3, welding process
In order to obtain high-quality under-weld welds, in addition to keeping the welding equipment in a normal state and selecting suitable welding materials, it is necessary to adopt a reasonable process and be operated by qualified welders with high technical level.
1 Windshield and cleaning requirements The argon arc welding site must have reliable wind-shielding measures and prevent the wind from entering the pipe, so as not to affect the protection effect. The filler, oil, dirt, rust and other debris within 10-15mm inside and outside the nozzle should be cleaned until the metallic luster is exposed. If necessary, wash with acetone.
2 The inner wall of the weld is filled with argon to protect the low carbon steel and low alloy heat-strength steel pipe joints. When the argon arc welding is used, the pipe can be protected from argon. For high-alloy heat-strength steel and austenitic stainless steel pipe joints , must be filled with argon protection. There are two ways to fill argon: one is that the entire pipe is filled with argon and is suitable for pipes with small diameters, such as stainless steel sampling pipes. Pipes with larger diameters usually use soluble paper or install pistons in the pipes on both sides of the weld to form a small gas chamber filled with argon only in the gas chamber. In order to prevent a large amount of leakage of argon from the gap, a ring of asbestos rope or a tape of tape should be embedded in the gap before welding, and the part that hinders the welding is pulled off at any time during the welding process. At the end of the bottom welding, quickly pull out the argon tube and make up the hole. The soluble paper as the wall of the air chamber is burned to ash at the time of heat treatment of the weld, and is removed with water after the water pressure test. The air-filled air flow rate of the inner wall varies with the size of the air chamber and the degree of air leakage, so as to achieve good protection, and it is not suitable for the weld bead due to too large flow.
3 Groove form and size The size of the groove form and the gap of the pipe end assembly have a great influence on the quality of the weld and the root crack. Commonly used groove forms are v-shaped, U-shaped, double-v-shaped, etc., and the pipe end assembly has a certain gap.
4 Tungsten extreme shape Tungsten extreme shape has a great influence on arc stability and weld formation. The ideal shape is that the tungsten end is ground at an obtuse angle or with a flat top. This can stabilize the arc combustion, reduce the diffusion of the arc column, and concentrate the heating of the weldment. The tungsten extremes should not be sharpened so as not to cause tungsten defects in the weld.
5 Welding joint assembly Spot welding pipe welding joint is generally assembled by fixture and fixed by spot welding at the root. For horizontal welded joints, pipes with a diameter of ≤60mm can be spot welded only at the flat welding position, with a length of about 10-20 mm; pipes with a diameter of >159 mm are generally spot welded at the flat welding and vertical welding positions. It is about 30 to 50 mm long. The number of welding points of the vertical welding joint is the same as that of the horizontal welding, and the spot welding position is determined according to the specific conditions. The welding wire used, the welding process and the technical requirements for the welder are the same as in the case of formal welding.
6 Pre-heating argon arc welding welds before welding are relatively pure, and low hydrogen, generally can not preheat, but if it is not preheated during winter construction or thick-wall pipe welding, cracks may occur on the bottom weld. The preheating parameters are selected depending on the diameter or wall thickness.
7 When starting welding and stopping welding, it is necessary to send argon in advance, and when stopping welding, it is necessary to delay argon to protect the weld from surrounding air. The arc is to be carried out in the groove. When the arc is introduced by the contact method, the operation should be stable, light and fast, and the tungsten extremum is prevented from being broken and the tungsten is trapped. When welding and closing the arc, add more welding wire to fill the arc pit. Leading the arc to the edge of the groove and then extinguishing the arc: the quality of the arc and the joint is often poor, and the welding process should avoid stopping the arc and reducing the number of joints.
8 Filling operation method The wire filling operation method is that the welding wire extends into the pipe from the opposite gap, the arc burns on the outer groove of the pipe, the welding wire is melted in the pipe, and the whole welding process is carried out in stages. The operation method has two advantages: first, the back surface of the bottom seam is evenly convex, and the undercut portion does not appear concave; the second is particularly suitable for the difficult position welding of the dense tube of the boiler. However, when operating, the clearance is required to be large. When the Ø2. 5 mm welding wire is used, the clearance is more than 3 mm, the wire filling amount is large, and the welding speed is correspondingly slower.
When the outer wire is welded, the weld fitting clearance is small, and the operation requirement is stable and fast. For small-diameter thick-walled pipes, the clearance is equal to or slightly smaller than the diameter of the wire, and the welding gun is basically not laterally oscillated during operation. For large-diameter thick-walled tubes, the gap is slightly larger than the diameter of the wire, and the welding gun swings during operation. In the case of skilled operation, the underside of the underlying weld is also formed uniformly, and the overhead weld is not concave. The advantage of this method is that the amount of wire is small and the welding speed is fast. However, for the densely packed tube weld of the boiler which is particularly difficult to weld, sometimes the external wire filling operation is quite difficult, and it is preferable to use the combination of the inner and outer filler wires for welding. When welding boiler tubes, the quality of the pipe joints is the most difficult to guarantee. Generally, two welders are symmetrically welded.
4, other matters needing attention
1 During the welding process, all the main force of the counterparts shall not be removed, so as to prevent the external welding from cracking the underlying weld.
2 When the bottom welding is about to end, the formation of the back of the weld should be carefully observed and the problem should be dealt with in a timely manner. After the completion of the full circle of bottom welding, visual inspection shall be carried out, and if necessary, magnetic particle inspection or coloring inspection shall be carried out. After the inspection is passed, the electrode arc welding cover surface is timely performed to prevent cracks.
3 The argon arc welding bottom weld is relatively thin. Therefore, the subsequent first layer electrode arc welding should use small diameter electrode.
V. Conclusion
The TIG welding process can solve the problem of non-welding at the bottom of the weld, improve the qualification rate of the weld non-destructive inspection, effectively ensure the welding quality, and the application effect is ideal. It is worthy of application and promotion in the installation and maintenance of power stations.
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