Pipeline welding issues

Generally, a welding allowance is left during welding, as there is thermal expansion and contraction at the weld joint. When assembling the steel frame, should more spacing be left between the columns?

It facilitates full penetration, allowing for single-sided welding with double-sided formation.

The answer above is correct; it’s necessary to ensure thorough welding, otherwise radiographic inspection will easily reveal lack of fusion at the root of the weld

In the case of thin-walled pipes, the main goal is to achieve double-sided formation through single-sided welding； In the case of thick-walled pipes, the main purpose is to avoid lack of fusion at the root of the weld. If the gap is too large, it can lead to burning through or the formation of weld beads, both of which have an adverse effect on the results of non-destructive testing.

The only purpose of leaving a gap is to ensure thorough welding, but the specification provides a range of values (such as 1.5–2.0 mm), rather than fixing it at 1 mm.

A gap of 1-2 mm is sufficient for process pipelines. When constructing large steel structures, we leave a gap of 2-4 mm; the main purpose of this is to ensure proper weld penetration. If too much gap is left, welding can only take place after the metal has grown together, otherwise the pre-welding inspections will not be passed. On-site workers* usually judge based on the diameter of welding rods and wires.

When performing butt welding on pipe joints, V-shaped or Y-shaped grooves are generally used. A butt gap of 1–2 mm is left during welding in order to ensure complete penetration of the welded parts, preventing defects such as incomplete fusion or lack of penetration at the root of the weld, thus achieving a result where one-sided welding yields a weld with a finished appearance on both sides. Everyone is welcome to share their experience in pipeline welding. Common groove types include: Y-groove, VY-groove, U-groove with bevel, I-groove, V-groove with backing plate, Y-groove with backing plate, double U-groove with bevel, double Y-groove with bevel, K-groove, I-groove (T-joint), Y-groove (T-joint), V-groove with backing plate (T-joint), single V-groove (T-joint), single V-groove with backing plate (butt joint), I-groove (lap joint), I-groove (gas tungsten arc welding), V-groove (gas tungsten arc welding), X-groove (gas tungsten arc welding), single V-groove, single V-groove with bevel, 2/3 double Y-groove, and a total of 21 types (see Table 1). During the welding process, we appropriately select different groove shapes for welding based on factors such as material, plate thickness, and welding method.

The purpose of reserving 1mm is mainly to ensure thorough welding and high weld strength. Leave a 1mm gap, so that the molten metal can effectively bond the two pieces of material during welding, and this also facilitates single-sided welding with double-sided formation. In fact, the gap reserved during welding work is usually around 2-3 mm; 1 mm is generally not reserved.