304 square tube is a common stainless steel material used in a wide range of applications. However, cracks and other weld defects during welding can seriously affect its quality and performance. To avoid these defects, multiple aspects must be carefully controlled.
Pre-welding preparation is crucial. First, thoroughly clean the weld area of the 304 square tube. 304 stainless steel may have impurities such as oil, rust, and scale on its surface. These impurities can affect the purity of the weld pool during welding and increase the risk of cracks and other defects. Use a stainless steel cleaner or organic solvent to wipe the square tube surface, then polish with a wire brush or sandpaper to ensure the weld area is clean and free of impurities. Also, check the dimensional accuracy and straightness of the square tube. Any bends or excessive dimensional deviations should be corrected to ensure a good fit during welding. This avoids stress concentration caused by excessive or uneven gaps, which can lead to cracks.
Selecting the right welding material is crucial. When welding 304 square tube, welding consumables should be compatible with the base metal's composition to ensure the weld metal possesses similar mechanical properties and corrosion resistance. Improper welding consumables, such as those with excessively high carbon content or other mismatched elements, can easily form a brittle martensitic structure in the weld during the cooling process, increasing the tendency to crack. Furthermore, the quality of the welding consumables is crucial. Ensure the surface is free of defects such as rust and oil, and dry the consumables according to specified requirements to remove moisture and prevent defects such as porosity during welding.
Properly setting welding process parameters plays a crucial role in preventing cracking. Welding current, voltage, and speed are three key process parameters. Excessive welding current increases heat input, leading to overheating of the weld metal and coarsening of grains. This also increases weld deformation and makes thermal cracking more likely. Excessive current, on the other hand, results in arc instability and insufficient penetration, which can easily lead to defects such as incomplete fusion. Excessively high voltage will cause the arc to be too long, increasing spatter and reducing weld quality. Excessively low voltage will cause the arc to be too short and the weld width to be narrow, also negatively impacting weld quality. Welding at too high a speed will accelerate the cooling of the molten pool, potentially leading to cold cracks. Welding at too slow a speed will widen the heat-affected zone, increasing the likelihood of weld deformation and cracking. Therefore, during actual welding, appropriate welding process parameters must be determined through experimentation based on factors such as the thickness and groove shape of the 304 square tube and must be strictly controlled during the welding process.
Welding technique is also crucial. Maintain smooth and precise movements during arc striking and arc termination. Striking the arc should be performed within the groove to avoid arbitrarily striking the arc on the parent material surface, which can cause arc crater cracks. When terminating the arc, ensure that the arc crater is completely filled to prevent defects such as shrinkage cavities. During welding, maintain consistency in the angle between the welding gun or electrode and the square tube, as well as in the welding direction, to ensure a uniform and aesthetically pleasing weld. Observe the shape and color of the molten pool and adjust welding parameters and technique accordingly.
The welding environment also has a certain impact on weld quality. Welding in a humid or windy environment can easily cause defects such as porosity in the weld. Therefore, welding should be performed in a dry, windless environment whenever possible. If adverse conditions cannot be avoided, appropriate protective measures should be taken, such as erecting a windproof shelter over the welding area and preheating the weldment.
Post-weld treatment is equally important. After welding, the weld should be visually inspected to promptly detect and address surface defects such as cracks, porosity, and slag inclusions. For welds with higher requirements, non-destructive testing such as radiographic and ultrasonic testing is also required to ensure internal weld quality. Additionally, appropriate post-heat treatment can be performed to eliminate residual weld stress and reduce the risk of cracks.
