Four key tips for pressure vessel welding techniques

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For important structures such as boilers and pressure vessels, it is required that joints be welded thoroughly and safely. However, due to constraints such as structural size and shape, double-sided welding is sometimes not possible. It is a special operation method that only allows for single-sided grooves, namely the technique of welding on one side while forming the structure on the other side; it represents one of the more challenging skills in manual arc welding. During vertical welding, due to the high temperature of the molten pool, under the effect of gravity, the droplets formed by the melting of the electrode and the molten iron in the pool tend to flow downward, resulting in weld beads and undercuts on both sides of the weld. At too low temperatures, slag inclusions are likely to form, while on the opposite side, defects such as lack of penetration and weld beads can occur, making it difficult to achieve proper weld formation. The temperature of the weld pool is not easy to determine directly; however, it is related to the shape and size of the weld pool. Therefore, during welding, by carefully observing and controlling the shape and size of the weld pool, it is possible to regulate its temperature and ensure welding quality. 1. The angle of the welding electrode is very important, and welding specifications are essential. When welding in the vertical position, the droplets formed by the melting of the electrode and the molten iron in the weld pool tend to flow downward, resulting in weld beads and undercuts on both sides of the weld, which deteriorates the quality of the weld. Master the correct welding parameters and adjust the electrode angle and travel speed according to changes during welding. The angle between the welding rod and the surface of the workpiece is 90° in the left-right direction. The angle with the weld is 70°–80° at the start of welding, 45°–60° during the welding process, and 20°–30° at the end of welding. The assembly gap is 3–4 mm; therefore, a smaller electrode diameter of Φ3.2 mm and a lower welding current should be used. The welding current should be 110–115 A for the root pass, 115–120 A for the intermediate pass, and 105–110 A for the cover pass. The current is generally 12%–15% lower than that used in flat welding, thereby reducing the volume of the molten pool and minimizing its exposure to gravity effects; this facilitates the transfer of droplets. Short arc welding is used to reduce the distance that the droplet travels before entering the molten pool, thereby causing a short-circuit transition. 2. Observe the molten pool and listen to the arc sound; keep the shape of the melt hole in mind. The root pass welding is a key factor in ensuring welding quality. When welding using the interruptive method, the interruptive rhythm for vertical welding is slightly slower than that for flat welding—30 to 40 times per minute. The arc burns for a slightly longer duration at each weld point; therefore, the weld bead formed in vertical welding is thicker than that in flat welding. During welding, start from the lower end; the angle of the root-pass welding rod should be around 70°–80°. Use two-strike penetration welding, initiating the arc on one side of the groove and moving it toward the root along the weld points to preheat and melt the material. When you hear the \"whooshing\" sound indicating that the arc has penetrated the groove, and see a molten hole form and a pool of molten material appear, immediately lift the welding rod to extinguish the arc. Then re-ignite the other side of the groove; the second molten pool should cover 1/2 to 2/3 of the first molten pool that has begun to solidify. In this way, by using arc extinction on the left and right sides, the entire weld seam can be formed. Arc extinction requires wrist flexibility; each time, the arc must be extinguished cleanly and decisively, giving the molten pool a chance to solidify instantaneously. During arc extinguishing, the molten holes formed at the broken edges can be clearly seen; the size of these molten holes in vertical welding is approximately 0.8 mm. The size of the molten holes is closely related to the shape of the back side of the weld – if the molten holes are too large, weld beads are likely to form on the back side; conversely, if there are no molten holes, the back side of the weld often remains under-welded. It is necessary to maintain a uniform size of the molten holes during welding, so as to ensure uniform penetration at the root of the groove, as well as a weld bead on the back side that is even in terms of width and height. When changing the welding electrode for root pass welding, it is necessary to clean the flux from the joint area each time, re-ignite the arc inside the groove, and then weld continuously about 10 mm along the formed weld while adjusting the angle of the welding electrode. When the angle reaches 90°, move the electrode slightly back and forth toward the center of the weld while also pressing the arc downward; once a melting hole is formed, extinguish the arc immediately, allowing the welding electrode arc to penetrate to the root of the weld where another melting hole is created, after which the arc is extinguished right away. Then, following the same process as using the first welding rod for the root pass, the arc is extinguished and reignited in an alternating left-right pattern; concentration is required at every step, with attention paid to the shape of the molten pool and the melted areas on both sides. The melted areas at the root of the groove can only be seen when the arc moves to the other side. If it is found that the edges have not been properly fused, the