitude of the three holes with a depth of 11.5mm reaches 80%, the reflected sound pressure values are different on the three locations, although they have the same depth. The background noise is mainly invisible (Fig.11 (a)), when the beam crosses over the base metal, apart from the interference signal on the coupled surface. The noise of the beam across to the weld center (Fig.11 (b)) is comparable to that through the base metal. However, the noise of the beam across the weld to fusion line is serious (Fig.11 (c)). In this case, some minor defects may be missed due to the lower SNR. Fig.12 shows the S-display ofφ3mm side transverse holes with a depth of 34mm at different locations of weld, which visually exhibits the change of background noise.
Fig.13 gives the sound pressure values and SNR of φ 3mm side transverse holes at different depths in all three cases. The results show that the sound pressure values ofφ3mm transverse holes in the fusion line on the opposite side of the weld increased by 7dB compared with that on the side of the base metal. At the same time, the SNR decreased by 8.7dB, the SNR of the beam across the weld to fusion line is only 10.6dB; this is consistent with the increase of the background noise in Fig.11(c). According to the ASME PBVC. V-2017 Standard, the signals with more than 20% of the amplitude need to be recorded and qualitative analyzed [10], suggesting that the SNR cannot be lower than 12dB during the detection.
There are two main reasons why three different positions at the same depth are different. One reason is the scattering due to the coarse microstructure. According to the Rayleigh scattering mechanism mentioned above, the scattering will occur when the weld grain size exceeds 240mm. However, the grain size of GH3535 alloy weld obtained by metallographic observation is up to 500μm. There will be obvious scattering, resulting in the reduction of the penetration ability of the acoustic beam, reducing the amplitude of the reflection echo, and increasing the grain noise. The grain size near the fusion line is coarser than that of base metal. Therefore, the echo and the SNR decrease when the beam crosses over the weld. The other reason is the effectiveness of beam deflection. The beam energy tends to be concentrated near the main beam. With the deflection angle away from the main beam, the energy gradually decreases(Fig.10). According to the distribution of the sound field in Fig.10, the distance between the probe and the fusion line can be shortened so that the acoustic beam energy covers the upper surface of weld.
Fig.14(a) and (b) show the S-display of φ3mm side transverse holes on the fusion line at the horizontal displacement from weld center-10mm and +10mm, separately. The weld was penetrated and covered by the acoustic beam. The SNR of the latter one is significantly higher than the former one. However, the latter one requires the weld reinforcement to be polished smoothly. One half width of the weld is covered by the acoustic beam and the other half needs to be inspected from the other side using the same process.