The welding technology of pipe butt joints in different locations in space is also an operational skill that welders must master. For the convenience of description, seamless steel pipes with a diameter of no more than 60mm are referred to as small-diameter pipes, and seamless steel pipes with a diameter of 133mm or more are referred to as large-diameter pipes.
For example, 20 seamless steel pipes are used, and the small diameter pipe size is φ42mm x 5mmx 100mm. The size of the large diameter tube is φ133mm x 10mm x 120mm, and its processing requirements are shown in Figure—Preparation of pipe butt welding test pieces
Use tools such as files, emery cloths, wire brushes or angle grinders to remove rust, oil, oxide scale and other impurities within 20mm of the bevel edges of the inner and outer walls of the pipe to expose the metallic luster. However, special attention should be paid not to damage the groove angle and blunt edge size during the grinding process, so as not to cause difficulties in bottom welding.
During assembly and positioning welding of pipe butt joints, the welding wires used are the same as those used for formal welding. During tack welding, the room temperature should not be lower than 15°C. The tack welds should be evenly distributed in 3 places, 10~15mm long, and should be connected by bridges to avoid damaging the edges of the groove. When welding to the tack weld, use an angle grinder to grind off the tack weld before welding. Groove clearance: The clearance at 6 o’clock is 2mm, and the clearance at 0 o’clock is 1.5mm. The blunt edge of the bevel can be customized.
Use the DC positive connection method (the tube is connected to the positive electrode, and the tungsten electrode of the welding gun is connected to the negative electrode).
The welding wire is selected from H05MnSiAITiZr, specification φ2.5mm, and cut into lengths of 800 ~ 1000mm, and the wire is polished with emery cloth to give a metallic luster; the protective gas is argon with a purity of 99.99%; the tungsten electrode brand is WTh-15, specification φ2.5mm x175mm ,The geometric shape of its end grinding is shown in Figure -Tungsten tip geometry.
Small-diameter pipes need to undergo appearance, ball passing, fracture, and cold bending inspections, while large-diameter pipes need to undergo appearance, X-ray flaw detection, and fracture inspection. The inspection requirements are as follows:
During the appearance inspection, the appearance is inspected with the operator’s eyes or a magnifying glass less than 5 times, and the location and size of the defects are determined with measuring tools. The following requirements should be met:
The surface of the weld should be in its original state without traces of processing or repair welding. Appearance dimensions comply with Table-Welding seam dimensions table.
Defects such as cracks, lack of fusion, slag inclusions, pores, welding nodules and lack of penetration are not allowed on the weld surface.
Allowable welding defects: the undercut depth is less than or equal to 0.5mm, the total length of the undercut on both sides of the weld should not exceed 10% of the effective length of the weld; back pit: when δ≤5mm, the depth should not be greater than 25%δ, and Not more than 1mm; when δ>5mm, the depth should not be more than 20%δ, and not more than 2mm.
Pipes with an outer diameter less than or equal to 76mm are subject to the ball passing test. When the outer diameter of the pipe is greater than or equal to 32mm, the ball diameter is 85% of the inner diameter of the pipe. When the outer diameter of the pipe is less than 32mm, the ball diameter is 75% of the inner diameter of the pipe.
Pipe specimens with an outer diameter of 76 mm or more must be inspected for flaw detection. The radiographic quality should not be lower than AB level, and the weld defect level should not be lower than ΙΙ level to be qualified.
Take two random specimens of small-diameter pipes for fracture testing. There are no cracks or lack of fusion on the section. Other defects should comply with the provisions of Table-Fracture inspection standards for small diameter pipe specimens. The inspection results of the two fracture specimens at each specimen position must meet the above requirements to be qualified, otherwise they will be unqualified.
Cut the cold-bent specimen according to the position specified in Figure-Sampling position of cold-bent specimens.
After the sample is processed according to the regulations, a cold bending test is performed. If both the back and face bending test pieces are qualified, it is considered qualified. If only one sample is qualified, another sample is allowed to be taken for retest. If the retest is qualified, it is considered qualified, otherwise it is considered unqualified.