Surface defects of rectangular tube include:
It exists on the outer or inner surface of the rectangular tube in a straight line or spiral, continuous or discontinuous state. The main reason for the folding is that the quality of the pipe, the folding itself or the surface is poor, there are inclusions, there are serious scratches and cracks, there are edges and corners at the grinding place, and it is folded after stretching. In order to avoid folding, the quality of the pipe should be improved, and attention should be paid to inspection and grinding.
2. Rectangular tube pit
This is one of the surface defects of rectangular tube, which is distributed in local depressions of different areas on the surface of the steel tube, some of which are periodic and some of which are irregular. Dimples are caused by scale or other factors. During the drawing or straightening process, hard dirt is pressed into the surface of the steel pipe, or the existing skin on the steel pipe surface is peeled off. When there are smooth or sharp spiral marks on the outer surface, check the straightener. Due to the incorrect position and angle of the straightening roller, the steel pipe rubs against the edge of the straightening roller during straightening, and there are worn grooves on the shoulder and the straightening roller. When the end of the steel pipe is bent too much, pits may appear on the outer surface of the steel pipe.
3. Pockmarked noodles
It is characterized by small pits on the surface of the steel pipe. The main cause of pitted surfaces is pitting during pickling. After annealing, the iron scale is over-thick and straightened, and then pressed into the surface of the steel pipe to form a pitting surface steel pipe. The storage environment is humid, and even pitting corrosion occurs after water ingress, and pits are formed after the rust spots are removed.
4. Bridge skin
Its characteristic is that the inner and outer surfaces of the rectangular tube are partly separated from the metal matrix, and some are blocky and discontinuous, rooting or not rooting on the tube wall, but cannot be peeled off naturally.
The detection method of surface defect detection of rectangular tube
1. Eddy current testing
Eddy current testing is suitable for checking the defects below the surface of rectangular tube which are generally no more than 6-7mm deep. There are two types of eddy current detection, the placed coil method and the through coil method. If there is a defect on the surface of the casting, the electrical characteristics of the eddy current will be distorted, thereby detecting the existence of the defect. The main disadvantage of the eddy current test is that it cannot directly display the size and shape of the detected defect. Generally, it can only determine the surface position and depth of the defect.
2. Radiographic detection
Generally, X-rays or γ-rays are used as the radiation source, so equipment and other auxiliary facilities for generating radiation are required. When the rectangular tube is placed in the radiation field, the radiation intensity of the radiation will be affected by the internal defects of the casting. The intensity of radiation emitted through the casting varies locally depending on the size and nature of the defect, forming a radiographic image of the defect, which can be visualized and recorded through radiographic film, or detected and observed in real time through a fluorescent screen, or detected by a radiation counter. In addition, the use of a microfocus X-ray system with an approximate point source also virtually eliminates blurry edges produced by larger focus devices, resulting in sharper image outlines. Using a digital image system can improve the signal-to-noise ratio of the image and further improve the image clarity.
3. Liquid Penetration
Liquid penetrant testing is used to check various opening defects on the surface of rectangular tube. The commonly used penetrating test is coloring test, which is to soak or spray the colored liquid with high penetrating ability on the casting surface. The penetrant penetrates into the open defect, quickly wipes off the surface penetrant layer, and then sprays the easy-drying revealer on the surface of the casting. After the penetrant remaining in the open defect is sucked out, the revealing agent is dyed to reflect the shape, size and distribution of the defect. In addition to coloring detection, fluorescent penetrant testing is also a commonly used method for liquid penetrant testing. It needs to be equipped with ultraviolet light for irradiation and observation, and its detection sensitivity is higher than that of coloring detection.
4. Ultrasonic testing
During the propagation of the sound beam with high-frequency sound energy inside the pipe, it is reflected when it hits the internal surface or defects to find defects. The amount of reflected acoustic energy is a function of the directivity and nature of the internal surface or defect and the acoustic impedance of such reflectors. Therefore, the acoustic energy reflected by various defects or internal surfaces can be used to detect the location of the defect, the wall thickness or the depth of the defect under the surface. Ultrasonic testing is a widely used non-destructive testing method. Its main advantages are high detection sensitivity, which can detect small cracks; it has large penetrating ability, and can detect thick sections.
5. Magnetic Particle Testing
It is suitable for detecting surface defects and defects several millimeters deep below the surface. It requires DC magnetization equipment and magnetic powder for detection operations. Magnetizing equipment is used to generate a magnetic field on the inner and outer surfaces of the casting, and magnetic powder or magnetic suspension is used to reveal defects. When a magnetic field is generated within a certain range of the square tube, the defect in the magnetized area will generate a leakage magnetic field. When the magnetic powder or suspension is sprinkled, the magnetic powder is attracted, so that the defect can be displayed.