What should be noted when welding square tubes

01. When welding square tubes, it is important to pay attention to the speed. 

If the cooling speed during welding is too fast, cracks are likely to occur. Therefore, during welding, one should avoid getting wet by rain and avoid welding in windy areas. When welding medium or high carbon steel or alloy steel, heating and insulation measures should be taken according to the composition and properties of the raw materials before the welding can be carried out. 

02. When welding square tubes, attention should be paid to the sequence. 

Improper welding sequence can also cause cracks. When the welding sequence is incorrect, it will form a dead knot of welding contraction force, hindering the natural contraction of the weld seam, resulting in large contraction stress and causing weld seam cracks. 

03. When welding square tubes, it is necessary to pay attention to the raw materials. 

The general causes of welding cracks are related to the chemical composition, crystalline structure, and smelting method of the raw materials of square and rectangular tubes; for example, the higher the carbon content of the steel or the higher the alloy content, the higher the hardness of the steel, and it is usually more prone to cracking during welding. 

04. When welding square tubes, it is necessary to pay attention to the surrounding environment. During the welding process, if the temperature of the surrounding area is low or if the ventilation conditions at the site are too good (resulting in rapid heat dissipation), it can also cause cracks. 

Summary: The welding rods used for welding square and rectangular tubes contain high levels of sulfur, phosphorus, and carbon. Weld seams made with such rods are also prone to cracking. Sulfur and phosphorus are harmful elements. Weld seams with high sulfur content have thermal brittleness, while those with high phosphorus content have cold brittleness. The sulfur and phosphorus content in the welding rods must be controlled below 0.0035. Currently, such problems with square and rectangular tubes are mostly related to the welding method and the temperature during welding. Low ambient temperatures and rapid cooling can also cause this phenomenon. Additionally, whether it is a one-time formed straight-out square and rectangular tube or a square and rectangular tube formed by first rounding and then squaring, the square and rectangular tubes will generate internal stress during the forming process. In warm seasons, the existence of internal stress does not cause much trouble, but in cold winter seasons, the presence of internal stress becomes more obvious. For example, cracking may occur during loading, welding, etc. The best solution is to perform heat treatment on the produced square and rectangular tubes (i.e., the heating and holding measures mentioned above). The heat treatment process for square and rectangular tubes involves sampling and testing the square and rectangular tubes to understand their mechanical properties (yield, tensile strength, etc.). Once the test results are obtained, the heating time and temperature in the furnace are set, and the square and rectangular tubes are placed in a carrousel furnace. The furnace temperature changes according to the specified heating system. Heating, holding at a constant temperature, holding, and cooling occur inside the furnace. After heat treatment, the internal stress of the steel pipe can be eliminated, significantly reducing the probability of cracking. 

Tips: When the temperature is very low, do not rush to weld; instead, preheat the area to be welded using a flame or other methods before starting the welding process. This will significantly reduce cracks. (For example, in the cold winter, if boiling water is poured directly into a cold glass, due to the large temperature difference, the glass may crack. If warm water is poured into the glass first to raise its temperature, and then boiling water is poured in, won't the cracking situation be greatly reduced?)