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Welding method of long-distance pipeline

DataTime:[10/4/2018]
Long-distance pipeline welding method:
First, manual welding
(1) Arc welding of electrodes is sensitive and cumbersome, and has strong conformability. At the same time, due to the continuous improvement of electrode performance, its deposition efficiency and mechanical properties can still meet the requirements of today's pipelines. In particular, welding welding is more commonly used. . The electrodes are cellulose-type electrodes and low-hydrogen type electrodes. The organic separation of the two methods of down welding and upward welding and the good root welding compliance of cellulose electrodes are still not replaceable by other welding methods in many places.
(2) Manual tungsten-arc welding has good welding quality and no back slag on the back. It is commonly used to stop the import and export of ballast machines, ball valves and other equipment, as well as the process pipelines with smaller diameter and thinner wall thickness. Welding device welding. The TIG welding method requires strict stop of groove settlement before welding and windproof measures during welding.

Second, semi-automatic welding
(1) Semi-automatic welding of self-shielded flux-cored wire This technology was first applied to the Kuqin Line Pipeline Project in 1996, and subsequently applied in pipeline projects in Sudan, Lancheng County, and Shibuyinlan City. This welding method is sensitive to operation, environmental compliance can be strong, welding deposition efficiency is high, welding quality is good, welders are easy to control, welding qualification rate is high, is an important filling, cap welding method in the current domestic pipeline engineering.

(2) Semi-automatic welding of CO2 gas protection With the improvement of the characteristics of the welding power source, the splash problem of CO2 gas shielded arc welding has been fundamentally dealt with after controlling droplets and arc patterns, and has begun to play an important role in pipeline welding, such as STT type CO2 inverse. Variable welding machine applications. This kind of welding method is sensitive to operation, welders are easy to control, have strong conformability to different grooves, good welding quality, high welding efficiency, and weld bead lubrication, but the welding process is greatly affected by the ambient wind speed. The STT semi-automatic root welding request nozzle group insists on the average disagreement of the counterpart gap during the process, otherwise it will produce defects such as the edge of the groove not being fused and the slag inclusion in the subsequent filling and capping bead.


Third, automatic welding
Automatic welding can be applied to root welding and filling and cap welding. The automatic root welding method adopts automatic internal welder or external welder single-side weld double-sided molding.
(1) The automatic welder used in the "West-East Gas Pipeline" pipeline project of the internal welder is a pipe root welding machine for Φ1016mm pipe diameter. The internal welder and the CNPC Natural Gas Pipeline Bureau, which were purchased by British NOREAST Company, respectively. PIW3640 internal welder. Its characteristic is that the range of applicable pipe diameter is narrow, the equipment investment is larger at one time, but the welding efficiency is very high, the root welding of the Φ1016mm steel pipe takes about 70 seconds. Since the welding is stopped in the steel pipe, the influence of the ambient wind speed during the welding process is relatively small, as shown in FIG. 1 .
(2) Automatic external welder single-sided welding double-sided forming root welding single-sided welding double-sided forming root welding equipment is mainly Italian PWT's CWS.02NRT automatic external welding machine, and the United States LINCOLN's STT power matching automatic external welding machine. The double-sided root welding equipment for single-side welding of automatic external welders handles the problem of single-sided double-sided root welding without backing. The root welding thickness reaches 4.5 mm, which is much higher than the welding thickness of internal welding machines (1 to 1.2mm), high welding efficiency, Φ1016mm steel root welding takes about 8 minutes. Due to the use of gas maintenance, the welding process is sensitive to the wind speed of the environment and windbreaks and other wind protection measures should be taken during construction.
 
Welding process
1, Welding preparation
1.1 Selection
Because the West-East Pipeline X is a DN813×9.5mm spiral steel pipe, the caliber is larger than the steel pipe normally used, so it is easier for the two steel pipes to have different circumferences, and the radius of curvature is not the same in some directions. This is relatively easy to produce a large amount of error in the construction, which directly affects the welding quality of the entire weld, so before the steel into the site it is recommended to measure the circumference of each pipe at both ends, and then have a rough distribution of the steel pipe The plan should try to make the difference between the circumferential lengths of the two steel pipes to be welded not exceed 3mm, which can reduce the defects caused by the wrong side.
 
1.2 Groove processing
Groove cleaning is very important for root welding, otherwise it is easy to produce defects such as internal bite, concave, and air hole. Groove cleaning requires removal of all dirt within 25 mm from the inside of the groove and both sides of the groove.
 
