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Keywords: welding; Common defects; preventive measure
First, the common defects in welding
1, welding crack defect
Cracks are serious defects in the welding process. Welding crack refers to the gap caused by the destruction of atomic bonding effect between welds in the welding process, which makes the welds form a new interface. According to the size of cracks in the welding process, weld cracks can be divided into macro cracks, micro cracks and micro cracks, in which macro cracks can only be found by naked eyes, while micro cracks refer to cracks that can only be seen under a microscope, and the last cracks can only be seen under an ultra-precision microscope, and can be divided into hot cracks and cold cracks according to the different crack formation temperatures. At the same time, according to the different mechanism of crack formation, it can be divided into layered tearing and stress corrosion cracking. Cracks formed in the welding process, especially cold cracks, have a disastrous impact on the performance of welds, which is most obvious in pressure vessel accidents. Through the statistics of pressure vessel accidents, it is found that apart from unreasonable design and material selection, the accident rate caused by cracks in welding seam is the largest.
2. Porosity phenomenon and its causes
When welding enters the melting stage, before the pool solidifies, the gap formed by endogenous and exogenous gases is called porosity. It directly leads to the reduction of metal fusion area, thus reducing the strength of weld under load. This phenomenon is particularly obvious in arc welding. Most of the reasons for this quality defect are low temperature of molten pool, excessive feeding of deposited metal, improper conveying angle of strip, unclean weld and too fast welding speed.
3. Slag inclusion and its causes
Slag inclusion is slag or other non-metallic inclusions remaining in the weld, and its shape is irregular, which is easy to occur at the edge of groove and the sudden change of weld shape. The main reasons for slag inclusion are improper transportation and unclear composition of molten pool; Oil, dust, rust, etc. The weldment or groove is not cleaned up, especially in multi-layer welding; The temperature of molten pool is low and the welding speed is too fast; The arc is too long or the polarity is incorrect; When submerged arc welding is used to seal the bottom, the position of welding wire will shift.
4, lack of penetration phenomenon and its causes
Incomplete penetration is the phenomenon of incomplete penetration at the root of the joint during welding. This kind of defect not only reduces the mechanical properties of welded joints, but also forms stress concentration points in the incomplete gaps and ends, which often leads to cracks after loading, which is a dangerous defect. The reasons are as follows: the gap between the blunt edges of the groove is too small; The welding current is too small or the bar conveying speed is too fast; Small slot angle; Incorrect deviation of conveying angle and circular arc.
5, flash phenomenon and reason
Flash refers to the metal that overflows onto the under-heated base metal or weld during welding and fails to fuse with the base metal or previous weld. This kind of defect makes the weld formation unsightly, and ash accumulation and incomplete penetration often occur in the parts with flash during vertical welding. The flash in the pipe not only reduces the strength, but also reduces the effective cross section in the pipe. The main reason for flash is that the temperature of molten pool is too high, solidification is slow, and molten iron falls under its own weight to form flash. If the welding current is too large, the covered electrode angle is wrong or the operation posture is improper during vertical welding of groove or lap; The chemical composition mismatch between welding wire and base metal is easy to cause this defect.
Second, the prevention and control measures of welding defects
1, preventive measures for hot and cold cracks
The main method to prevent hot cracks: stress can be reduced during welding, which requires welding technicians to strictly abide by the relevant provisions of the process flow for welding. Before welding metal materials, the welding process should be strictly analyzed and the most suitable process should be selected. In addition, technicians who weld metal materials should strictly control the process parameters and reasonably control the cooling rate after welding. In order to reduce the generation of cracks in welding, multi-layer bead welding of metal materials can also be carried out by means of small current; Control method of cold crack: firstly, covered electrode should be selected reasonably, and the low hydrogen type should be selected in the selection process to avoid a large amount of hydrogen diffusion in the weld. Another way to reduce the hydrogen content is to clean the grooves to remove oil stains and moisture. Secondly, it is necessary to avoid welding materials from being affected with damp, abide by the storage rules of welding materials, and select welding materials in accordance with relevant regulations. Finally, the stress and toughness of the welded joint are reasonably adjusted by reducing the hydrogen content in the weld to reduce the internal stress, tempering the hardened structure, following a reasonable procedure in the welding process, and annealing the metal material by stages.
