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The development of science and technology inevitably leads to innovations in applied technologies. The widespread application of ultrasonic technology in medicine, **, aerospace, scientific research, and construction has facilitated the rapid development of several new detection and application technologies. This paper provides a detailed discussion and research on the application of ultrasonic testing technology in defect identification, hoping to offer certain references for the identification applications of ultrasonic testing technology. 1. The significance of defect identification using ultrasonic testing technology. Ultrasonic testing technology is one of the most widely used non-destructive testing methods today. Its applications offer a range of advantages, such as high sensitivity, excellent penetrability, fast detection speed, portability and ease of use, as well as being harmless to the human body. In the application of ultrasonic testing in construction, it has a significant advantage in terms of its ability to penetrate steel materials, and is primarily used for detecting thick steel plates and welds. Regarding defects on the surface of steel plates, although some defects are quite deep, as long as ultrasonic waves can directly reach the defect interface, very clear defect echoes can be obtained. Therefore, ultrasonic testing technology holds great significance in the inspection of pressure vessel welds and the detection of high-risk defects such as underpenetration cracks. 2. Identification of defects in ultrasonic testing (1) Detection and identification of planar defects For planar-type defects, when detected from different directions, the height of the defect echoes also varies significantly. When detection is performed perpendicular to the plane of the defect, the defect echo is relatively high ； When detecting defects on parallel planes, the defect echoes are low, and in some cases there are no defect echoes at all. Therefore, for crack-type defects, ultrasonic flaw detection typically yields relatively large echo heights, with wide amplitudes and multiple peaks. When the probe is translated, a continuous phenomenon of reflected waves occurs, and the wave amplitude also varies accordingly ； By rotating the probe, it can be observed that the wave peaks shift up and down, and these variations can all be used as a basis for detecting planar defects. (2) Detection and identification of point defects: In terms of direction, the echo from point defects does not show significant changes; its waveform remains stable, and the amplitude of the reflected waves detected from different directions is roughly the same. However, in actual testing, once the probe is moved, the echo may disappear. Depending on the differences in impedance of inclusions in different materials, the manifestation of ultrasonic flaw detection also varies. Pores usually contain gas, which has a low acoustic impedance and a high reflectivity, resulting in a sharp and steep waveform ； Defects of the metal inclusions or non-metallic inclusions type have a higher acoustic impedance, which results in lower reflected waves; when the surface of the inclusions is rougher, the waveform becomes wider and takes on a serrated shape ； The amplitude of the reflected waves, where the pores are more dense, varies depending on the size of these pores; as the probe moves in a fixed pattern to carry out measurements, the amplitude shows fluctuations. (3) Detection and identification of burr defects: The ultrasonic detection and identification of burr defects is mainly reflected in the reflected waves; generally, the reflected waves associated with this type of defect appear in front of the primary and secondary waves. This phenomenon can be observed during the inspection process whenever the probe scans both sides of the weld; when the probe is moved to a position where the strongest reflection signal is generated, fixing the probe allows the detection sensitivity of the instrument to be reduced appropriately. Use a finger dipped in some oil to gently tap the edges of the weld where undercutting occurs, and observe the reflected signal. When there is a noticeable fluctuation in the reflected signal, it indicates that it is an undercutting-related reflection, confirming that the defect type is undercutting. (4) Detection and identification of crack defect types: Generally, cracks exhibit high echo heights, wide amplitudes, and a multi-peak pattern. By translating the ultrasound probe, the reflected waves are observed to appear in a continuous manner, with the amplitude varying to some extent ； When rotating the probe for testing, the wave peaks shift up and down. Furthermore, crack defects also tend to occur in the heat-affected zone of welds, and in most cases these cracks are perpendicular to the weld. When conducting inspections, it is necessary to examine in a direction parallel to the weld, so that ultrasonic waves can reach the cracks more easily, facilitating the detection of such defects. (5) Detection and identification of underfill defects: Underfill defects occur mainly because the weld metal does not fill the root area of the joint. This type of defect is mainly found in the root area of the weld; its ends are blunt, it has a certain length, and it also belongs to the category of planar defects. When performing translational testing with the probe, it is found that the waveform of the reflected wave from incomplete penetration defects is relatively stable ； When performing flaw detection on both sides of the weld, reflections with relatively consistent amplitude are generally obtained, which enables the identification of the type of defect. (6) Detection and identification of the lack of fusion weld defect type. The so-called lack of fusion weld defect type refers to defects that occur when the weld bead and the base material, or between weld beads, are not fully melted and bonded during the welding process. When using ultrasound for flaw detection, ultrasonic waves can be directed perpendicularly onto a surface, resulting in echoes with high amplitudes. However, in actual inspection processes, if the flaw detection method and the choice of refraction angle are not appropriate, it may also lead to missed detections. The characteristics that serve as the basis for detecting, identifying, and judging defects in unfused fusion welding are as follows: when the probe is moved for inspection, the waveform remains relatively stable ； When probing both sides, the amplitude of the reflected waves changes, and there are cases where detection is only possible on one side. 3. Identification of pseudo-defect types (1) Instrumental noise-type pseudo-defect wave patterns. These pseudo-defect waves usually occur when no probe is connected, as a result of poor performance of the equipment or instruments, as well as excessive adjustment of the probe’s sensitivity; they appear on the fluorescent screen as single-peak or multi-peak waveforms. When the probe is connected and in use, the position of this waveform on the fluorescent screen remains unchanged; such false defect waves can be eliminated by reducing the sensitivity of the probe. (2) Pseudo-defect waves caused by grooves on the weld surface. The defect waves resulting from grooves on the weld surface are mainly reflected waves; when using ultrasonic waves to inspect the weld surface, groove reflection waves are generated due to these surfaces grooves. This type of waveform generally appears at the position where the first or second wave is delayed; it is not intense, but rather gentle and sluggish in nature. (3) Pseudo-defect waves caused by overlapping welds: During the preparation of grooves in steel materials, issues of weld misalignment can arise due to asymmetry in the cutting operations or deviations during welding. As a result of the misalignment of the upper and lower welds, during ultrasonic testing, the reflection waves from the weld corners resemble those of actual weld defects. However, by conducting testing on the other side, no reflection waves appear in the primary wavefront, which can be used as a standard to avoid misdiagnosis. Conclusion In summary, there are numerous causes for defects, and a wide variety of defect types exist; different defect types exhibit distinct characteristics during ultrasonic flaw detection. However, similar situations may also occur in some cases. Therefore, in the practical application of ultrasonic flaw detection techniques, it is necessary to continuously gain experience. During actual inspections, one must be familiar with the various detection methods, echo patterns, and reflection wave characteristics of different defect types, in order to accurately identify the defect type and provide guidance for taking appropriate corrective actions.