In the welding process of stainless steel water pipes, porosity and cracks are two common and highly detrimental defects. Their appearance not only affects the density and strength of the weld but can also shorten the service life of the water pipe and even cause safety accidents. To avoid these defects, a comprehensive approach is needed, addressing aspects such as welding materials, process parameters, operating techniques, and environmental control.
Porosity primarily originates from impurities such as oil, rust, and moisture on the surface of the welding materials or workpiece. These impurities decompose at high temperatures, producing gases. If these gases fail to escape completely during the solidification of the molten pool, porosity will form. To avoid porosity, firstly, the welding area must be thoroughly cleaned, including a certain range on both sides of the bevel to remove oil, rust, and moisture, ensuring the weld surface is clean and free of impurities. Secondly, the selection and storage of welding materials are crucial. Welding rods and wires must be dried according to the instructions and properly stored to prevent moisture absorption and deterioration. Furthermore, short-arc welding should be used during the welding process to reduce the chance of air entering the molten pool, while simultaneously controlling the welding speed to ensure sufficient time for the molten pool to release gases.
The causes of cracks are more complex, potentially triggered by welding stress, material composition, and microstructural changes. Based on the temperature and time of crack initiation, cracks can be classified as hot cracks, cold cracks, and reheat cracks. Hot cracks mostly occur during the solidification process of the weld metal and are closely related to the content of harmful impurities such as sulfur and phosphorus. To prevent hot cracks, it is necessary to strictly control the sulfur and phosphorus content in stainless steel water pipes and welding materials, while simultaneously adjusting the chemical composition of the weld metal, improving the weld microstructure, refining grains, increasing plasticity, and reducing segregation. Cold cracks mostly occur when the weld cools to a lower temperature and are related to hydrogen content, hardened microstructure, and welding stress. Preventing cold cracks requires selecting low-hydrogen welding materials, thoroughly removing oil and moisture before welding to reduce hydrogen sources, and simultaneously reducing the hardening tendency and welding stress of the weld through preheating, controlling interpass temperature, and post-weld hydrogen removal treatment. Reheat cracking is related to post-weld heat treatment or high-temperature operating environments. Preventive measures include using low-strength welding materials, reducing residual welding stress, and appropriately selecting preheating and heat treatment temperatures.
In addition to the above targeted measures, the appropriate selection of welding process parameters is also crucial to avoiding defects. Welding current, voltage, speed, and heat input parameters need to be comprehensively determined based on factors such as the water pipe material, thickness, and welding position to ensure a stable welding process and good weld pool formation. Simultaneously, the welding technique also significantly affects weld quality. Welders must be proficient in electrode manipulation techniques, maintain a stable arc, and avoid arc blow or excessively rapid electrode manipulation that could lead to gas retention or stress concentration.
Environmental control is equally important. Welding of stainless steel water pipes should avoid humid, windy, or low-temperature environments, as these conditions may increase the risk of gas intrusion or excessively rapid weld cooling. If welding must be performed in adverse environments, appropriate protective measures must be taken, such as constructing windbreaks and heating the workpiece, to improve welding conditions.
Finally, post-weld quality inspection and defect repair are also crucial aspects of ensuring water pipe quality. After the weld is completed, visual inspection and non-destructive testing are required to promptly identify and repair defects such as porosity and cracks. Any defects exceeding the standard must be completely removed and re-welded to ensure the weld quality meets the required standards.
