There are four main common internal defects in rectangular square tubes: first, slag inclusion, which is an impurity phase or foreign particles surrounded by a solid metal matrix; second, decarburization, which is the surface layer of steel and iron-based alloy materials or parts. The phenomenon of all or part of the carbon in the cast being lost; the third is looseness, which is the non-density caused by the final solidification of the melt in the intergranular area and the release of gas, resulting in the production of many small pores and gases; the fourth is segregation, which is caused by the square Uneven distribution of chemical components in various areas inside the rectangular tube. In addition, bubbles, cracks, delamination, white spots, etc. are also common internal defects.

     For the inspection methods of internal defects of rectangular steel square tube, we will introduce four methods:

     (1) Microscopic examination 

     Also called high-magnification inspection, it is an inspection method in which the prepared square and rectangular tube samples are observed and measured under a phase microscope at a specified magnification to inspect the structure and defects of metal materials. Generally inspect inclusions, grain size, decarburized layer depth, intergranular corrosion, etc.

     (2) Ultrasonic inspection

     Ultrasonic waves are used to propagate linearly in the same uniform medium, but at the interface between two different materials, partial or complete reflection will occur. Therefore, when ultrasonic waves penetrate into materials with pores, cracks, shrinkage cavities, and inclusions, they will be reflected at the metal interface. The larger the heterogeneous interface, the stronger the reflection ability, and vice versa. In this way, the location and size of internal defects can be reflected by the waveform of the fluorescent screen of the flaw detector. Commonly used ultrasonic flaw detection include X-ray and radiographic flaw detection.

     (3) Macroscopic inspection

     Use the naked eye or a low-power magnifying glass below 10 times to observe the internal structure and defects of the square and rectangular tube material. It can be mainly used to detect internal bubbles, slag inclusions, delamination, cracks, coarse grains, white spots, segregation, looseness, and other problems.

     (4) Non-destructive inspection

     Non-destructive testing includes magnetic flaw detection, fluorescent flaw detection, and coloring flaw detection. Magnetic flaw detection is used to detect surface cracks, inclusions, white spots, folds, shrinkage holes, scars, etc. on the ferromagnetic materials of square and rectangular tubes. Fluorescent flaw detection and coloring flaw detection are used to detect small cracks and pores on the surface of non-magnetic materials such as non-ferrous metals, stainless steel, and heat-resistant alloys.