Large-diameter pipes are much easier to weld than small-diameter pipes. When welding pipes with a wall thickness of more than 12mm and a diameter of more than 89mm in production or installation engineering, it is necessary to consider the large-diameter pipe welding method. Due to the large wall thickness of the weldment, multi-layer multi-pass welding must be used. After welding, perform X-ray inspection and cold bending test. The main reason for the failure of X-ray inspection of welds is due to the existence of slag inclusions and Unfused defects; in the cold bending test, the pass rate of back bending is slightly poor. Therefore, when the operator is practicing, he should pay attention to control the diameter of the fusion hole during the welding process. Usually, the diameter of the fusion hole should be 1~2 mm larger than the gap to ensure good fusion of the welding root.
When the large-diameter pipe is rotated horizontally butt welding, the pipe is generally rotated around its own axis in a horizontal position for welding, as shown in Figure Relationship between welding wire position and bead formation during horizontal rotary welding of large-diameter pipes-a. If the thin-walled tube is welded, the welding wire should be in a horizontal position, which is equivalent to vertical downward welding; if it is a thick-walled tube, it should be welded in the flat welding position, and the reverse direction of the welding wire deviates from the highest point l distance (l is called displacement) , and the displacement l should be appropriate. The influence of the welding wire position on the shape of the weld when the thick-walled pipe is welded is shown in Figure Relationship between welding wire position and bead formation during horizontal rotary welding of large-diameter pipes-b.
The most critical issue in welding horizontally rotating tubes is the position of the welding torch, which will seriously affect the weld formation. Especially when welding thick-walled pipes, the welding torch should be on the upper part of the pipe, opposite to the direction of rotation of the pipe, at a position of l displaced from the centerline. The size of the displacement l has a significant impact on the weld formation. When l is too small, the build-up height of the weld bead is too large; when it is too large, the weld bead overflows and the fusion on both sides of the weld is poor, so the displacement l should take an appropriate value.
The weldment material is 20 steel, and the weldment and groove dimensions are shown in Figure Welding material preparation diagram. If the weldment is not straight or has other defects, it should be leveled.
Use cleaning tools to remove oil, rust, moisture and other dirt within 20mm of the groove surface of the pipe and the inner and outer surfaces of its ends until the original luster of the metal is exposed.
Assembly Clearance Weldments are assembled with a 3 mm assembly gap and a blunt edge of 1 mm. Figure Large-diameter pipe butt weldment transfer tool shows the assembly mold for the large pipe butt joint.
Tack welding Tack welding adopts three-point positioning, and each tack welding point is separated by 120°, as shown in Figure Tack weld position, the tack welding is carried out in the groove, and the length of the welding point is 10~15 mm. Tack welding should also ensure penetration and no defects, and both ends of the welding point should preferably be pre-ground into a gentle slope. The amount of misalignment must not exceed 2 mm. The welding method in which the large pipe is rotated horizontally is adopted.
Welding requirements Large diameter pipe butt welding adopts single-sided welding and double-sided forming welding technology.
Both tack welding and formal welding are carried out by CO2 gas shielded welding method. H08Mn2SiA welding wire is selected. The diameter of the welding wire is 1.0 mm.
Welding is carried out by semi-automatic CO2 gas shielded welding machine.
1) Welding torch angle and welding method Left welding method is used for welding large diameter pipes, multi-layer multi-pass welding, and the welding torch angle is shown in Figure Welding torch angle for large diameter pipe welding.
2) From the welding point, place the weldment on the welding turret, so that a tack welding point is at the position of 1 o’clock on the clock.
3) After the preparations for welding are completed, the welding can be started.
Welding parameters. When CO2 gas shielded welding is used for horizontal rotation welding of large diameter pipes, the welding parameters in Table Welding Parameters of Horizontal Rotary Welding of Large Diameter Pipes can be selected for reference.
Bottom welding. Adjust the welding machine according to the welding parameters of the bottom welding, start the arc on the positioning weld at 1 point, and weld from right to left to the 11 point arc break. Immediately use your left hand to turn the tube clockwise by an angle, and turn the arc extinguishing point to 1 point before welding. The above welding process is repeated continuously until a complete circle of welds is welded. It is best to use a mechanical rotating device to weld while rotating; or one person rotates the tube, and the other person welds; it is also possible to use the method of holding the welding torch with the right hand and rotating with the left hand to continuously complete the whole circle of bottoming weld welding. The following points should be paid attention to when bottom welding:
①When the tube is rotated, the molten pool must be kept in a horizontal position, and the speed of the tube rotation is the welding speed.
② The bottoming weld bead must ensure that the reverse side is well formed, so the diameter of the fusion hole should be controlled during the welding process, and it should be 0.5~1 mm larger than the gap.
③ Remove the welding slag and spatter of the bottoming weld bead, and grind the local protrusions on the weld bead.
Filler welding. Adjust the welding parameters of the filler welding, weld the filler bead according to the bottoming method, and pay attention to the following matters:
①The lateral swing of the welding torch should be slightly larger, and it should stay on both sides of the groove properly to ensure that both sides of the weld bead are well fused, and the surface of the weld bead is flat and slightly concave.
② Control the height of the last layer of filling bead so that it is 2-3 mm lower than the base metal, and the edge of the groove must not be melted.
Cover welding. Adjust the welding parameters of the cover surface, finish welding the cover surface weld bead, and pay attention to the following matters:
①The lateral swing of the welding torch should be larger than that of the filling welding, and it should stay on both sides for a while, so that the molten pool exceeds the edge of the groove by 0.5~1.5 mm to ensure good fusion on both sides.
②The speed of turning the pipe should be slow, and the welding should be kept in a horizontal position to make the weld bead beautiful in appearance.