Most of the large-diameter thermal pipelines are now made of low-carbon steel spiral pipes. The general requirement is: in the weld quality inspection of the rotating weld and the fixed weld, the sampling should reach level II or above. The operation process is introduced as follows:
First, the preparation before welding of large-diameter steel pipes
The steel pipe material is Q235, and the specification is 820mm×10mm spiral welded pipe. Use ZX7-500 AC arc welding machine and E4303 welding rods. Dry at 150℃ for 1h before use. Take it as you use it. Before welding, the dirt, oil, rust, moisture, etc. on the surface of the 60mm wide pipe on both sides of the groove should be cleaned to reveal the metallic luster.
Second, welding of rotating steel pipe joints
1. Grouping and positioning: Grouping and positioning welding of steel pipe joints is the key to ensure welding quality and promote good back forming of steel pipe joints. If the groove form, grouping gap, and blunt edge size are not appropriate, it is easy to cause defects such as concave, weld nodules, and incomplete penetration. When positioning welding, according to the situation of the steel pipe joint, 8 to 10 positioning plates (200mm×80mm×10mm) are evenly distributed around the pipe, and the grouping gap is 4mm. When positioning, the concentricity between the pipes should be ensured, and the misalignment should be <1.5mm. When welding the positioning plate, the same direction of the fillet weld of the pipe plate should be welded to facilitate removal.
2. Welding of the base weld layer. It is completed by arc welding single-sided welding and double-sided forming operation method. Use a 3.2mm diameter welding rod and weld by arc-breaking method. The welding current is 115~125A. When welding, strictly follow the principle of “arc start in the middle and arc extinguish on the right side” (that is, after the electrode arcs in the center of the groove gap, it quickly swings horizontally to the left side of the groove, then swings back to the right side of the groove and extinguishes the arc downward). Each time this action process is completed, the interval is about 1.5s. During the welding process, the welding position is adjusted to the best welding operation state by rotating the pipe fitting, that is, it is always carried out between 8:30 and 10:30 (4:30 to 2:30) of the steel pipe joint, and the positioning plates are knocked off one by one as the weld extends.
3. Welding of the filling layer. Still use a welding rod with a diameter of p3.2mm, a welding current of 115~130A, and continuous arc welding. Before welding, first clean the base layer welding slag, and then keep the welding position at 9 to 11 o’clock (3 o’clock to 1 o’clock) by rotating the pipe fitting. During welding, the welding rod is swung in the shape of “8″ with the principle of slightly slower welding on both sides and faster welding in the middle. This method can make the weld of the filling layer flat, and no deep grooves (angles) appear on both sides of the groove, to prevent defects such as slag inclusion between layers. The welding arc should be short and the welding rod should be swung evenly to increase the temperature of the molten pool and give the slag and pores on the previous weld a chance to re-melt, so as to avoid defects such as slag inclusion and pores, which is conducive to the welding of the surface layer. The upper surface of the weld of the filling layer must be 0.5~1.5mm lower than the pipe surface, and the groove contour should be retained.
4. Welding of the cover layer. Use a welding rod with a diameter of 4mm and a welding current of 150~160A. The welding operation is the same as that of the filling layer. The welding rod should be swung evenly so that the weld can be formed beautifully. The two sides of the cover weld should exceed the edge of the groove by 2mm, and the weld excess height should be about 2mm.
5. Welding of the bottom layer. For the reverse side with poor forming or technical requirements, use a 4mm diameter welding rod and a welding current of 160~180A to re-melt the weld inside the pipe and perform bottom welding. This can make the width and height of the weld inside the pipe consistent, the forming is beautiful, and the transition is smooth. The defects such as depressions, weld nodules, and shrinkage holes on the weld inside the pipe are removed, and the technical requirements of the drawings are met.
Third, the welding of fixed steel pipe joints
The groove form of the fixed steel pipe joint is basically the same as that of the rotating steel pipe joint. The difference is that the joint extinguishes the arc upward on the right side so that the left and right are alternately broken arc welding. The arc can be extinguished until it exceeds 12 points and 10mm. During the welding process, the welding speed should not be too fast, otherwise, it will affect the forming inside the pipe, and it can also cause defects such as slag inclusions and shrinkage holes: when changing the welding rod joint, the operation is the same as when welding the rotating pipe. Correct use of the welding rod angle and mastering the arc length is the key to ensuring welding quality. The electrode angles of the base layer, filling layer, and cover layer are basically the same, but the arc length and the rod feeding method are different. The shape of the molten pool is mainly controlled by the length of the arc and the rod feeding method. The welding of the pipe is completed in two half-circles, and the arc starting and arc ending points are selected 10mm in front of the center of the half-circle of the pipe.