1.3 Groove grinding
Groove polishing technology requirements: grinding the inside of the groove to the emergence of metallic luster while not hurting the groove angle. Spiral Weld Grinding Technology Requirements: It is required that the spiral welding seam be flush with the base metal within 10mm on both sides of the grinding groove and gently transition at an elevation of 30 degrees. The higher requirements for the grinding of the spiral bead are due to three-dimensional stress and notch effects caused by unwelded spiral welds and pipeline welds that create cross welds, which can cause cracks, and the angles between unsanded spiral welds and the parent metal With rust, pores are easily formed on the cover. In addition, spiral welds of spiral steel pipes for welding excuses should be at least 100 mm away to avoid stress concentration and defects.
 
1.4 Groove preparation
In the groove preparation process, special attention should be paid to the angle and blunt edge of the groove. If the size of the groove is too large or too small, the weld quality will be affected. Excessive blunt edges will generally show incomplete penetration, and too small blunt edges will easily burn. Wear will affect the quality of root welding. Therefore, it should be strictly in accordance with the process requirements, generally choose between 1.5 ~ 2.0mm.
 
2, Root welding
The purpose of down welding for long-distance pipelines is to use large welding specifications and relatively low consumption of welding materials in order to achieve improved ergonomics and cost savings, and many welders still use the customary pipe all position up-welding large gaps, small blunt The use of edge-to-edge parameters as a downhole welding technique is neither scientific nor economical. Such counterpart parameters not only increase the unnecessary consumption of welding consumables, but also increase the consumption of welding consumables and increase the probability of occurrence of welding defects. Moreover, the repair of the root defect is more difficult than the defects produced in the filler cap. Therefore, the choice of the root joint parameter is quite important. Generally, the gap is between 1.2-1.6 mm, and the blunt edge is between 1.5-2.0 mm.
When performing root welding, the electrode is required to make an angle of 90 degrees with the axis of the pipe and point to the axis. The correct electrode position is the key to ensure that the back surface of the root welding is formed, especially to ensure that the root bead is located at the center of the weld and eliminated. Bite and unilateral incomplete penetration, when the adjustment of the longitudinal angle of the electrode can change the penetration of the electrode, because generally can not get a complete groove gap and blunt edge, so the welder must be adjusted by the welding rod longitudinal angle adjustment to adjust the arc Penetration force to adapt the joint groove and welding position. The electrode should remain in the center of the joint unless an arc blow occurs. The welder can eliminate the arc blow by adjusting the angle between the welding rod and the axis of the pipe and keeping the short arc. Otherwise, the inner side of the single-sided groove where the arc is blown will be bitten inside, and the other side will be incompletely welded.
For control of the weld pool, in order to obtain a well-formed root weld, it is critical to maintain a small, visible weld pool during the root weld. If the weld pool becomes too large, it will immediately lead to Bite or burn. In general, the size of the bath is 3.2 mm long, which is ideal. Once a small change in the size of the bath is found, the proper bath size should be maintained by adjusting the angle and current of the welding rod.
Root rooting is the key to ensure the quality of root welding in the entire weld. The point of root rooting is to clearly define the convex weld bead and the track line. If the clear root is excessive, it will cause the root weld to be too thin, making it easy to heat weld. Burning and burning is not enough, and slag and pores are easily generated. Qing root should use 4.0mm thick disc wheel. Our welders usually like to use 1.5 or 2.0mm reworked cutting discs as welding slag removal tools, but 1.5 or 2.0mm cutting discs are often prone to deep grooves, resulting in incomplete fusion or slag inclusion in the subsequent welding process, resulting in rework, At the same time, the 1.5-mm or 2.0-mm cutting discs are not as good as the 4.0 mm thick disc-shaped discs in terms of slag loss and slag removal efficiency. For clearing requirements, the track line is cleared and the fish back is trimmed to be flat or dimpled.
 
3, Hot welding
Under the premise of ensuring the quality of the root welding roots, thermal welding can be performed. Usually, the gap between hot welding and root welding cannot be longer than 5 minutes. Semi-automatic welding usually uses a trailing angle of 5 degrees to 15 degrees, and the angle between the welding wire and the management axis is 90 degrees. The principle of hot bead is not to make or make a small pair of lateral swings. Under the condition of ensuring that the arc is located in the front of the weld pool, it is carried down from 4 o'clock to 6 o'clock with a puddle; the 8 o'clock to 6 o'clock position should be appropriate. Swing in the lateral direction to avoid excessive convex welds in the upside-down position.
For the clearance of arc starting and arc collecting holes, a pause can be made at the arc starting point to facilitate the gas floating out of the weld pool, or the use of overlapping arc starting and arc closing is the most effective method to solve arc starting and arc collecting holes; After completion, the convex bead was removed using a 4.0 mm thick disc wheel.
If root burnout occurs during hot welding, semi-automatic welding must not be used for repair, otherwise dense holes will appear in the repair bead. The correct process is to find that the semi-automatic protection welding is stopped immediately after burn-through, and the two ends of the root welding burn-through, especially the burn-through place, are grinded into a gentle slope transition. According to the requirements of the root welding process, the hand-made cellulose electrode is used for the burn-through. The repair welding is performed, and when the temperature of the weld at the repair weld is reduced to 100 degrees to 120 degrees, the normal hot welding semi-automatic welding process is continued for welding.
The selection principle of the hot bead process parameters is based on the principle of no burn-through of the root bead. High wire feed speeds and welding voltages matching the wire feed speed should be used as much as possible. The advantage is that high welds can be obtained. Speed, high wire feed speed can achieve large penetration depth, large arc voltage can obtain a wide molten pool, which can make the residual residue after cleaning the residue in the root pass, especially the hidden slag melted in the root bead wire. Out, floating to the surface of the bath, and can get concave bead to reduce the hot bead clearance slag labor intensity.
The slag of the hot-bead slag in principle requires that the wire-wheel be cleaned, and the slag that cannot be partially removed must be removed by the grinding wheel. The local convex bead requires grinding with a 4.0 mm thick disc-shaped grinding wheel to remove the protruding parts (mainly occurring at 5:30-6:30 o'clock), otherwise columnar blow holes are likely to occur. It is not allowed to have welding slag on the weld bead because the presence of welding slag will affect the conductivity of the filling welding arc, causing momentary arcing and forming local dense pores.
 
