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Downward welding defects in gas pipelines and preventive measures – China’s oil and gas pipeline network. Downward welding is a manual arc welding process that began to be developed in the mid-1960s; it is currently a widely used welding method in the construction of gas pipelines. Our company employed the downward welding technique in the Liu Tang pressure regulation station (55/1) and the Gangxia light oil connection line as part of the Nanjing natural gas high-pressure pipeline project. The characteristic of this welding method is that, with the pipeline kept horizontal and fixed in place, the welding heat source starts at the top center and moves vertically downward until it reaches the bottom center. The sequence of welding positions is: flat welding, vertical flat welding, vertical welding, overhead vertical welding, and overhead welding. The downward welding process uses cellulose-based downward welding electrodes. Thanks to their unique flux formulation, these electrodes offer advantages over the traditional top-down welding method, specifically: (1) faster welding speed and higher production efficiency. Due to the low molten iron concentration in this type of electrode, no slag flows, which increases efficiency by 50% compared to welding from bottom to top. (2) It offers good welding quality: the root of the weld produced by cellulose electrodes has a full shape, the arc exerts strong force leading to uniform penetration, the back side of the weld seam has an attractive appearance, and it possesses strong wind resistance, making it suitable for use in field conditions. (3) It reduces the consumption of welding materials; compared with the traditional bottom-up welding method, the electrode consumption is reduced by 20%–30%. (4) The first-pass welding qualification rate can reach over 90%. The defects that tend to occur during downward welding and the measures to prevent them are as follows: 1 Defects that occur easily during welding 1.1 Causes of slag inclusions (1) Inadequate cleaning of the root area after the root pass welding, which results in the slag at the root not being completely removed during rapid heating and welding. (2) The method used for root cleaning in the backing weld was improper, resulting in overly deep grooves on both sides of the root pass, giving it a \"W\" shape. During rapid thermal welding, the slag that flows into deep grooves does not have time to flow out, resulting in slag inclusions. (3) Closing the arc too quickly at the 6 o’clock position can also lead to slag inclusions. 1. 2 Causes of porosity (1) During overlay welding, the molten pool overheats, absorbing a large amount of surrounding air. (2) During cover welding, the electrode swings too much, resulting in poor protection of the molten pool. (3) If the root gap is too small, needle-shaped bubbles in the roots are likely to form. (4) The welding rod was not used up within the specified time or was exposed to air for an extended period. 1. Causes of cracks at the 3 o’clock position (1) If the construction area has significant unevenness and the soil pads are not placed in place in a timely manner, the pipes end up under stress; as a result, stress cracks are likely to appear at the welding start point (especially at the 6 o’clock position). (2) During welding, if the joint holder is released or removed too early, preventing the molten iron in the pool from solidifying properly, cracks are likely to form at the start of welding. (3) The welder used improper arc extinguishing methods such as a straight-line approach at the 6 o’clock position, resulting in the molten pool not being filled and arc crater cracks forming. 1. 4 Causes of inward concave deformation (1) Excessive clearance between mating surfaces. (2) During the root welding, the electrode is not fed deep enough. (3) The welding current is too high; during heat welding, the arc movement at the 5-7 o’clock position is too slow. 2 Measures to be taken against common defects In accordance with the requirements for pipes and welding materials used in engineering projects, a welding procedure qualification must be carried out for each project, and welding procedure specifications must be prepared; welders are required to follow these specifications strictly when carrying out welding tasks. Proper protection of welding materials is essential; during transportation and storage, electrodes must not be dropped, struck, scraped, or otherwise damaged, to ensure their integrity. 2.1 Pre-welding preparation requirements: (1) Before assembly, the groove and the areas on its inner and outer surfaces within a range of at least 25 mm must be cleaned of oil, paint, dirt, rust, burrs, and other contaminants using an electric wire brush. There should be no cracks, delaminations, or other defects, and the surface should have a metallic luster. (2) Before assembly, the groove must be ground to ensure that the groove angle and root margin meet the design parameters and welding process requirements. (3) Before welding, use a grinder to grind the spiral welds within 15 mm of both ends of the steel pipe into a gentle slope to ensure good fusion of the weld seams in those areas. (4) The pipe alignment dimensions must meet the requirements of the welding process. 2.2 Measures taken during welding process (1) Measures to prevent slag inclusions After the root pass welding, a professional grinder should be assigned to remove the slag; the cleaning must be thorough, and each joint area must be made smooth. When cleaning the roots, the root weld bead should be shaped into a \"U\"-shaped groove, avoiding a \"W\"-shaped groove. At 6 o’clock, when ending the arc, the molten pool should be filled first before moving the arc to the formed weld to terminate it; the arc should be extinguished using a horizontal swishing method. (2) Methods to prevent porosity ① When covering the surface, use a low current level (the current should be lower than that used for filling welding); employ a low current, short arc, and fast welding speed to avoid overheating and prevent surface porosity. ②Appropriate welding technique should be used during welding; otherwise, the molten pool will move too far forward, leading to prolonged short circuits and the electrode sticking to the weld bead. This is unfavorable for deoxidation and can result in pores. However, the swinging width of the electrode should not exceed twice its diameter, as this too can cause pores. ③To prevent the defect of too small alignment gap, as an overly small alignment gap leads to excessive dilution of the base material during welding, which hinders gas expulsion and results in the formation of pin-shaped bubbles at the root. ④During use, welding rods should be stored in welding rod holders and taken out as needed; it is strictly prohibited to leave them exposed to prevent moisture absorption. (3) Measures to prevent cracks ① When constructing in uneven areas, the soil embankments should be placed in position in a timely manner, or chain blocks should be used to strictly control any stress on the welds during the welding process. ②It is strictly prohibited to loosen or remove the alignment tool during welding. ③When the welder stops welding at 6 o’clock, he must ensure that the weld pool is filled, and the arc should be extinguished using a horizontal method; it is strictly prohibited to use a vertical method to extinguish the arc. (4) Measures to prevent inward concave defects ① Strengthen quality inspection, control the alignment quality, and ensure that the alignment gap and chamfer meet the requirements of the welding process. ②The mound must be raised high enough (usually not less than 400 mm) to facilitate the welder’s work and ensure an appropriate depth of penetration ； ③During the root welding, the current should not be too high; whereas during hot welding, the wire feeding speed at around 5–7 o’clock should be moderate and not too slow.