Welding defects and causes, very important experience!

【Summary Description】 Welding is a key work in the construction of large-scale installation projects, and its quality and efficiency directly affect the safe operation of the project and the manufacturing period. Due to the different level of skilled workers, the welding process is uneven, and there are many defects. The types and causes of defects are sorted out to reduce or prevent the generation of welding defects and improve the quality of the project. 1. weld size is not required The welding wave is thick, the shape is uneven, the weld reinforcement height is too low or too high, and the welding wave width is different. Fillet weld unilateral or excessive sag is the weld size does not meet the requirements, the reason is: 1. The weld groove angle is improper or the assembly clearance is uneven. 2. The welding current is too large or too small, and the welding specification is improperly selected. 3. The speed of the strip is uneven, and the angle of the welding rod (or welding rod) is improper. 2. crack The shape of the crack end is sharp, the stress concentration is serious, and it has a great influence on the alternating and impact load and static tension, which is the most dangerous defect in the weld. According to the causes can be divided into cold cracks, hot cracks and reheat cracks. (Cold crack) refers to the crack produced below 200 ℃, which is closely related to hydrogen, and its main causes are: 1. It is inappropriate to select preheating temperature and slow cooling measures after welding for large and thick workpieces. 2. The selection of welding materials is not appropriate. 3. The welding joint is rigid and the process is unreasonable. 4. The weld and its vicinity produce brittle and hard tissue. 5. improper selection of welding specifications. (Hot crack) refers to the crack (mainly solidification crack) generated above 300 ℃, which is mainly caused: 1. Influence of ingredients. It is easy to appear when welding pure austenitic steel, some high nickel alloy steel and non-ferrous metals. 2. The weld contains more harmful impurity elements such as sulfur. 3. Improper selection of welding conditions and joint forms. (reheat crack) stress relief annealing crack. Refers to the high-strength welding zone, due to post-weld heat treatment or high temperature use, in the heat affected zone produced intergranular cracks, the main reason is: 1. Improper heat treatment conditions for stress relief annealing. 2. The influence of alloy composition. Elements such as chromium molybdenum vanadium boron have a tendency to increase reheat cracking. 3. Improper selection of welding materials and welding specifications. 4. Unreasonable structural design causes large stress concentration. 3. stomata During the welding process, the cavity formed in the weld metal or on the surface due to the gas not being able to escape in time is caused: 1. Welding rod, flux drying is not enough. 2. The welding process is not stable enough, the arc voltage is too high, the arc is too long, the welding speed is too fast and the current is too small. 3. The oil and rust on the surface of filler metal and base metal are not removed. 4. The arc striking point is not melted by the backward method. 5. The preheating temperature is too low. 6. The positions of arc striking and arc extinguishing are not staggered. 7. The welding zone is poorly protected and the molten pool area is too large. 8. The AC power source is prone to stomata, and the stomata tendency of DC reverse connection is the smallest. 4. weld bead During the welding process, the molten metal flows to the weld outside the unmelted base metal to form a metal tumor, which changes the cross-sectional area of the weld and is detrimental to the dynamic load. Its causes are: 1. The arc is too long and the bottom welding current is too large. 2. The current is too large during vertical welding, and the strip is not properly swung. 3. The weld assembly gap is too large. 5. crater There is obvious lack of meat and depression in the end of the weld. Its causes are: 1. The welding arc closing operation is improper, and the arc extinguishing time is too short. 2. During automatic welding, the wire feeding and the power supply are cut off at the same time, without stopping the wire first and then cutting off the power. 6. undercut After the arc melts the base metal at the edge of the weld, there is no replenishment of the weld metal and the gap is left. The undercut weakens the stress section of the joint, reduces the strength of the joint, causes stress concentration, and may cause damage at the undercut. Its causes are: 1. The current is too large, the arc is too long, the speed of the strip is improper, and the arc heat is too high. 2. Submerged arc welding voltage is too low, welding speed is too high. 3. The inclination angle of welding rod and welding wire is incorrect. 7. slag inclusion Non-metallic inclusions are present within the weld metal or at the fusion line. Slag inclusions have an effect on mechanical properties, and the degree of influence is related to the number and shape of inclusions. Its causes are: 1. Each layer of welding slag is not cleaned during multi-layer welding. 2. There is thick rust on the weldment. 3. improper physical properties of electrode skin. 4. The shape of the welding layer is poor, and the groove angle design is improper. 5. The ratio of weld width to weld depth is too small, and the undercut is too deep. 6. The current is too small, the welding speed is too fast, and the slag is too late to surface. 8. incomplete penetration There is local incomplete fusion between the base metal or between the base metal and the deposited metal. It generally exists in the weld root of single-sided welding, which is very sensitive to stress concentration and has a great influence on the performance of strength fatigue. Its causes are: 1. Poor groove design, small angle, large blunt edge and small gap. 2. The angle of welding rod and welding wire is incorrect. 3. The current is too small, the voltage is too low, the welding speed is too fast, the arc is too long, and there is magnetic bias blowing. 4. There is thick rust on the weldment and it is not cleaned up. 5. Welding deviation during submerged arc welding.

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