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Method for Detecting “Internal Defects” in Fillet Welds Using Phased-Array Ultrasonic Testing
Release Date:
2022/11/10
Welding is a manufacturing process and technology that joins metallic materials by means of heating, high temperature, or high pressure. Various types of welded joints are widely used in petrochemical plants and play a crucial role in the fabrication of pressure-containing special equipment. Consequently, the quality of welded joints is of paramount importance for the safe operation of such equipment.
Welding is a manufacturing process and technology that joins metallic materials by means of heating, high temperature, or high pressure. Various types of welded joints are widely used in petrochemical plants and play a crucial role in the fabrication of pressure-containing special equipment. Consequently, the quality of welded joints is of paramount importance for the safe operation of such equipment.
1. Introduction to Phased Array Ultrasonic Testing
Phased-array ultrasonic testing utilizes computer-controlled phased-array transducers to generate and steer ultrasonic beams with precise phase adjustments, thereby producing highly flexible and controllable synthetic beams for array scanning of the test object. This technique offers superior detection sensitivity, resolution, and signal-to-noise ratio compared with conventional ultrasonic testing.
2. The advantages of phased-array ultrasonic testing are specifically manifested in the following aspects:
① The inspected images can be displayed using 3D imaging, providing an intuitive visualization of the defect’s three-dimensional location and enhancing positioning accuracy, thereby significantly reducing the risk of misidentification, false positives, and missed defects.
② The reflection of defect echoes and their three-dimensional imaging based on weld structure visualization enable more precise defect assessment, facilitate the differentiation between non-defect signals and defect signals, and are advantageous for inspecting complex geometries and in-service components.
③ The sound-field focusing approach has evolved from segmented dynamic focusing to real-time or adaptive dynamic focusing, thereby enhancing the instrument’s vertical resolution, further improving detection sensitivity, and enabling more accurate defect measurement.
④ There is no need to switch probes for different angles; a single scan along the weld seam is sufficient to inspect the entire weld. Furthermore, simulation and modeling techniques can be employed to process the images and data.
These advantages have led to the widespread application of phased-array ultrasonic testing in the inspection of complex-structured components of pressure-containing special equipment.
3. Fabrication of Welding Simulation Specimens and Acoustic Field Simulation
Prior to phased-array ultrasonic testing of diagonal welds, simulation techniques are employed to determine the optimal distance between the probe and the weld edge, ensuring complete coverage of the entire weld area at various beam angles. During actual inspection, to verify the reliable detection of defects in critical weld regions, artificial-defect welded test specimens can be fabricated for verification trials.
To investigate full-range scanning of fillet welds, achieve complete acoustic-field coverage of the weld, and ensure detection sensitivity and defect detection rate, an L-shaped weld (single V-groove) containing a fusion-line crack defect and a T-shaped weld (K-groove) containing a lack-of-fusion subsurface defect were fabricated. The physical specimens and their structural parameters are shown in Figure 1 and Table 1, respectively.
During inspection, the phased-array probe does not need to be moved back and forth; it is only necessary to determine the distance between the weld and the probe’s front end to ensure that the sound beam fully covers the entire inspection area, after which the probe is swept along the weld direction. A 4 MHz, 16-element probe with a 0.5 mm element pitch and a 36° angled wedge is selected. The phased-array ultrasonic inspection sweep angle ranges from 35° to 69°, with the probe’s front face positioned approximately 45 mm from the weld edge; when the probe is moved forward and backward, this distance increases to about 55 mm from the weld edge, thereby ensuring that both the primary and secondary wave incidence points of the sound beam fully cover the weld and the heat-affected zone. The schematic diagram of sound-beam coverage is shown in Figure 2.
4. Phased Array Inspection Calibration Settings
In phased-array ultrasonic testing, gain compensation across all angles is critical, as the echo reflectivity varies with angle; therefore, DAC (distance-amplitude) curve correction should be performed first.
Sensitivity setting: Refer to the sensitivity settings specified in GB/T 32563-2016. The method for selecting the inspection sensitivity is to adjust the echo from the (diameter × depth) 2 mm × 40 mm transverse hole on the DAC curve, at its maximum sound path, to 40% of the full-screen height.
5. Phased Array Ultrasonic Testing
Phased-array ultrasonic testing is performed on welded simulated test specimens in accordance with the process parameters, and the effectiveness of the phased-array ultrasonic inspection procedure is verified by analyzing and comparing the inspection results.
The ultrasonic inspection image of the L-shaped weld (single-V groove) is shown in Figure 3. From the figure, defects can be clearly identified; their maximum echo location is measured and then compared with the actual defect position on the L-shaped test specimen, revealing that the positions are essentially consistent. During phased-array ultrasonic testing, since the defect is located within the weld, the A-scan signal typically exhibits multiple high-amplitude wave peaks. As the probe is moved vertically or parallelly across the weld, continuous reflected waves are observed; when the probe is swung, a strong bottom-corner reflection signal appears, accompanied by intermittent, weaker end-diffraction signals superimposed on this reflection. Based on these characteristics, the defect can be identified as a crack.
The ultrasonic inspection image of the T-shaped weld (K-groove joint) is shown in Figure 4. The defect can be clearly identified in the figure, and its maximum echo location is accurately measured, with the displayed position essentially coinciding with the actual defect location. During phased-array ultrasonic testing, it was observed in the A-scan display that when the probe is moved laterally, the echo amplitude changes only slightly; the waveform variation is similar to that of an incomplete penetration defect. The A-scan waveform exhibits a smooth, linear rise followed by a decline, with no extraneous small peaks, indicating a smooth reflecting surface. Therefore, the defect can be classified as an unfusion-type defect.
Thus, the developed phased-array ultrasonic testing procedure for simulated weld defects is feasible and capable of quantitatively detecting subsurface defects within the test object.
6. Conclusion
(1) When the probe is positioned appropriately, advanced inspection techniques such as computer-based geometric simulation and three-dimensional imaging can be employed for welded joints with complex geometries, thereby ensuring the detection sensitivity of phased-array ultrasonic testing and the reliable identification of defects at various locations.
(2) When inspecting workpieces with complex geometries, good acoustic beam accessibility is essential. In phased-array ultrasonic testing, a single probe can replace multiple probes used in conventional ultrasonic techniques for scanning, thereby enhancing inspection efficiency and speed.
(3) The use of phased-array ultrasonic imaging technology enables effective identification of defect size, shape, and distribution, thereby enhancing the accuracy of defect characterization.
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