arc should be moved slightly downward to achieve proper fusion. The arc should be extinguished each time when still 1/3 of the molten pool remains unfrozen, after which the arc should be reignited. When ending the arc, it is important to note that when each welding rod is left with a length of 80–100 mm, the rod melts more rapidly due to overheating. At this point, the time taken to extinguish the arc should be increased, allowing the molten pool to solidify momentarily, thereby preventing the formation of weld bumps as a result of the molten pool moving downward under the welding rod. When only 30–40 mm of the electrode remains, prepare to extinguish the arc. Continuously drip molten metal two or three times on one side of the weld pool, so as to gradually cool it down. This helps prevent defects such as shrinkage cavities and crater cracks from occurring on both the front and back surfaces of the weld bead. 3. Proper control of the molten pool temperature can improve weld quality; it is required that the intermediate-layer weld bead be smooth and even. For the middle two layers, the electrode diameter is φ3.2 mm, the welding current ranges from 115 to 120 A, the electrode angle is approximately 70° to 80°, and a zigzag welding technique is used. The temperature of the molten pool is controlled by adjusting the electrode angle, the length of the arc, the welding speed, and the time spent on each side of the groove. Ensure good fusion on both sides and maintain a flat, oval shape for the molten pool. When welding the third layer, do not damage the edges of the groove; leave a depth of about 1 mm to ensure that the entire filler weld bead is smooth. Taking the edge of the groove as the reference line, a foundation is laid for the overlay layer. By moving back and forth, pause slightly longer on each side of the groove to melt 1–2 mm of the groove edge, and ensure even temperature across the molten pool and both sides of the groove. Pay close attention to the shape of the molten pool, aiming to keep it in a crescent shape; spend less time on the side with more molten pool and more time on the side with less. While welding, calculate the height and width of the weld seam. Since the weld bead in vertical welding is thicker than that in flat welding, pay attention to observing the shape of the molten pool and the thickness of the weld bead. If the lower edge of the molten pool bulges outward from a gentle slope, it indicates that the temperature of the molten pool is too high. In such cases, the arc burning time should be shortened while the arc extinguishing time should be extended to lower the temperature of the molten pool. The arc crater must be filled before replacing the welding rod to prevent arc crater cracks. 4. Maintaining the correct welding technique is essential for achieving a good weld shape. When performing cover welding, a zigzag or crescent-shaped welding motion can be used; the movement must be steady, with a slightly faster speed in the middle of the weld bead and a slight pause at the edges of the groove. The process specifications require a welding rod diameter of φ3.2 mm, a welding current of 105–110 A; the angle of the welding rod should be maintained at around 80°. The rod should be moved back and forth to melt 1–2 mm of the groove edges, and slight up-and-down movements should be made when stopping on either side. But as the welding rod moves from one side to the other, the arc in the middle rises slightly, allowing for an observation of the shape of the entire molten pool. If the molten pool is flat and oval in shape, it indicates that the molten pool temperature is appropriate; normal welding can then be carried out, resulting in good surface formation of the weld. If it is observed that the bottom of the weld pool becomes bulged and rounded, it indicates that the temperature of the weld pool is slightly too high. In such cases, the welding technique must be adjusted immediately: increase the dwell time of the electrode on both sides of the groove, accelerate the transition in the middle section, and keep the arc length as short as possible. If the molten pool cannot be restored to a flat elliptical shape and the bulging becomes more pronounced, it indicates that the temperature of the molten pool is too high; in such cases, the arc must be extinguished immediately to give the molten pool time to cool down, before welding can resume. When covering the surface, it is important to ensure that the weld edges are in good condition; if undercutting is detected, move the welding rod slightly or pause for a moment to address the defect, so that the surface can be smooth. When starting to weld the cover pass, the temperature of the workpiece is relatively low, making it prone to defects such as poor fusion, slag inclusion, separation at the joint, and excessive height. Therefore, the quality of the cover pass directly affects the surface formation of the weld. Therefore, when welding at the joint, preheating should be used; about 15 mm above the starting point of welding, the arc is ignited from top to bottom using the scratching method, and the arc is stretched to 3–6 mm in length in order to preheat the starting area of the weld. Then lower the arc and make 2–3 additional passes at 2/3 of the original arc crater to achieve good fusion, after which proceed with normal welding. Although the positions of the welds vary, they follow common patterns. Practice has shown that by selecting appropriate welding process parameters, maintaining the correct electrode angle and mastering the three key actions related to using the electrode, as well as strictly controlling the temperature of the molten pool, high-quality welds with an attractive appearance can be achieved when welding in the vertical position.