Key operating principles for single-sided welding with double-sided formation in vertical plate butt welding using alkaline electrodes

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Alkaline electrodes are generally low in hydrogen content, and the welds produced with them possess good comprehensive mechanical properties; cracks are less likely to occur under load. Therefore, they are widely used in industries such as pipelines, pressure vessels, petroleum, chemicals, and shipbuilding, where high standards are required for weld quality – a role that acidic electrodes cannot fulfill. Although the improvement of welding materials is essential for ensuring product quality, the individual skill of the welder has a direct impact on it as well. This article mainly focuses on the key operating principles for single-sided welding with double-sided formation in vertical plate butt joints using alkaline electrodes. The single-sided welding with double-sided formation technique refers to a method that uses ordinary welding electrodes and specific welding techniques to carry out welding on the front side of the groove, ensuring that qualified welds are formed on both sides of the groove after welding. The single-sided welding with double-sided formation technique is not only a required course for welder skill training, but also an essential test item in welding skill competitions at various local levels, as well as a operation technique that welders working on pressure pipelines and pressure vessels must master. 1. Pre-welding preparations 1.1 Wearing of work clothes Before starting welding, welders must wear appropriate work clothes, safety shoes, protective gloves, and safety helmets. When welding, it is essential to use a face shield, as well as to fasten the collar and tie up the sleeves in order to protect the body from arc radiation and spatter. 1.2 Preparation of the weldments: Two pieces of low-carbon steel plate Q235 were used, with the weldment dimensions being 300mm×150mm×12mm. A V-shaped groove was created, with a groove angle of 30°±2° on each side; the weldments had to be flat and free from any deformation. Use tools such as manual grinders and wire brushes to remove rust, oxides, and other contaminants from both sides of the groove as well as within a 20 mm range on each side, until a metallic shine is visible; finally, use a file to dull the edges by 0.5–1 mm. 1.3 Preparation of electrodes and welding power supply: Use E5015 alkaline electrodes with a diameter of Φ3.2mm. Before welding, these electrodes must be baked at 350–400°C for 1–2 hours, followed by another 1–2 hours of holding at that temperature, after which they should be placed in an electrode storage tube for later use. The welding power source can be a ZXG-500 DC arc welder. 1.4 Weldment assembly and tack welding: Arc interruption welding is used for assembly. The typical gap for the groove can be set at 2.5 mm at the starting point of welding and 3.2 mm at the ending point. Since it is required that the gaps remain at 2.5 mm and 3.2 mm after tack welding, the shrinkage and deformation that will occur after welding must be taken into account prior to welding. The misalignment between the weldments should be less than 1 mm, with the counter-deformation angle ranging from 3° to 5°. The weldment is placed on the welding rack, its height is adjusted, and preparation is made for the actual welding process. 2. Welding process
2.1 Root pass welding
For root pass welding, the interrupted arc welding method is used. This method is a single-sided welding technique that enables formation of welds on both sides; it controls the shape, temperature, and thickness of the molten pool by regulating the duration during which the welding arc remains lit or extinguished, as well as the movement of the welding rod. During welding, the arc penetrates the groove gap and melts the areas on both sides of the groove as well as the previous weld pool, thereby forming a new weld pool; the temperature, shape, and position of this weld pool are then controlled by the effects of arc extinction, the surface tension of the weld pool, and electromagnetic contraction forces. The E5015 welding electrode has a diameter of Φ3.2 mm, and the welding current is 110A. The downward angle between the electrode and the test plate is generally 70°–80°. An arc is initiated on the tack weld; when continuous welding reaches the end of the tack weld, the arc length is increased for a brief preheating period. Then, the arc is lowered to touch the root of the groove before being extinguished (this serves to form the first molten hole). This process creates the first weld pool. Ideally, the resulting molten hole should extend approximately 0.5–1 mm into both sides of the groove. Arc welding is carried out by igniting the arc above the first molten pool and then lowering it; the arc is extinguished once the size of the molten hole reaches that of the previous one. This process of alternating between arc ignition and extinction continues until the root pass is completed. Key points to note during the operation: Keep the size of the molten hole relatively constant throughout the welding process ； During welding, a \"popping\" sound is produced when the arc breaks through the root of the groove, indicating that the root of the groove has been fully melted ； The absence of sound indicates that there is no full penetration at the root of the groove. During welding, the new weld pool should cover approximately 2/3 of the previous weld pool. One-third of the arc should burn on the reverse side of the test plate, ensuring that the height of the weld on the reverse side of the groove remains between 0 and 2 mm ； Replacing the welding rod must be done quickly, and re-preheating is required. 2.2 Filler layer welding: Electrode E5015, diameter 3.2 mm, welding current 105 A; the downward angle between the electrode and the test plate is generally 70°–80°. Before filling the weld, the slag in the weld bead should first be removed using a slag hammer, and then the area should be scrubbed repeatedly with a wire brush to prevent slag inclusions from forming in the weld. Ignite the arc about 10 mm from the start of the weld, pull the arc back to the start point and preheat it for a moment, then press down to proceed with normal welding. A serrated, laterally swinging electrode tip is used to achieve an aesthetically pleasing weld shape. When swinging the welding rod, it is necessary to move more quickly in the middle of the groove and pause briefly at the sides, so as to facilitate the removal of slag from the weld and prevent a situation where the middle of the groove is higher while the sides are lower, leading to incomplete fusion. Filling welding is generally performed twice, using the same procedures as before. Key points to note during the operation: The interlayer temperature must be carefully controlled during the second filling weld ； The thickness of the fill weld in the second layer should be 1–1.5 mm below the surface of the base material (it cannot be too deep to prevent undercutting during welding in the lower layers), and it must not melt the edges of the groove (so that the boundaries are visible during the overlay welding, allowing the weld to remain straight). 2.3 Welding of the cover layer: Use electrode E5015 with a diameter of 3.2 mm; the welding current should be 103 A. The angle at which the electrode is tilted relative to the test plate is generally 70° to 80°. Before performing the overlay welding, the slag in the weld bead should first be removed using a slag removal hammer, and then the area should be scrubbed repeatedly with a wire brush to prevent slag inclusions from forming in the weld. The method for arc initiation and filler welding is the same; the electrode is moved in a zigzag pattern from side to side. When it reaches the edge of the groove, a brief pause is required to allow it to melt along the groove boundary, thereby maintaining a weld width of 1–2 mm. The swinging frequency of the welding rod should be slightly faster than that in flat welding, and the upward speed must be uniform, so that each new molten pool covers about 2/3 of the previous one. After replacing the welding electrode, wait for the test plate to cool slightly, then start the arc about 10 mm above the crater. Pull the arc up and down along the filler weld, moving it in a zigzag pattern toward the crater, gradually filling it until normal welding can be carried out. Finally, be sure to use spot welding to seal the arc and fill the arc crater. After welding is completed, the slag is removed to obtain a high-quality single-sided welded, double-sided formed weld for vertical plate butt joints.