4, Fill welding
The welding bead can only be filled under the premise of guaranteeing the quality of hot bead welding. The welding requirements of the filler welding are basically the same as those of the thermal welding. The feeding speed is slightly higher than the hot welding, and the voltage is slightly higher than that of the hot welding. After the completion of the filled bead, 2 to 4 points are required, and the 8 to 10 point filler weld is basically flush with the surface of the base metal, and the maximum remaining edge of the groove must not exceed 1.5 mm to ensure vertical welding when the surface weld is applied. Positions will not have air holes or be lower than the parent material. If necessary, filler welds will be added to add a vertical weld. The vertical filler weld only in the filler bead between 2-4 o'clock and 10-8 o'clock. When the filler weld is completed, the filling surface at the above position is much different from the groove surface, such as direct cover, complete weld bead Later, in the above position, there will be an increase in the number of vertical welds when the surface of the weld is lower than the surface of the base metal. The stand-up welding must be completed once after starting the arc, and the arc cannot be interrupted during the welding process because the welded joint at this location is prone to dense joints. The vertical fill welding usually does not swing laterally. With the molten pool down, the surface of the weld bead can be slightly convex or flat in the vertical welding position. This can avoid the concave shape of the surface of the cover weld and the center of the bead is lower than the base material. The selection principle of the welding process parameters for the vertical welding is relatively high welding wire feeding speed and a relatively low welding voltage, so that air holes can be avoided.

5, Cover welding
Cap welding can only be performed on the premise of guaranteeing the quality of the filler weld. Due to the high deposition efficiency of the semi-automatic protection welding, special attention must be paid to the selection of the welding process parameters during cap welding. The key to the selection of process parameters is the wire feed speed, voltage, back drag, dry extension and welding speed. In order to avoid the generation of blowholes, higher wire feed speeds should be used, lower voltages (voltages lower than the normal and wire feed speeds should be about one volt lower), longer dry stretches, and welding speeds to ensure welding arcs Always in front of the weld puddle is the principle. At 5 o'clock - 6 o'clock, 7 o'clock - 6 o'clock, it can increase the dry elongation and change the welding to push welding, so that a thinner bead layer can be obtained so as to avoid excessive height in the upside welding position of the welding bead. In order to eliminate the occurrence of joint blowholes on the surface of uphill and upright welds, it is usually necessary to weld once at the upright welding position. It is forbidden to generate welding joints at 2 o'clock, 4:30 o'clock, 10 o'clock and 8:30 o'clock. In order to avoid the generation of stomata. In order to avoid the occurrence of cracks in the joints at the site of the upward slope, welds are to be made between 4 and a half minutes - 6 o'clock and 8 o'clock and 6 o'clock, followed by 12 o'clock - 4:30 o'clock, 12 o'clock. The weld between the 8 o'clock and 8 o'clock positions can effectively avoid the occurrence of joint pores on the climbing site. The welding process parameters of cap welding are basically the same as those of heat welding, but the wire feeding speed is slightly higher.
 
6, Control of semi-automatic welding welding defects
The key to the operation of semi-automatic welding is to take advantage of the situation. Always keep the welding arc in the front of the welding bath and the thin layer of fast multi-pass welding is the key to overcome all welding defects. Never avoid a large single-layer thickness. , and pay attention to the stability of the welding process, welding quality is mainly related to the wire feeding speed, welding voltage, dry elongation, rear drag angle, welding walking speed of the five major welding process parameters, change any one, the remaining four parameters have to be done Corresponding adjustments.
Source: China Stainless Steel Pipelines Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)

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