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Welding defects: Common defects in welds include cracks, pores, slag inclusions, lack of penetration, lack of fusion, undercutting, weld beads, depressions, burn-through, misalignment, and welding stress. Cracks: During or after welding, due to the combined effect of residual stresses and other embrittlement factors, the atomic bonds in a localized area of the welded joint are disrupted, resulting in the formation of new interfaces; these lead to local fractures in the weld or in the heat-affected zone of the base material. Thermal cracking occurs during welding, and one of the conditions for its formation is the creation of low-melting-point structures as a result of alloy elements and impurities (mainly C, S, P) during solidification. Thermal cracking can arise when the carbon content in the weld metal exceeds 0.15% or the sulfur content exceeds 0.04%; therefore, it is necessary to strictly control the levels of C, S, P, and other sensitive alloy elements. Cold cracking occurs when the welded joint cools to a certain temperature (near room temperature), and it mainly takes place in the heat-affected zone. Reheat cracking occurs when a welded part is reheated to a certain temperature after welding (such as during post-weld heat treatment, multi-layer welding, or high-temperature operation). It results from the high concentration of stress at grain boundaries due to high-temperature creep and carbide precipitation strengthening, and it generally occurs in low-alloy steels and austenitic stainless steels. Porosity: These are voids that form in the weld as a result of excess gases absorbed by the welding pool at high temperatures, or gases generated by metallurgical reactions that fail to escape before the weld cools and solidifies. Porosity reduces the bearing area of the weld, thereby decreasing its strength and ductility. Slag inclusions: Slag or non-metallic inclusions that remain within the weld metal after welding; their effects are similar to those of pores. Non-fusion and lack of penetration: Non-fusion occurs when the base metal and the weld metal do not melt together in certain areas, or when the weld metal itself does not melt together ； Underwelding is a phenomenon in which the base metal at the root of the weld joint is not fully melted, and it occurs at the root of the groove. Based on the location where lack of fusion occurs, it is commonly classified into root fusion deficiency, groove fusion deficiency, and interlayer fusion deficiency. Root fusion deficiency refers to the defect in which the weld metal and the base metal do not fuse together at the root edge of the groove ； Weld joint lack of fusion refers to the defect in which the weld metal and the base metal at the sides of the groove do not melt together to form a single entity ； Interlayer lack of fusion is a defect in multi-layer welding, where the weld metals of different layers do not melt together to form a single unified structure. Non-fusion and lack of full penetration can lead to stress concentration, which is highly hazardous; such conditions are generally not allowed in welds. Undercutting: A depression that forms on the surface of the workpiece along the edge of the weld, resulting from the failure to fill the joint with molten metal in a timely manner after the metal there melts. Edge biting is mainly caused by improper welding parameters (such as excessive current or an overly long arc) and incorrect operation (such as an incorrect electrode angle). Under vertical, horizontal, and overhead welding conditions, undercutting is likely to occur. When edge biting is severe, stress concentration occurs, which can easily lead to cracks and reduce the area of the weld, thereby decreasing the strength of the workpiece. On important components, burrs are generally not allowed. Weld bead: A metal lump that forms when molten metal flows outside the weld area and onto the unmelted base material during welding. The main cause of weld beads is inadequate skill in operation. Depression: A portion on the surface of the weld that is lower than the base material. Such as the arc pit caused during arc termination. Burn-through: As the melting depth exceeds the thickness of the base metal, the molten metal flows out from the back side of the groove, resulting in a penetration defect. The main reasons for this defect are excessive welding current, too slow welding speed, and excessive groove gap. Misalignment: Welding is carried out forcibly due to the failure of the base materials on both sides of the weld to be aligned, resulting in non-parallel center lines. Welding stress: The welding process generates significant residual stresses, as well as deformations in various directions. Welding stress can be classified into three categories based on its underlying causes: first, the internal stress resulting from uneven expansion and contraction during the uneven heating and cooling in the weld zone and heat-affected zone, known as thermal stress ； The second is structural transformation, especially during phase changes, where structural stress arises due to volume changes. Third are the restraining stresses caused by the constraints of the welding structure itself. When the stress generated during welding is high enough, it can lead to welding hot cracks under certain conditions ； The residual stresses resulting from welding can cause welding cold cracks in hardenable steels, and may even lead to brittle fracture of the structure. When welding stress is high enough, it can cause deformation or cracks.