How to effectively improve the welding technology level of steel pipes

Important structures such as boilers and pressure vessels require steel pipe joints to be safely welded through, but due to restrictions on structural size and shape, double-sided welding is sometimes impossible. A special operation method that can only open a single-sided groove is single-sided welding and double-sided formation technology, which is a more difficult operation skill in manual arc welding.

When welding steel pipes vertically, due to the high temperature of the molten pool, under the action of gravity, the molten droplets formed by the melting of the welding rod and the molten iron in the molten pool are easy to flow down to form weld nodules, and undercuts are formed on both sides of the steel pipe weld. When the temperature is too low, slag inclusions are easy to occur, and defects such as incomplete penetration and weld nodules are easy to form on the reverse side, making it difficult to form the steel pipe weld.

The temperature of the molten pool is not easy to judge 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 molten pool temperature and ensuring the welding quality.

1. The welding rod angle is very important: steel pipe welding specifications are indispensable

During vertical welding of steel pipes, the molten droplets formed by the melting of the welding rod and the molten iron in the molten pool are easy to flow down to form weld nodules and undercuts on both sides of the steel pipe weld, which deteriorates the formation of the steel pipe weld. Master the correct steel pipe welding specifications and adjust the welding rod angle and rod feeding speed according to the changes in the situation during steel pipe welding.

The angle between the steel pipe welding rod and the surface of the weld is 90° in the left and right directions, and the angle with the steel pipe weld is 70°~80° when starting welding, 45°~60° in the middle, and 20°~30° at the end. The assembly gap is 3~4mm, and a smaller welding rod diameter of Φ3.2mm and a smaller welding current should be selected. The base welding is 110~115A, the intermediate transition layer is 115~120A, and the cover layer is 105~110A. The current is generally 12% to 15% smaller than that of flat welding to reduce the volume of the molten pool and reduce the influence of gravity, which is conducive to the transition of the molten droplet. Short arc welding is used to shorten the distance from the molten droplet to the molten pool to form a short circuit transition.

2. Observe the molten pool and listen to the arc sound: remember the shape of the molten hole

The root welding of the steel pipe weld is a key to ensure the welding quality. The arc extinguishing method is used for welding. The arc extinguishing rhythm of vertical welding is slightly slower than that of flat welding, 30 to 40 times per minute. The arc burns slightly longer during each point welding, so the weld meat of vertical welding is thicker than that of flat welding.

When welding, start welding from the bottom end. The angle of the base electrode is about 70° to 80°. Two-point penetration welding is used. The arc is ignited on one side of the groove preheated and melted along the spot welding point to the root. When the arc penetrates the groove, the "puff" sound is heard, and the molten hole is seen and the molten pool seat is formed, the electrode is immediately lifted to extinguish the arc.

Then re-ignite the other side of the groove, and the second molten pool should suppress the first molten pool that begins to solidify by 1/2 to 2/3. In this way, the entire steel pipe weld can be obtained by using left and right arc extinguishing. Arc extinguishing requires the flexibility of the wrist, and the arc should be extinguished cleanly every time to give the molten pool a chance to solidify instantly.

When extinguishing the arc, the molten hole formed by the blunt edge that was punctured can be clearly seen. The molten hole of vertical welding is about 0.8mm. The size of the molten hole is closely related to the back forming. If the molten hole is too large, it is easy to form a weld nodule on the back. On the contrary, if there is no molten hole, the back is often not welded through. During operation, it is required to keep the size of the molten hole uniform, so as to ensure that the root of the groove is evenly penetrated, the back weld is full, and the width, narrowness, height, and lowness are uniform.

When changing the welding rod joint for base welding, the coating of the steel pipe joint should be cleaned every time, and the arc should be re-ignited in the groove. The welding rod angle should be changed continuously along the formed steel pipe weld at about 10mm. When it reaches 90 degrees, it should be inserted into the center of the steel pipe weld and slightly swing left and right, and at the same time, the arc should be pressed down. When the arc sound is heard, a molten hole is formed, and the arc should be extinguished immediately, so that the welding rod arc is inserted into the root of the steel pipe weld, and the molten hole is formed and the arc is extinguished immediately.

