Key points for welding large-diameter pipelines

　　The welding of large diameter pipelines usually requires manual welding or automatic welding methods. Currently, the use of automatic welding methods is more common.

　　The following are the welding points for large-diameter pipelines:

　　1、 Weld from top to bottom, the root weld must be fully penetrated and the surface formed. The joint points between adjacent layers shall not overlap and shall be staggered by more than 30mm. Due to the shallow hot welding pool of the second layer, it is not easy to precipitate slag. After the root welding is completed, the root weld bead should be ground and cleaned. To avoid arc craters caused by grounding wire ignition, a specially designed grounding wire is used in the following form: each layer of weld bead and each weld junction should be continuously welded without interruption. Before the molten pool cools down, the next layer of welding (or the next welding) should be carried out, and the interval between hot welding should not exceed 10 minutes. Using a straight line running method for hot welding, adjust the dry elongation of the welding wire to the appropriate length, and use the direct short-circuit arc ignition method for arc ignition. During the welding process, the welding wire does not swing. The interlayer temperature does not meet the requirements and should be reheated.

　　2、 To avoid the overflow of liquid metal from the molten pool, low-speed welding should be used during overhead welding, and the welding slag on the weld bead should be cleaned before the next welding step. When filling welding, due to the increase in groove width, the welding wire needs to be swung appropriately. To avoid defects such as overflow of the molten pool, porosity, and slag inclusion, the welding speed should be controlled appropriately to keep the molten pool moving forward. Fill welding should have sufficient welding layer, and after cover welding, the cross-section of the completed weld should be uniform and consistent throughout the entire weld joint. The groove should be filled before cap welding, and the remaining groove depth should be controlled within 6mm. The wire feeding speed for cap welding should be the same as or slightly lower than that for filler welding.

　　3、 For welds that require post heating or heat treatment, post heating and heat treatment should be carried out in accordance with the welding process specifications. Slow cooling of the weld seam is required, and it is not allowed to remove the coating immediately after welding. The coating can only be removed and repaired after the slow cooling is completed. Composite slow cooling devices are commonly used for post weld slow cooling, which consist of heat-resistant materials, insulation materials, protective layers, etc. The completed weld joint is wrapped in asbestos blanket and covered with felt while it is hot, and tied tightly with rubber bands. The insulation time is more than half an hour.