1. Operation method of base layer
① Welding of the first half of the steel pipe joint: Use a welding rod with a diameter of 3.2mm and a welding current of 115~125A. When welding, first preheat the starting point (10mm before 6 o’clock) with a long arc. When sweat-like molten iron appears in the groove, quickly lower the arc, first swing horizontally from the right to the left, then swing from the left to the right and extinguish the arc downward to form the first molten pool seat. When the arc is started again, in order to avoid a dent on the back of the pipe wall, the arc should be aligned with the inner angle of the groove, and the welding rod should be pushed up as much as possible to make the arc act completely on the pipe wall. From 3 o’clock to 1 o’clock or 9 o’clock to 11 o’clock is a vertical welding and climbing position, the arc length should be changed to 2~3mm (from the inner surface of the groove), otherwise a large weld defect will be formed in the pipe wall. From 11 o’clock to 12 o’clock (from 1 o’clock to 12 o’clock) is close to the flat welding position, and the welding operation should be left and right alternating arc breaking welding, that is, first start the arc on the left side, when the left groove is fused, immediately extinguish the arc upward on the left side, and then start the arc on the right side, when the right groove is fused, immediately extinguish the arc on the right side, keep the size of the molten hole basically the same (it is best to melt about 2mm on both sides of the groove), and the electrode inclination is similar to that of vertical welding (tilted downward 80°~85°). When changing the electrode joint, the speed should be fast. First, stretch the arc to preheat the pipe. When you see sweating in the molten pool, quickly lower the arc for welding. When the electrode swings to the center of the groove, consciously press the arc inward and stay for about 2s. After hearing the “puff” sound, resume normal arc welding. This can effectively avoid defects such as pits or poor joints in the weld of the steel pipe joint. When sealing the joint, pay special attention to two issues: grind the starting weld of the circumferential seam into a slope; avoid another joint about 20mm away from the sealed joint.
② Welding of the second half of the circle: Before welding, use a hand grinding wheel to grind the two ends of the first half of the weld into a gentle slope, and grind off 5~10mm at the beginning and end, but do not damage the edge of the groove. The welding process is the same as the first half of the circle. However, it should be noted that when the second half of the weld is welded to about 10mm from the end of the first half of the weld and the end is closed, the arc must not be extinguished again. When the weld is closed, the electrode should be slightly pressed down. When the “tooth-beating” sound is heard, it means that the root has been melted through and the head has been connected. At this time, the arc should not be extinguished. The electrode should be swung back and forth at the joint to extend the time and fill the molten iron to fully fuse the seal to avoid the formation of shrinkage.
2. Filling layer welding: The filling layer electrode changes with the change of pipe diameter. The continuous arc welding, welding current and rod movement techniques are the same as the rotating pipe.
3. Cover layer welding: The cover layer welding is basically the same as the filling layer.
4. Bottom layer welding: The bottom layer welding is different from the rotating pipe and requires full position welding.
Fourth, post-weld treatment
After the thermal pipeline is welded, X-ray flaw detection → water pressure test → weld joint anti-corrosion treatment → backfilling after passing the masking or outer insulation layer. This process method is used to weld thermal pipelines. The process is simple, easy to master, with high welding efficiency, low weld repair rate, good welding quality, and easy to promote.
Fifth, summary
1. The key to the base welding is to ensure the internal forming quality of the pipe by controlling the size of the molten hole. If it is too large, it is easy to cause weld nodules; if it is too small, it is easy to cause defects such as incomplete penetration. The diameter of the molten hole = the gap between the groups + 3mm (1.5mm melted on each side of the groove) is appropriate.
2. When welding the base, the arc-extinguishing electrode position is never allowed to be selected at the center gap of the groove. It should be selected on any side of the groove or on the weld meat after welding so that there will be no shrinkage cavity at the root.
3. It is advisable to use the “8″-shaped rod method for filling layer welding. This can weld a smooth and neat weld without angles on both sides of the groove, and can avoid defects such as interlayer slag inclusions and pores.
4. In order to obtain a smooth transition and beautifully formed weld, it is advisable to use a crescent-shaped welding strip for the cover layer, and to complete it with an appropriate welding speed (where the filling layer is high, the welding speed is slightly faster, and where the filling layer is low, the welding speed is slightly slower).
5. When welding the bottom, pay attention to ventilation measures.
Post time: Apr-08-2025