As critical connecting components in pressure equipment, the internal quality and surface integrity of flanges directly affect the safety of piping systems. Non‑destructive testing (NDT) techniques can effectively identify internal defects and surface cracks without causing any damage to the flange, making them indispensable for flange quality control.
Ultrasonic testing is one of the most commonly used methods for detecting internal defects in flanges. This technique utilises the propagation characteristics of ultrasonic waves in metallic materials; by analysing changes in reflected waveforms, it identifies hidden defects such as inclusions, porosity, and incomplete penetration. In ultrasonic inspections of the largest surfaces of flanges, technicians collect waveform data from each inspection position and perform band segmentation. Based on the waveform at a reference position, combined with the number of waveform bands and variations within the neighbourhood, the anomaly degree is calculated, allowing preliminary identification of suspected defect locations. Ultrasonic testing is highly sensitive to internal cracks and inclusions in forged flanges.
Array eddy current testing demonstrates unique advantages in detecting surface and near‑surface defects in forged flanges. Research has shown that using array eddy current technology to detect surface and near‑surface defects in steel tubular tower forged flanges is not only reliable but also more efficient than traditional penetrant and magnetic particle testing. This technique is particularly suitable for rapid screening of large‑batch flange products.
Radiographic testing (typically X‑ray or gamma‑ray) holds an irreplaceable position in flange weld inspection. However, flanges present difficulties in radiographic examination due to their special geometric configuration. For the circumferential welds of flange pipe stubs, engineers have developed improved inspection procedures that significantly shorten inspection cycles while ensuring radiographic reliability.
Magnetic particle testing and penetrant testing are mainly used for detecting surface‑breaking defects. Magnetic particle testing is suitable for detecting surface and near‑surface cracks in ferromagnetic flanges; penetrant testing is applicable to surface‑breaking defects in flanges of various metallic materials. In the inspection of bolts for high‑pressure steam lead pipes in turbines, technicians consider the structural characteristics, material properties, and schedule constraints, using a combination of longitudinal‑wave straight probes and transverse‑wave angle probes for ultrasonic inspection, with magnetic particle testing for verification, achieving both reliability and efficiency.
Time‑of‑flight diffraction (TOFD) ultrasonic testing is increasingly used for inspecting flange circumferential welds. According to relevant standards, when using a pair of probes, it is recommended that the probe centre spacing be set such that the beam intersection point is located at two‑thirds of the wall thickness depth. This technique has a high detection rate for crack‑type defects in welds.
Different NDT methods each have their strengths and limitations. In practice, multiple methods are often combined to achieve comprehensive quality control of flange products.
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