Then, the same as the base welding method of the first welding rod, the arc is extinguished and punctured alternately from left to right. Each action should be focused, and attention should be paid to observing the outline of the molten hole and the melted gaps on both sides. The melted gap at the root of the groove can only be seen when the arc moves to the other side. If the blunt edge is not well fused, slightly move the arc downward to achieve good fusion. The arc extinguishing time should be controlled each time when one-third of the molten pool has not solidified before the arc is re-ignited.

When the arc is closed, it should be noted that when each welding rod is only 80-100mm long, the welding rod will melt faster due to overheating. At this time, the arc extinguishing time should be increased to make the molten pool solidify instantly to prevent the high-temperature molten pool from falling and forming a weld nodule.

When the welding rod is only 30-40mm long, prepare to extinguish the arc and drip one side of the molten pool two or three times continuously to make the molten pool slowly cool down. This can prevent defects such as shrinkage holes and arc pit cracks on the front and back of the steel pipe weld.

3. Good control of the molten pool temperature: the quality of the steel pipe weld can be improved

The middle layer welding wave is required to be flat. The middle two layers are based on the welding rod diameter φ3.2, the welding current is 115-120A, the welding rod angle is about 70°-80°, and the sawtooth running method is used to control the molten pool temperature by using the welding rod angle, arc length, welding speed and the dwell time on both sides of the groove. Make the two 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, leave a depth of about 1mm, and make the entire fill weld flat. The depth is based on the edge of the groove above, laying the foundation for the cover surface. Use left and right swings. Generally, the two sides of the groove stop a little longer 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 are balanced. Mainly observe the shape of the molten pool, control the molten pool into a crescent shape, stay less on the side with more molten pool, and stay more on the side with less molten pool. Calculate the height and width of the steel pipe weld while welding. Because the weld meat of vertical welding is thicker than that of flat welding, pay attention to the shape of the molten pool and the thickness of the weld meat. If the lower edge of the molten pool convexes from the flat side, it means that the temperature of the molten pool is too high. At this time, the arc burning time should be shortened and the arc extinguishing time should be extended to reduce the temperature of the molten pool. The arc pit must be filled before replacing the electrode to prevent the occurrence of arc pit cracks.

4. Ensure the correct wire feeding method: the steel pipe weld can be well-formed

When welding the cover, the sawtooth or crescent wire feeding method can be used. The wire feeding should be stable, the speed should be slightly faster in the middle of the steel pipe weld, and it should stay for a while at the edges of both sides of the groove. The process specifications are that the electrode diameter is φ3.2, the welding current is 105~110A, the electrode angle should be kept at about 80°, the electrode swings left and right, so that the edge of the groove melts 1~2, and the two sides tremble slightly up and down when pausing.

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 flat and oval, it means that the molten pool temperature is more appropriate, normal welding is carried out, and the surface of the steel pipe weld is well formed.

If it is found that the bottom of the molten pool is bulging and rounding, it means that the molten pool temperature is slightly high, and the wire feeding method should be adjusted immediately, that is, the electrode stays on both sides of the groove for a longer time, speeds up the intermediate transition speed, and shortens the arc length 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. The arc should be extinguished immediately, and the molten pool should be allowed to cool down. Welding can be continued after the temperature of the molten pool drops.

When covering the surface, the edge of the steel pipe weld should be ensured to be good. If the undercut electrode is found, move it slightly, or stay for a while longer to make up for the defect. The surface transition can be smooth. When the cover joint is welded, the temperature of the weldment is low, which is prone to defects such as poor fusion and slag inclusion, joint disconnection, and excessive height. Therefore, the quality of the cover directly affects the surface forming of the steel pipe weld.

Therefore, the preheating method is used for welding at the joint. About 15mm above the welding end, the arc is ignited from top to bottom by the scratching method, and the arc is stretched by 3~6mm to preheat the welding start of the steel pipe weld. Then lower the arc and swing it 2~3 times at 2/3 of the original arc pit to achieve good fusion and then turn to normal welding.

Although the positions of steel pipe welds are different, they also have common rules. The practice has proved that by selecting appropriate welding process parameters, maintaining the correct electrode angle, mastering the three actions of electrode movement, and strictly controlling the temperature of the molten pool, excellent steel pipe weld quality and beautiful steel pipe weld formation can be obtained during vertical welding.