Important structures such as boilers and pressure vessels require safe penetration of joints. However, due to restrictions on the size and shape of the structure, double-sided welding is sometimes not possible. Only the special operation method of opening single-sided grooves can be used to form single-sided welding and double-sided forming technology. It is a difficult operation skill in manual arc welding.
During vertical welding, because the temperature of the molten pool is too high, under the action of gravity, the droplets formed by the melting of the electrode and the molten iron in the molten pool are easy to drip down to form weld bead and undercut on both sides of the weld. When the temperature is too low, slag inclusion is easy to occur, and defects such as incomplete penetration and weld bead are easy to form on the reverse side, which makes it difficult to form the weld. The temperature of the molten pool is not easy to determine directly, but it is related to the shape and size of the molten pool. Therefore, as long as you carefully observe and control the shape and size of the molten pool during welding, you can achieve the purpose of controlling the temperature of the molten pool and ensuring the welding quality.
Based on the master’s more than ten years of experience, this rule can be summarized in a few sentences:
1. The angle of the electrode is very important, and the welding specification is indispensable
During vertical welding, the droplets formed by the melting of the welding rod and the molten iron in the molten pool are easy to drip to form weld bead, and undercut on both sides of the weld, which deteriorates the weld formation. It is necessary to grasp the correct welding specifications and according to the welding conditions Change the angle of the welding rod and the speed of transporting the rod.
The angle between the electrode and the surface of the weldment is 90° in the left-to-right direction, and the angle to the weld is 70°~80° at the start of welding, 45°~60° in the middle, and 20°~30° at the end. The assembly gap is 3~4mm, the smaller electrode diameter Φ3.2mm and the smaller welding current should be selected, 110~115A for the bottom welding, 115~120A for the intermediate layer, and 105~110A for the cover layer.
The current is generally 12% to 15% smaller than that of flat welding to reduce the volume of the molten pool, so that it is less affected by gravity, which is beneficial to the droplet transfer. Short-arc welding is used to shorten the distance from the droplet to the molten pool to form a short-circuit transition.
2. Observe the molten pool, listen to the arc sound, keep the shape of the molten hole in mind
The bottom welding at the root of the weld is a key to ensuring the welding quality. The arc extinguishing method is adopted for welding. The arc extinguishing rhythm of vertical welding is slightly slower than that of flat welding, 30-40 times per minute, and the arc burns slightly longer at each point of welding, so the welding meat of vertical welding is thicker than that of flat welding.
When welding, start welding from the lower end. The angle of the bottom electrode is about 70°~80°. Two-click penetration welding is used. The arc is ignited on the side of the groove and the spot welding point is preheated and melted to the root, and the arc penetration is heard. The “pounce” sound from the bevel, see the molten hole and the formation of a molten pool seat, immediately lift the electrode to extinguish the arc.
Then re-ignite the other side of the groove, and the second molten pool should press 1/2~2/3 of the first molten pool that begins to solidify, so that the entire weld can be obtained by the left and right arc extinguishing breakdown. The arc extinguishing requires the flexibility of the wrist, and the arc is extinguished cleanly every time, so that the molten pool has the opportunity to instantaneously solidify.
When the arc is extinguished, the molten hole formed by the pierced blunt edge is clearly seen. The molten hole of vertical welding is about 0.8mm. The size of the molten hole is closely related to the back surface forming. If the molten hole is too large, it is easy to form a weld on the back, and vice versa. The back of the weld hole is often incomplete, and the size of the weld hole is required to be uniform during operation, so as to ensure uniform penetration at the root of the groove, full weld bead on the back, and uniform width and height.
When changing the welding rod joint, clean the coating of the joint part every time, re-ignite the arc in the groove, and change the angle of the welding rod continuously at about 10mm along the formed weld seam, and extend into the weld seam when it reaches 90 degrees The center swings slightly from side to side and presses down the arc at the same time. When the arc sound is heard, a molten hole is formed, and the arc is immediately extinguished, so that the electrode arc extends into the root of the weld, forming a molten hole and immediately extinguishes the arc.
Then it is the same as the first welding rod bottom welding method. The left and right alternately circulate the arc extinguishing and breakdown. Each action must be concentrated. Pay attention to the outline of the molten hole and the notch that is melted on both sides, and the notch that is melted at the root of the groove. When the arc is moved to the other side, it can be seen. It is found that the blunt edge is not fused well and the arc is slightly lowered to achieve a good fusion. Start the arc again.
