Common Welding Problems of Large Diameter Spiral Pipe

Large-diameter spiral pipe, also known as large-diameter welded tube, is a welded pipe made of steel plate or steel strip after crimping and forming. Large diameter spiral pipe has simple production skills, many varieties and specifications, and less equipment investment, but its general strength is lower than that of seamless steel pipe. Since the 1930s, with the rapid development of advanced strip rolling production and the advancement of welding and testing techniques. The quality of welds has been continuously improved, and the types and specifications of large-diameter spiral pipes are increasing day by day, and seamless steel pipes have been replaced in more and more areas. Large-diameter spiral pipes are divided into straight seam welded pipes and spiral welded pipes according to the form of welds.

Longitudinal welded pipe is simple to manufacture and develops rapidly. The strength of the spiral welded pipe is generally higher than that of the straight seam welded pipe, and the welded pipe with a larger diameter can be produced with a narrower blank, or the welded pipe with different diameters can be produced with a blank with the same width. However, compared with the straight seam pipe of the same length, the weld length is increased by 30~100%, and the production speed is lower. Therefore, most of the welded pipes with smaller diameters are straight seam welded pipes, and the welded pipes with large diameters are mostly spiral welded pipes. The flaw detection of steel pipes before leaving the factory is indispensable, and it can be said to be very important.

Common weaknesses in general steel pipe welds include: porosity, slag inclusions, incomplete penetration, incomplete fusion and cracks. It is only based on the shape of the defect wave obtained on the fluorescent screen and the change in the height of the reflected wave to analyze the position of the fault and the welder to comprehensively estimate the error. The assessment and judgment of the nature of internal malpractices, as well as the causes and preventive measures for the occurrence of defects generally summarize the following points:

1. Slag inclusion

The echo signal of point-shaped slag inclusions in the steel pipe is similar to that of point-shaped pores. The strip-shaped slag-inclusion echo signals are mostly jagged with low amplitude, and the waveform is mostly dendritic. There are small peaks on the side of the main peak, and the amplitude of the probe translation changes. When detecting from all directions, the reflected amplitudes are not mutually exclusive. The reasons for this failure are: the welding current is too small, the speed is too fast, the slag has no time to float, and the edge to be welded and the welds of each layer are not cleaned cleanly. The chemical composition of the base metal and the welding material is inappropriate, containing more sulfur and phosphorus. Preventive measures include: correct selection of welding current, the slope, curvature, and length of the welded parts should not be too small, the groove must be cleaned and marinated before welding, and the welding slag must be eradicated layer by layer during multi-layer welding; Reasonably determine the welding speed of the strip angle, etc. .

2. Incomplete penetration

The reflectivity is high, and the amplitude is also high. When the probe moves in translation, the waveform is relatively stable, and roughly similar reflection amplitudes can be obtained when both sides of the weld are detected. This kind of defect not only reduces the mechanical performance of the welded joint, but also forms a stress concentration point at the notch and end of the incomplete penetration, which often causes cracks after loading, which is a harmful defect. The reasons for this are usually: the gap between the pure edges of the groove is too small, the welding current is too small or the speed of the strip is too fast, the slope is small, the angle of the strip is wrong, and the arc is blown. The prevention sequence includes: reasonable selection of groove type, assembly clearance and correct welding skills.

3. Stomata

The echo height of a single pore in the large-diameter spiral tube is low, and the waveform is a single slit, which is relatively stable. From the left and right detection, the reflected waves are roughly the same, but when the probe is moved a little, it disappears, and a cluster of reflected waves will appear in the dense air holes. The wave height varies with the size of the air holes. . The main reason for this defect is that the welding consumables are not dried according to the specified temperature. The coating of the welding rod is deteriorating, the welding core is corroded, the welding wire is not cleaned cleanly, the current is too large and the arc is too long during manual welding; Assuming that there are pores in the weld of the large-diameter spiral pipe, it will not only damage the compactness of the weld metal, but also reduce the effective cross-sectional area of the weld and reduce the mechanical performance. In addition, when there are chain-like pores, there will be a contrast between bending and attack toughness.

The steps to prevent this kind of defect are: Do not use electrodes with cracked, peeled, degraded and corroded welding cores. Rusty welding wires must be used after derusting. The welding materials used should be dried according to the specified temperature, the groove and both sides should be cleaned, and the appropriate welding current, arc voltage and welding speed should be selected.

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