Rectangular steel tube surface defect detection

Surface defects of rectangular steel tubes include folding, rectangular tube pits, pockmarked surfaces, and bridge skin. This article introduces several methods for detecting surface defects in rectangular tubes.

Methods for Surface Defect Detection of Rectangular Steel Tubes

- Eddy Current Flaw Detection

Eddy current testing is suitable for inspecting defects beneath the surface of rectangular tubes, with a typical detection depth not exceeding 6-7 mm. There are two primary methods: the coil placement method and the through-coil method. If there are defects on the casting surface, the electrical characteristics of the eddy currents will distort, indicating the presence of defects. The main disadvantage of eddy current testing is that it cannot directly display the size and shape of the detected defects, usually only indicating the surface location and depth.

- Radiographic Testing

Radiographic testing typically uses X-rays or gamma rays as the radiation source, requiring radiation-generating equipment and other auxiliary facilities. When the rectangular tube is placed in a radiation field, the intensity of radiation is affected by defects within the casting. The radiation intensity that passes through the casting varies locally depending on the size and nature of the defect, forming a radiographic image of the defect. This image can be visualized and recorded using radiographic film, observed in real-time using a fluorescent screen, or detected with a radiation counter. The use of micro-focus X-ray systems with near-point sources can also significantly reduce the blurring caused by larger focus devices, resulting in sharper image contours. Digital imaging systems can improve the signal-to-noise ratio of images, further enhancing clarity.

- Liquid Penetrant Testing

Liquid penetrant testing is used to detect various surface-opening defects on rectangular tubes. A common method is color testing, where a colored liquid with high penetration ability is applied to the surface of the casting. The penetrant seeps into open defects, and the surface layer of the penetrant is quickly wiped off. A quick-drying developer is then sprayed onto the casting surface. The remaining penetrant in the open defect is drawn out, staining the developer and revealing the shape, size, and distribution of the defect. Besides color testing, fluorescent penetrant testing is also commonly used. This method requires an ultraviolet lamp for illumination and observation, offering higher detection sensitivity than color testing.

- Ultrasonic Testing

When a high-frequency sound wave propagates through the tube, it reflects off the inner surface or defects, thereby detecting these flaws. The amount of reflected sound energy depends on the directionality and nature of the interior surface or defect and the acoustic impedance of the reflectors. Therefore, the sound energy reflected from various defects or internal surfaces can be used to determine the defect’s location, wall thickness, or depth below the surface. Ultrasonic testing is a widely used non-destructive testing method, known for its high detection sensitivity and ability to detect tiny cracks, as well as its deep penetration capability for inspecting thicker sections.

- Magnetic Particle Testing

Magnetic particle testing is suitable for detecting surface defects and defects a few millimeters below the surface. The testing process requires DC magnetizing equipment and magnetic particles. The magnetizing equipment generates a magnetic field on the inner and outer surfaces of the casting, while magnetic particles or magnetic suspension are used to reveal defects. When the rectangular tube generates a magnetic field within a specific range, defects in the magnetized area will create a leakage magnetic field. When magnetic particles or suspension are applied, the magnetic particles are attracted, making the defects visible.