When closing the arc, it should be noted that when each electrode is only 80~100mm long, the electrode will melt faster due to overheating. At this time, the arc extinguishing time should be increased to make the molten pool instantaneously solidify to prevent the high-temperature molten pool from falling down and forming weld bumps. . When the welding rod is only 30~40mm left, prepare to do the arc extinguishing action, drop one side of the molten pool continuously two or three times, so that the molten pool can be slowly cooled down, which can prevent shrinkage holes and crater cracks on the front and back of the weld bead. defect.
3. The temperature of the molten pool is well controlled, and the quality of the weld can be improved
The middle layer welding wave is required to be flat. The middle two layers are based on the electrode diameter Φ3.2mm, the welding current is 115~120A, and the electrode angle is about 70°~80°. The zigzag method is adopted to use the electrode angle, the length of the arc, the welding speed and the residence time on both sides of the groove. To control the temperature of the molten pool. Make both sides fuse well, and ensure the shape of the oblate molten pool.
When welding the third layer, do not damage the edge of the groove, and leave a depth of about 1mm to make the entire fillet weld bead smooth. The edge of the groove above the depth is used as the reference line to lay the foundation for the cover surface. In general, use left and right swings. Under normal circumstances, stop a little more on both sides of the groove to melt the edge of the groove by 1～2mm, and ensure that the temperature of the molten pool and both sides of the groove is balanced. .
Mainly observe the shape of the molten pool, and control the molten pool into a crescent shape. The side with more molten pool stays less, and the side with less molten pool stays more. The height and width of the weld are calculated while welding. Because the weld meat of vertical welding is thicker than that of flat welding, pay attention to the shape of the weld pool and the thickness of the weld. The arc extinguishing time is used to reduce the temperature of the molten pool. The arc crater must be filled before replacing the welding rod to prevent arc crater cracks.
4. Make sure that the method of transporting the strip is correct, and the weld can be formed properly
For surface welding, zigzag or crescent-shaped strips can be used during welding. The strips should be transported steadily, the speed should be slightly faster in the middle of the weld bead, and the edges on both sides of the groove should be stopped for a while. The process specification is that the diameter of the electrode is Φ3.2mm, the welding current is 105~110A, the angle of the electrode should be kept at about 80°, and the electrode should swing from side to side to melt the edge of the groove by 1~2mm, and it will vibrate slightly when the sides stop.
But when the electrode moves from one side to the other, the arc in the middle is slightly lifted to observe the shape of the entire molten pool. If the molten pool is in a flat oval shape, it means that the temperature of the molten pool is appropriate, normal welding is carried out, and the weld surface is well formed. If you find that the bottom of the molten pool is rounded, it indicates that the temperature of the molten pool is slightly higher. The method of transporting the rod should be adjusted immediately, that is, the electrode staying time on both sides of the groove is increased, the intermediate transition speed is increased, and the arc length is shortened as much as possible.
If the molten pool cannot be restored to a flat elliptical state, and the bulge increases, it means that the temperature of the molten pool is too high, and the arc should be extinguished immediately to allow time for the molten pool to cool down, and then continue welding after the temperature of the molten pool drops.
When covering the surface, make sure that the edge of the weld is good. It is found that the undercut electrode moves slightly, or stays longer to compensate for the defects, and the surface is too smooth to be smooth. When the cover joint is welded, the temperature of the weldment is low, which is easy to produce defects such as poor fusion, slag inclusion, joint disjoint, and excessive height. Therefore, the quality of the cover directly affects the surface formation of the weld. Therefore, the preheating method is used to weld the joints, and the arc is ignited from top to bottom by the scratch method about 15mm above the welding end, and the arc is elongated by 3~6mm to preheat the weld starting point. Then lower the arc, and swing 2 to 3 times at 2/3 of the original arc pit to achieve a good fusion and then switch to normal welding.
Although the positions of the welds are different, they also have the same laws. Practice has proved that selecting the appropriate welding process parameters, maintaining the correct electrode angle and mastering the three actions of the good luck bar, strictly controlling the temperature of the molten pool, welding In vertical welding, excellent weld quality and beautiful weld formation can be obtained.
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