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Welding defects ⑴ Local deformation and cracks Welding deformation occurs when the welding stress exceeds the yield limit； Cracks will occur when the strength limit is exceeded. ⑵ External defects of the weld: Excessive weld reinforcement, too small an angle of the groove, or insufficient welding current ； It causes stress concentration. To improve fatigue life, the reinforcement height of the weld should be leveled. An overly concave weld reduces the working cross-section of the weld, thereby decreasing the strength of the joint. Undercutting: A depression forms on the workpiece along the edge of the weld. It reduces the working cross-section and causes severe stress concentration. Weld beads have no effect on static load strength, but they cause stress concentration, which reduces dynamic load strength. Through-burning: Some of the molten metal leaks out from the opposite side of the weld, and in severe cases a hole is formed, reducing the strength of the joint. ⑶ Internal defects in the weld: incomplete penetration, severe stress concentration, which reduces strength and is a cause of cracking. Slag inclusions reduce the working cross-section, cause stress concentration, and diminish the strength and impact toughness of the weld. Stomata reduce the effective working cross-section and decrease mechanical strength. Based on the time of occurrence, welding cracks are divided into hot cracks and cold cracks. Thermal cracking occurs during the crystallization process from liquid to solid state. The weld contains low-melting substances such as FeS, which can cause cracking between the grains under high welding stresses. When weldments and electrodes contain large amounts of impurities such as S and Cu, thermal cracks are likely to occur. Thermal cracks are characterized by their distribution along grain boundaries. Cold cracks are formed during the cooling process. Due to the formation of a hardened microstructure in the heat-affected zone or weld, high stress can cause cracking within the grains. Cold cracks are most likely to occur when welding steel materials that are prone to hardening, such as those with high carbon content or many alloying elements. An excessive amount of hydrogen incorporated into the weld can also cause cold cracks. In addition, there are also reheat cracks and others. Cracks are the most dangerous type of defect; their presence is generally not allowed. Once detected, they must be removed and the area rewelded. Welding inspection includes: visual inspection, non-destructive testing, and mechanical property tests (hydrostatic and pneumatic testing), with non-destructive testing being the primary method. ⑴ Visual inspection: Observe with the naked eye or under a 5–20x magnifying glass. Surface defects in the weld can be detected, such as undercutting, weld beads, surface cracks, pores, slag inclusions, and weld penetration. The external dimensions of the weld can be measured using a weld inspection device or a template. ⑵ Non-destructive testing: Defects such as internal inclusions, pores, and cracks are inspected using X-ray testing, ultrasonic testing, magnetic testing, and other methods. For internal defects not deep beneath the weld surface and surface microcracks, magnetic testing and penetrant testing can be used. X-ray inspection involves using X-rays to take photographs of welds, and determining internal defects based on the images on the film. Then assess whether it meets the technical requirements. Ultrasonic testing uses the principle of ultrasonic wave propagation to detect internal defects. Then assess whether it meets the technical requirements. Ultrasonic flaw detection is simpler than X-ray radiography and is widely used. But it is based solely on experience, and no permanent evidence can be left.