Semi-automatic welding technology of flux-cored wire for pipeline welding

Subject terms Pipeline flux cored wire welding technology With the development of welding technology, more and more welding technologies are applied to long-distance pipelines, and the welding process is also more advanced. In the past twenty years, the welding technology used for long-distance pipelines has developed from manual upward welding to manual downward welding flux cored wire semi-automatic welding. Currently, we are actively researching and developing fully automatic welding technology. Among these welding technologies, flux cored wire Because of its high working efficiency and excellent welding quality, semi-automatic welding has become the first choice in the field construction of long-distance pipelines. Because the construction of long-distance pipelines is mainly field construction, the construction conditions are poor, and the requirements for welding methods and welding processes are harsh. The welding methods used externally for pipeline construction mainly include manual arc welding (including manual upward welding and manual downward welding), flux cored wire semi-automatic welding and gas shielded automatic welding. Among them, manual welding and flux cored wire semi-automatic welding are relatively mature welding methods developed in China, and are also the main welding methods for pipeline welding production at home and abroad.

Compared with manual arc welding, flux cored wire semi-automatic welding has more unique advantages in the construction of long-distance pipelines. The specific characteristics are as follows: semi-automatic welding of flux-cored wire is semi-automatic production. In the welding of pipeline girth welds, the welder can generally weld from the top of the steel pipe to the bottom of the steel pipe. Compared with manual welding, the time of the auxiliary process is currently used in pipeline welding. Very high mechanical properties of deposited metal. The effective utilization rate of flux cored wire is high, generally up to 100%. Usually, when manual welding, the 5060mm electrode head is removed (the arc is also consumed), so the effective length of manual electrode About 85% of the total length of the electrode. In addition, because the flux-cored wire contains only 15% to 20% of the powder, its deposition rate can reach 85% to 90%, while the deposition rate of the electrode is only 60% to 65% In other words, 1t flux-cored welding wire can almost top 1.5t electrode.

Due to the unique advantages of flux cored wire semi-automatic welding technology in the field construction of long-distance pipelines, this welding method has been widely used in pipeline construction at home and abroad. However, due to the development of flux cored wire for root welding It is very mature, so in the actual welding production, the widely used welding process is a hybrid welding process of manual down welding for root bead welding, flux cored wire semi-automatic welding for filler welding and cover welding. Many engineering practices have proven that this hybrid welding process fully combines the advantages of good manual welding root welding and good efficiency of semi-automatic welding of flux cored wires. 3.1 Welding preparation greatly saves the replacement of auxiliary electrodes such as starting arc and closing arc. The equipment for flux cored wire welding mainly includes constant voltage DC welding power source, constant speed wire feeder, welding gun and cable. Because manual down welding is often used for root welding, the groove form is similar to that of manual down welding. Show). The angle of the groove is 60, the gap is 2 ~ 2.5mm, and the blunt edge is 3.2. The main points of operation After the root welding is welded by manual arc welding, the welding slag and spatter covered on the weld bead must be cleaned up with a grinding wheel or wire brush , So as not to cause defects such as slag inclusion during the welding of flux cored wire semi-automatic welding. In addition, the following points should be noted when welding: arc starting: the dry elongation (that is, the length of the welding wire extending from the welding gun) should Maintain it at about 15mm. After lightly touching the weldment with the tip of the welding wire to start the arc, the dry elongation of the welding wire should be kept at about 20mm. The welding wire angle: During the welding process, maintaining the correct welding wire angle is conducive to maintaining welding Stability of process and welding quality. When performing flux-cored wire welding, the angle between the welding wire and the axis passing through the center of the steel pipe should be kept at 4560 and inclined backwards, as shown. When the angle is too large, the penetration will increase, and if the angle is too small, the penetration will decrease.

Joints: In pipeline welding, the welded joints are prone to defects. When performing flux cored wire semi-automatic welding, a skilled welder can generally weld all the way from the top of the pipe to the bottom of the pipe. When welding, care should be taken to avoid defects. At the joint, the slag at the crater should be removed first, and the arc is ignited at the top of the crater. After the arc is ignited, it should be paused and gently oscillated until the crater is filled, and then welded at the normal welding speed. Speed: During the welding process, the correct welding speed should be maintained in order to obtain a good quality weld bead. The specific welding speed should be determined according to the diameter and wall thickness of the pipe. The welding speed will be small Slag; excessive welding speed will make the weld bead unsatisfactory.

Weld bead layers: The number of weld bead layers is mainly determined according to the wall thickness of the pipe. In the common process, it generally includes root bead filling bead and cover welding bead. But in the welding process, if there is only one layer of filler welding At the time, the filling position of the vertical welding position of the pipe may sometimes be unsatisfactory. At this time, a layer of filler welding can be added at this position to fill the weld bead: when welding the filler bead, the general method of transporting the meter is straight pull . When the wall thickness of the pipe is large, row welding can be used to fill the wider groove. The use of direct transport strips is not only easy to operate, but also obtains good welding quality and mechanical properties. In addition, in order to make the edges of the weld bead well fused, it is also possible to perform welding with a slight swing cover weld during transport: When welding the cover, the welded joint should be staggered from the welded joint during filling welding. Before the cover welding, the height of the weld bead and the surface of the pipe should be less than 1.5mm. When welding to the bottom of the pipe, the angle of the welding wire can be greater than 60 to facilitate the control of the shape of the weld bead. 3 Technical measures (1) Joint cleaning: The flux cored wire semi-automatic welding process is a low-hydrogen type process. The cleaning of the joint is very important for all welds. The cleaning area should include the groove and the range of 25mm on both sides of the joint. The cleaning tool can be selected. In addition, the residual stress of the unexpanded diameter pipes produced by the cold expansion company using the HU-METAL molding method is generally small, and the location also varies with the residual stress level of the steel pipe of the production plant and the similar steel grade and specifications. Comparing Table 1 and Table 3, it can be seen that the residual stress level of the cold-expanded UOE tube is close to the partial residual stress test results of the cold-expanded UOE tube in Table 3. Specifications Residual stress (MPa) of the steel grade place of origin welded surface The outer surface of the surface and the inner surface of the surface indicate that this method can make the overall residual stress of the welded pipe tend to be uniform, and has a more obvious effect on reducing the residual stress, because when forming, the steel plate is first bent and deformed by the active roller (that is, the deformation roller). The bending curvature of the roller is greater than the curvature of the finished tube. The bent steel plate will elastically deform after leaving the active roller. The forming method of HU-METAL is not controlled by Xi Bu, and the elasticity can be completely restored. In this way, the selection of the diameter of the active roll that can be achieved by controlling the opening amount of the tube blank is related to factors such as the material, yield strength, and thickness.

The HU-METAL forming method has the characteristics of good edge quality after rolling, welding without requiring prior adjustment, and low investment. It also has a cold stress level that is similar to the same steel grade and the same specification without undergoing diameter expansion and hydraulic test. The advantages of the expanded diameter UOE steel pipe are relatively close (continued from page 8) or other mechanical methods. There should be no oil, sewage, paint and other stolen materials in the cleaning area (2) Preheating into the interlayer temperature: In order to avoid excessive hardness and prevent cracks, When welding high-strength steel pipes, preheating and interlayer insulation measures should be taken. Among the three main welding methods used in long-distance pipeline welding construction, the flux-cored wire semi-automatic welding technology is easy to operate, requires fewer supporting equipment and efficiency High characteristics, very suitable for the construction of long-distance pipelines, so for a long time in the future, it will still play an important role that other welding methods cannot replace.