Stainless Steel Pipe Fittings

What is the hazard of the weld seam of the welded pipe?

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The weld seam has a high residual weld height
Stress corrosion cracks are easily formed at the weld toe
The stress concentration of the butt joint is mainly caused by the weld height, and the weld of the butt joint has the greatest stress at the weld toe.
The magnitude of the stress concentration factor depends on the weld height h, the angle θ at the weld toe, and the radius r of the corner. When the weld height h increases, the θ angle increases and the r value decreases, which increases the stress concentration factor.
The greater the residual height of the weld, the more severe the stress concentration and the lower the strength of the welded joint. After the welding, the remaining height is reduced, as long as it is not lower than the base metal, the stress concentration is reduced, and sometimes the strength of the welded joint can be increased.
The outer weld seam is high, which is not conducive to corrosion protection.
If the epoxy resin glass cloth is used for anti-corrosion during operation, the outer weld seam is high, which will make it difficult to press the weld toe. At the same time, the higher the weld, the thicker the anti-corrosion layer should be. The thickness of the anti-corrosion layer is calculated based on the apex of the outer weld, which increases the anti-corrosion cost.
Spiral submerged arc welding is often prone to “fish ridge-back” outer welds, which makes it more difficult to guarantee the quality of corrosion protection. Therefore, it is also important to adjust the spatial position and welding specifications of the welding head to reduce or eliminate the outer seam of the “fish-back”.
The outer weld seam is high, which affects the shape of the pipe after water pressure expansion
When the straight seam submerged arc welded pipe is expanded by water pressure, the steel pipe is wrapped by the left and right outer parts of the inner cavity and the steel pipe with the same diameter expansion diameter. Therefore, the residual height of the weld is too large, and the shear stress of the weld is large when the diameter is expanded, and the phenomenon of “small straight edge” is likely to occur on both sides of the weld.
However, experience has shown that when the residual height of the outer weld is controlled at about 2 mm, there will be no “small straight edge” phenomenon when the water pressure is expanded, and the shape of the pipe will not be affected. This is because the residual height of the outer weld is small and the shear stress on the welded joint is also small. As long as the shear stress is within the elastic deformation range, the rebound will occur after unloading and the tube will return to its original state.
The inner weld seam is high, increasing the energy loss of the conveying medium
If the inner surface of the submerged arc welded pipe for transportation is not treated with anti-corrosion treatment, the residual weld height of the inner surface of the submerged arc welded pipe is large, and the frictional resistance to the transport medium is also large, thereby increasing the energy consumption of the transfer line. Control measures for weld height
Steel pipes with large wall thickness should be opened
For steel pipes with a wall thickness greater than 14.3 mm, X-shaped bevels shall be opened and pre-welded. If the pre-welding conditions are not mature, the roots should be cleaned by gouging after internal welding, or the roots should be automatically ground by grinding, or the roots should be milled. The outer welds are processed into U-shaped grooves before welding and then welded. .
Adjust the welding line energy
Check the welding line energy is appropriate, generally check with the acid etching of the welded joint. One is to check the degree of overlap between the inner and outer welds, and the other is to check the width of the weld bead. The specification for the amount of overlap is generally greater than 1.5 mm, but the author believes that the amount of overlap between the inner and outer welds is suitably 1.3 to 3.0 mm. If it exceeds 3.0 mm, the line energy is large.
The line energy is large, not only the penetration depth is large, but also the weld seam height is also large. If the groove or the U-shaped groove is not opened, the weld seam height is even larger. This is because the greater the welding line energy, the more the welding wire melted per unit time. For high-strength steel, the line energy of welding should be strictly controlled. When welding high-strength steel sheets, in order to reduce the line energy of each layer, multi-pass welding (two or more passes) is generally used, and the shape factor of the weld is to be within 1.3 to 2.0 mm.
Finer front wire should be used for multi-wire welding
When multi-wire welding is adopted, if the matching of the original outer welding three wire diameter is 4mm+3.2mm+3.2mm (DC-AC-AC), the front wire is preferably changed to Ф3.2mm. Because the same current is used, the use of Ф3.2mm wire is greater than the Ф4mm wire. That is to say, the front wire adopts a welding wire of Ф3.2mm, and even if some line energy is reduced, the same penetration effect when using Ф4mm can be achieved, which is because the filament has a higher current density than the thick wire.
Experience has shown that under the same conditions, the front wire of Ф3.2mm is about 20% larger than the front wire of Ф4 mm. This is more effective when the outer weld is not opened or the groove is not cut.
However, when welding a steel pipe with a wall thickness greater than 14.3 mm and requiring a front wire current of about 1000 A, a front wire of Ф4 mm should be used, otherwise the stable combustion of the arc may be affected.
Spiral welding must adjust the position of the inner and outer welding heads
The spiral submerged arc welded pipe shall be adjusted to the position of the inner welding head during internal welding to minimize or eliminate the “saddle shape” inner weld; during the outer welding, the spatial position of the welding head shall also be adjusted to minimize or eliminate the “shoulder shape”. “The outer weld seam, which is mainly achieved by the adjustment of the off-center value of the external weld joint. For the spiral welded pipes of different calibers, the off-center values of the outer welded joints are different.
Precautions

  1. For the butt weld of the submerged arc welded pipe, the first is to make the weld be smooth and the corner radius of the weld is large, otherwise the stress will be generated at the weld toe of the weld stress concentration part. Corrosion cracks. It is more suitable to control the residual height of the submerged arc weld below 2.5 mm.
  2. When spiral submerged arc welded pipe is used for internal welding, the spatial position of the inner and outer welds should be carefully adjusted to minimize the inner weld and avoid the “saddle shape”, and the outer weld does not have “fishback shape”. It is recommended that the “saddle shape” of the inner weld be to be specified when formulating the standard for conveying spiral welded pipe.

Source: China Welded Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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Steel Pipe

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