Pipeline 6GR Welding Definition
6GR pipe welding involves fixing the pipe with an obstacle ring at an inclination angle of 45°.
The tube axis forms an angle of 45° with the horizontal plane and its position is fixed and unchanging.
The outer diameter of the obstacle ring is approximately 300 mm larger than the outer diameter of the thick-walled pipe and is located at the outer edge of the thick-walled pipe.
The distance between the joint of the two tubes and the end face of the thick-walled tube is less than or equal to 13 mm.
“6” indicates the oblique welding position, “G” represents the groove as it is the first letter of the word “groove” and “R” means obstacle ring.
Related Reading: What do 1G, 2G, 3G, 4G, 5G and 6G mean in welding?
Pipe assembly and positioning
The assembly and positioning of the welded pipes is crucial to ensure the quality of the 6GR weld and the proper shape of the back of the pipe joint.
If the groove type, mounting clearance and blunt edge size are not suitable, it can easily lead to defects such as collapse, overlapping and incomplete penetration.
The shape of the tube to be welded is shown in Figure 1.
To ensure proper pipe alignment, pipe butt jointing must be performed on a specialized pipe aligner and positioning must be ensured with rib plates (attached at 2 points, 10 points and 6 points).
The ribbed plate is made of 20 steel and its shape is shown in Figure 2.
The positioning location of the ribbed plate is shown in Figure 3.
Features and difficulties in operating the 6GR
The 6GR tube has an obstacle ring and the difference in thickness between the inner walls of the two tubes is 6mm.
The technical requirement states that the back of the pipe must be flush with the inner wall of the thick-walled pipe.
However, the back of the pipe must actually be formed as a full penetration fillet weld.
Backup welding can be challenging.
After extensive exploration and practice, we found a practical method that meets the requirements.
Support weld
The main challenge in backing welding is the oblique and oblique welding position of the pipe. Due to the uneven heating of the tube, it becomes difficult to transfer the molten drop to the back weld as gravity causes the molten drop to flow downwards. To overcome these challenges, it is important to adjust the welder's arc starting current to position 5 and the arc blowing force current to position 7 before welding. This facilitates arc starting, improves arc stiffness, and helps utilize the arc's blowing force to transfer molten iron to the root of the thick-walled pipe, ensuring proper formation of both sides of one-sided welding.
The welding rod should strike the arc approximately 5 mm after 6 o'clock. After ignition, the welding rod should be moved slightly up and down the groove. After the root melts and forms a molten hole, the welding rod should quickly move up and down the arc. The angle of the welding rod should be inclined toward the side of the thick-walled pipe. The small oblique serrations are used to move the electrode back and forth, and the movement must be fast, uniform and stable.
The welder must pay close attention to the welding rod, using his senses to “look”, “listen” and “feed”. “Observing” involves observing the temperature of the molten pool and the shape of the molten hole, making sure they remain the same and ensuring that the length of the arc melts the root of the thick-walled pipe. The size of the molten pool should be 1/2 inside and 1/2 outside the groove. The time for each arc follow-up should be based on the proximity of the molten pool to solidification. If the follow-up is too fast, the liquid weld pool will increase and the liquid metal will flow downward and form weld beads. If tracking is too slow, the molten liquid pool will be compressed inward, leading to a depression in the back weld. Generally, the liquid metal feeding time should be controlled within 1 to 1.5 seconds each time.
“Listening” involves paying attention to the “plop” sound from the edge of the tube groove caused by the arc breaking. Feeding” involves adjusting the arc length and molten pool size through appropriate arc length, electrode angle, welding speed and electrode handling method to accurately send the molten iron to the groove root. If these three elements are properly coordinated, good reverse conformation can be achieved.
During arc extinguishing, a molten hole must be created above the molten pool, then the molten iron must be slowly brought back 10 to 15 mm, stopping the arc on the groove side to slow down the solidification speed of the molten pool and prevent cold shrink holes. The joint should form a slope to facilitate the next joint. It is not permitted to weld and stop the arc in the center of the weld pool or directly stop the arc in the weld pool, as this may generate a cold contraction cavity on the surface of the weld pool, making it a difficult defect to repair.
When the arc crater is still red-hot, the arc should be started on the slope 10 to 15 mm below the weld pool and welded to the arc crater. As the temperature of the arc crater gradually increases, the electrode must be pushed along the prepared hole to the root of the groove. After hearing the “plop” sound, a short pause should be taken, the welding rod should be moved sideways, and then normal welding should be resumed. The key to this stage is to carry the welding rod in place, replace it quickly, accurately observe the fusion hole, and maintain stable movement. The joint should be hot connected as much as possible to ensure smooth and beautiful appearance
Filler welding
When completing the butt welding, use an angle grinder to smooth out any uneven joints. Set the arc blowing force (thrust) to position 5 and use this force to remove the slag on both sides of the weld. This will help prevent slag inclusions.
The electrode must be moved in a zigzag pattern, stopping on both sides of the groove. The molten pool must always be kept horizontal.
Remember to stagger the start and end points of adjacent layers by 10 to 15 mm.
After fill welding is completed, the weld should be 1 to 2 mm below the base metal to facilitate welding of the cover layer.
Cover welding
Before welding, adjust the arc blowing force (thrust) to position 2. This reduces the arc blowing force, making the arc more flexible and helps prevent undercut defects in the weld. It also results in a smoother, more visually appealing weld.
A triangular shape should be formed in the overhead welding position using the first half of the welding rod. Welding must then be carried out in the horizontal direction using a crescent-shaped electrode.
When the welding rod reaches the edge of the groove, pause briefly while keeping the molten puddle horizontal.
After starting the arc at the oblique welding position above the second half of the welding rod, pull the triangular area to be welded horizontally toward the bottom edge of the groove. The molten pool must overlap the weld made on the first half of the rod. Use a transverse serrated electrode movement method for this.
In the second half of the closing method, reduce the weld pool one by one as the electrode is transported to the closing position. Fill in the triangular area before stopping the arc.
The welding cover layer must have a flat arc stopping and extinguishing part, and the crater must be filled before extinguishing the arc.
