Four key points for improving the welding skills for pressure vessels

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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 joint 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 molten pool is not easy to determine directly, but it is related to the shape and size of the molten pool. Therefore, by carefully observing and controlling the shape and size of the molten pool during welding, it is possible to control its temperature and ensure good welding quality. Based on the decades of experience of seasoned craftsmen, this principle can be summarized in a few sentences: First, the angle of the welding rod is very important, and welding parameters are essential. When welding in the vertical position, the droplets formed by the melting of the welding rod 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 welding speed according to changes during welding. The angle between the welding electrode and the surface of the workpiece is 90° in the horizontal direction. The angle with the weld seam 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 current for the root pass is 110–115 A, 115–120 A for the intermediate pass, and 105–110 A for the finish pass. The current is generally 12%–15% lower than that in flat welding, in order to reduce the volume of the molten pool and thereby minimize the influence of gravity, which 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. II. Observe the molten pool and listen to the arc sound; keep in mind the shape of the molten hole. The root pass welding at the base of the weld is crucial for ensuring welding quality. Arc extinguishing is used for welding; the arc extinguishment rate in vertical welding is slightly slower than that in horizontal welding, at 30–40 times per minute. The arc burns for a longer time during welding at each point, which results in the weld metal in vertical welding being thicker than that in horizontal 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 extinguishing requires the flexibility of the wrist, to extinguish the arc cleanly each time, thereby giving the molten pool a chance to solidify instantly. During arc extinction, the molten holes formed at the punctured 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 bulges are likely to form on the back side, whereas in the absence of molten holes, the back side often fails to be fully 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 consistent width, height, and shape of the weld bead on the back side. 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 continue welding while varying the angle of the electrode at a distance of about 10 mm from the resulting weld. When the angle reaches 90 degrees, move the electrode slightly back and forth inside the center of the weld and simultaneously press the arc downward; once a melting hole is formed, extinguish the arc immediately, allowing the 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 root welding method as with the first electrode, the arc is extinguished and reignited in an alternating back-and-forth manner; concentration is required at every step, with attention paid to the contour of the molten pool as well as 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 time between arc extinguishments should be controlled such that one-third of the molten pool remains unfrozen before the arc is reignited. When ending the arc, it should be noted that when only 80–100 mm of each electrode remains, the electrode heats up excessively, causing it to melt more rapidly. In such cases, the arc-off time should be increased to allow the molten pool to solidify momentarily, thereby preventing the formation of weld beads due to the sagging of the hot molten pool during welding. When the welding rod is reduced to 30–40 mm in length, prepare to extinguish the arc by dripping molten metal continuously on one side of the pool a couple of times, thereby causing the pool to cool down slowly. This helps prevent defects such as shrinkage cavities and crater cracks on the front and back sides of the weld seam. III. Good control of the molten pool temperature improves weld quality; it is required that the weld beads in the intermediate layer be even. For the middle two layers, use electrodes with a diameter of φ3.2 mm; the welding current should be set between 115 and 120 A. The electrode angle should be approximately 70°–80°. Employ a zigzag electrode movement pattern, and control the molten pool temperature by adjusting the electrode angle, arc length, welding speed, and the duration of time the electrode remains at both sides 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 edge of the groove; leave a depth of about 1 mm to keep the entire filler bead smooth. Taking the edge of the groove as the reference line, a foundation is laid for the weld surface. 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 weld pool and both sides of the groove. Pay close attention to the shape of the weld pool, aiming to keep it in a crescent shape; spend less time on the side with more material and more time on the side with less material. While welding, calculate the height and width of the weld seam. Since the weld bead formed in vertical welding is thicker than that in horizontal welding, it is important to pay attention to the shape of the molten pool and the thickness of the weld bead. If the lower edge of the molten pool bulges out from a flat edge, it indicates that the temperature of the molten pool is too high; in such cases, the arc combustion time should be shortened while the arc extinction time should be prolonged in order to reduce the temperature of the molten pool. Before changing the electrode, the crater must be filled to prevent crater cracks. IV. Proper 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-shaped, it indicates that the temperature of the molten pool is appropriate; normal welding can be carried out, and the weld surface will have good formation. If it is observed that the bottom portion of the weld pool becomes bulged and rounded, this 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 increases, it indicates that the temperature of the molten pool is too high; the arc should be extinguished immediately to give the molten pool time to cool down, and welding should resume only after the temperature has dropped. 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 joint, the temperature of the welded parts is low, which makes it easy to encounter defects such as poor fusion, slag inclusions, joint separation, and excessive thickness; therefore, the quality of the cover directly affects the surface finish of the weld. Therefore, when welding at the joint, the preheating method is employed: at a point approximately 15 mm above the starting point of welding, an arc is ignited from top to bottom using the scratch-start method, and the arc is stretched to a length of 3–6 mm 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 angle of the electrode, 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.