The internal quality of welded parts directly affects their structural strength, sealing performance, and operational safety. Therefore, detecting internal welding defects is an indispensable and crucial step in the welding process. Common welding defects include porosity, slag inclusions, lack of fusion, cracks, and incomplete penetration. These defects may be hidden inside the weld and are difficult to detect through visual inspection, requiring precise identification using specialized non-destructive testing (NDT) techniques.
Radiographic inspection is a classic method for detecting internal defects in welding parts. Its principle involves using X-rays or gamma rays to penetrate the weld. The difference in radiation attenuation is recorded using film or digital imaging equipment, thus forming an image of the internal structure. Porosity typically appears as circular or elliptical dark spots in radiographic images, slag inclusions appear as irregularly shaped dark areas, and cracks may appear as thin, linear defects. The advantages of radiographic inspection are its intuitive imaging, long-term preservation of results, and suitability for thicker welding parts. However, radiographic inspection carries radiation safety risks, requiring strict adherence to operating procedures and performance in a dedicated protected environment. It also demands a high level of professional qualification from the inspectors.
Ultrasonic testing is another widely used internal defect detection technology. It identifies defects by utilizing the propagation characteristics of high-frequency sound waves in welds. When a sound wave encounters a defect interface, it is reflected. The reflected signal is received by the probe, converted into an electrical signal, and processed to form a waveform. Porosity appears as a short, sharp peak signal in the ultrasonic waveform, while slag inclusions produce a more chaotic signal, and cracks may trigger multiple reflections. Ultrasonic testing has high sensitivity, can detect minute defects, and poses no radiation risk, making it suitable for on-site inspection. However, this method requires highly skilled operators who need training to master waveform interpretation techniques, and it also has certain limitations regarding the surface roughness and shape of the parts.
Magnetic particle testing is mainly used for welding parts made of ferromagnetic materials. By magnetizing the weld area, a leakage magnetic field is generated at the defect, attracting magnetic particles and forming visible traces. This method is highly effective in detecting surface and near-surface defects such as cracks and lack of fusion. It is simple to operate and has a low cost. Magnetic particle testing can be divided into wet and dry methods. Wet testing has higher sensitivity and is suitable for precision parts; dry testing is suitable for large structural components. However, magnetic particle inspection can only detect ferromagnetic materials and has limited ability to detect deep internal defects, usually requiring combination with other methods.
Penetrating inspection is suitable for detecting surface opening defects in non-porous materials. Its principle is to use capillary action to allow penetrant to seep into the defect, which is then revealed by a developer. This method can detect surface defects such as tiny cracks and pores, is flexible in operation, and is suitable for complex-shaped parts. The penetrant inspection process includes pre-cleaning, penetrant penetration, cleaning, development, and observation. Each step requires strict time and operational procedures to ensure the accuracy of the results. However, this method cannot detect internal defects and has a long inspection cycle, requiring combination with other internal inspection methods.
Eddy current testing utilizes the principle of electromagnetic induction. An alternating magnetic field generates eddy currents in conductive materials. Defects alter the eddy current distribution, which is then sensed by the detection coil. This method is suitable for detecting surface and near-surface defects in conductive materials, such as cracks and holes. It offers fast detection speed and does not require a coupling agent, making it suitable for automated production line applications. However, eddy current testing is sensitive to material conductivity, is only applicable to metallic materials, and has limited ability to detect deep defects, usually requiring combination with other methods.
Acoustic emission testing is a dynamic testing technique that monitors the internal state of welding parts in real time by capturing stress wave signals generated by defect propagation during material stress. This method is suitable for online monitoring of in-service equipment, and can detect dynamic defects such as crack initiation and propagation in advance, providing a basis for preventive maintenance. Acoustic emission testing has high sensitivity, but is easily affected by environmental noise and must be performed in a quiet environment. Furthermore, the accuracy of defect location needs to be verified in conjunction with other methods.
Combining multiple testing methods is an effective strategy to ensure the internal quality of welding parts. For example, for important structural components, X-ray or ultrasonic testing can be used for comprehensive screening first, followed by surface defect inspection using magnetic particle or penetrant testing, and, when necessary, acoustic emission monitoring can be combined to monitor the in-service condition. Through the synergy of multiple technologies, the detection rate of welding defects can be comprehensively improved, providing multiple guarantees for the reliability of welding parts.
