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PIPE WELD—Large-diameter horizontal fixed all-position welding 3/3

In the manufacture and maintenance of ships, boilers, chemical equipment, etc., the butt joints of pipes occupy a certain proportion, and many water pipes, oil pipes and steam pipes need to be welded. The requirements for pipe welding are first to ensure the tightness of the weld, that is, to ensure that the pipe does not leak under the working pressure, and secondly, the back of the weld is not allowed to burn through and slag leakage. The metal flow and solidification caused by burning through will protrude on the inner wall of the pipe, which will affect the normal flow rate of the liquid or gas.

When the pipe is welded horizontally, the welding direction along the circumference of the pipe is constantly changing, and the three welding positions of flat welding, vertical welding and overhead welding have undergone changes, which requires the angle of the welding torch and the swing amplitude of the welding torch to be changed continuously during welding To control the size of the melting hole, to achieve single-sided welding and double-sided forming. The main purpose of the butt joint of the pipe is to ensure that the root is penetrated without burning through, the appearance is good, and the compactness meets the requirements. The selection of welding parameters should be just right.

Preparation of Pipe weld Weldments

The welding material adopts 20 steel pipe, the size is 133 mm×10 mm×100 mm, and the groove size and assembly requirements are shown in Figure Pipe weld Groove size and assembly requirements. If the weldment is not straight or has other defects, it should be leveled.

pipe weld
Pipe weld Groove size and assembly requirements

Use cleaning tools to clean the groove and the oil, rust, moisture and other contaminants within 20 mm of the front and back sides of the groove until the metal luster is exposed, and the burrs are removed.

Pipe weld Weldment assembly technical requirements

  • Assembly requirements. The assembly gap of the weldment is 2 ~ 2.5 mm, the center line (axis) of the two welded pipes should be on the same line, and the deviation (coaxiality) of the two center lines should be less than or equal to 0.5 mm, as shown in Figure Pipe weld Groove size and assembly requirements . When welding, leave a blunt edge of 0~1 mm. The misalignment of the weldment is not more than 1.2 mm.
  • tack welding. Three-point tack welding is usually used for tack welding of weldments, and the position is around 3.9.12 o’clock (with the clock as a reference). Tack welding requires penetration of the root and good formation of the reverse side. It is not suitable for tack welding at 6 points, because 6 points are the starting point. Start solder joints. If the pipe diameter is less than 76 mm, two-point tack welding can also be performed. The length of the tack weld is 10~20mm. The tack weld should be carefully inspected. If any defects are found, the tack welding should be performed again.

  • The horizontal butt welding of CO2 gas shielded large-diameter pipes adopts single-sided welding and double-sided forming technology for welding.

Pipe weld Welding materials

The tack welding during welding and the welding seam during formal welding are all welded by gas shielded welding method. H08Mn2SiA welding wire is selected. The diameter of the welding wire is 1.2mm. The surface of the welding wire should be cleaned before use. CO2 gas purity requirement reaches 99.5%

Pipe weld welding equipment

CO2 gas shielded welding adopts semi-automatic CO2 gas shielded welding machine for welding.

Pipe weld Operation process

Pipe weld Welding parameters

 When CO2 gas shielded welding is used for horizontal butt welding of large-diameter pipes, the welding parameters in Table Welding parameters of horizontal fixed welding of large diameter pipes.can be selected for reference.

pipe weld

Welding levels and welding methods

  • The horizontal butt welding of the CO2 gas shielded large-diameter pipe adopts the left welding method, and the welding level is three layers and four passes.

  • Before and during welding, the contact tip and nozzle should be checked and cleaned, and the wire feeding should be checked. It can be welded clockwise or counterclockwise. The angle between the welding torch and the welding direction and the two sides of the weldment is shown in Figure Position of butt wire fixed horizontally.

  • welding operation. The welding is divided into two semi-circles, and the single-sided welding and double-sided forming are from bottom to top.

pipe weld Position of butt wire fixed horizontally
Position of butt wire fixed horizontally

Bottom welding

Start welding after arc strike about 10 mm past 6:00, and the welding torch swings in a small zigzag shape, as shown in Figure Pipe weld bottom welding-a. The amplitude should not be too large, as long as the base metal on both sides of the groove is melted, the welding wire swings to the sides for a while.To prevent the welding wire from penetrating the molten pool or failing to weld thoroughly, do not leave the molten pool. Swing the welding wire horizontally at about 1/3 of the front half of the molten pool (as shown in Figure Pipe weld bottom welding-b,I is the length of the molten pool) and rise gradually.  The speed of welding gun forward should be changed according to the change of welding position. In vertical welding, the molten pool should have more cooling time to avoid welding tumors.  It is necessary to control the uniform size of the molten hole and avoid the disconnection of the molten pool.  When welding to 12 point when the arc, equivalent to flat welding arc.  

Before half a circle after welding, grind the beginning and end of the weld at 6 o’clock and 12 o’clock into an oblique (gentle) slope with a length of 10~20mm. Start the arc at 6 points in the grinding area, and then pull it back to the end of the grinding area to start welding; swing the welding torch according to the shape of the grinding area, and hear a “pop” breakdown sound when welding to the limit of the grinding area. That is, the back surface was well formed. Then weld the second half circle like the first half circle,Until the welding is 10 mm away from 12:00, the welding wire is oscillated in a straight line or in a very small zigzag shape, and the arc is stopped after welding through the grinding area.

pipe weld
Pipe weld bottom welding

Fill layer

Before the filler layer is welded, the spatter on the surface of the underlying weld should be cleaned up, and the convex part of the joint should be ground flat with an angle grinder, the nozzles should be cleaned, and the welding parameters can be adjusted before the filler layer welding.

The welding gun for welding the filling layer is the same as that for the bottom layer, and the welding wire should swing left and right at the center 1/2 of the molten pool, using a zigzag or crescent swing, as shown in Figure Weaving of filler wire. The welding wire stops a little on both sides, and the speed is slightly faster in the central part, and the amplitude of the swing refers to the width of the previous welding seam.

Before welding the second half of the filler layer, the beginning and end of the first half of the weld must be ground into a slope shape, especially at 6 points. The method of the half circle after welding is basically the same as that of the first half circle, mainly because the beginning and the end are required to be well formed. After the filler layer is welded, the thickness of the welding seam reaches 1 to 2 mm from the surface of the pipe, and the edge of the bevel surface of the pipe cannot be melted. If local unevenness is found, it should be filled and ground.

pipe weld
Weaving of filler wire

Cover welding

Clean the surface of the filler weld before welding. The welding operation method of the cover layer is the same as that of the filling layer, but the lateral swing of the welding torch should be larger than that of the filling layer, so as to ensure that the edges and corners of each side edge of the deep groove of the molten pool are 0.5~1.5mm, and the arc stays on the edge of the groove for a short time. Swing speed is slow.

In the joint, the arc ignition point should be above the center of the weld, and the arc should be stabilized slightly after the arc ignition, that is, the arc should be pulled to the center of the molten pool for welding. When welding the cover layer, the welding speed should be uniform, and the size of both sides of the deep groove of the molten pool should be the same to ensure the appearance of the weld.

Requirements and standards for pipe weld

See the table for the welding requirements and standards for horizontal butt fixing of CO2 gas shielded large-diameter pipes

pipe weld
Welding requirements and standards for horizontal butt fixing of CO2 gas shielded large diameter pipes