2. Preventive measures for blowholes
The coating at the covered electrode end of the coating is not well melted, and its protection is poor, so it is easy to produce pores. At the same time, if there is oil stain and moisture on the surface of the weldment, it is easy to produce blowholes. Therefore, it is necessary to strengthen the cleaning of weldment surface. According to the actual production situation, the coated covered electrode with strong anti-rust ability is selected, such as low-hydrogen coated covered electrode with poor anti-rust ability and iron oxide coated covered electrode with good anti-rust ability. If the derusting workload is heavy, it is difficult to completely remove it. On the premise of not affecting the weld strength, covered electrode with good antirust ability can be selected, which is not easy to produce blowholes. DC reverse welding is recommended, which can effectively prevent the occurrence of blowholes.
3, measures to prevent slag inclusion
In order to prevent slag inclusion, reasonable welding specification and groove size should be selected. Properly increase the welding current, shorten the arc if necessary, and increase the residence time to fully heat the molten metal and slag. Adjust the covered electrode angle and strip conveying mode according to the situation. Thoroughly clean the slag between the groove and the welding layer, and level the concave and convex parts. Proper selection of base metal and covered electrode can reduce the melting point and viscosity of welding slag.
4. Preventive measures for incomplete penetration
High current welding is the basic method to prevent incomplete penetration. In addition, the use of alternating current instead of DC in fillet welding to prevent magnetic bias blowing, reasonable design of groove and strengthening cleaning, and short arc welding can also effectively prevent incomplete penetration.
5, the prevention and control measures of flash.
Flash is not just a shadow? The appearance is beautiful, and it is easy to cause stress concentration, slag inclusion and incomplete fusion at the flash, leading to cracks. Generally, when there is a "small bulge" in the lower part of the molten pool, covered electrode can swing left and right and jump arc to control it. When lap joint or side seam is welded vertically, the welding current should be reduced appropriately compared with flat welding. When the covered electrode swings from side to side, he walks faster in the middle and slower on both sides. The vertical welding seam with groove can be strengthened by small diameter covered electrode, and the welding current should be reduced appropriately.
Third, measures to improve welding quality
1, fully implement the welding laws and regulations, and strengthen each welder's awareness of welding specifications.
Through the implementation of the evaluation method for workshop personnel, every inspector and welder will have a correct working attitude. Especially the management of welders, because the working attitude and psychological state of welders determine the appearance quality of each weld. At the same time, inspectors are required to learn welding management regulations to familiarize them with welding knowledge and common sense and on-site welding requirements.
2. Preparation of welding materials and welding workpieces
Ensure that the welding materials must be correctly matched according to the welding process requirements and the material properties of the workpiece to be welded before welding, and at the same time, dry them in strict accordance with covered electrode instructions to ensure that they can be used as needed. If conditions permit, choose covered electrode with better brand. Prepare the welding groove according to the standard before work, strictly clean the welding joint, and assemble it reasonably to facilitate welding.
3. Improve the professional and technical level of welders.
Every welder should be familiar with welding specifications, correctly understand and master various covered electrode conveying methods at various welding positions. Strengthen theoretical training, be familiar with welding theory and understand welding defect knowledge. Before welding the inevitable difficult parts in production practice, simulation exercises should be carried out to ensure the welding quality in actual work.
Concluding remarks
Welding technology plays an increasingly important role in the development of engineering structures, and there are many unfavorable factors affecting welding quality, which is difficult to control. However, the welding quality is directly related to the safe operation and service life of equipment production. Therefore, welding workers must be very aware of the causes of common welding defects and take timely measures to prevent them.