Prevention of Surface Depressions in SAW Steel Pipe Welds

Surface depressions in submerged arc welded (SAW) steel pipe welds refer to defects where localized areas of the weld surface fall below the intended, normal weld profile. This is a common surface defect encountered in submerged arc welding.
In actual production, the following measures can be taken to prevent the formation of such depressions: Select appropriate flux: While ensuring the internal quality of the weld, choose a flux characterized by low oxidation potential, low viscosity, low surface tension, and low bulk density. Sintered fluxes (such as SJ101 and SJ301) are generally preferable to fused fluxes (such as HJ431 and HJ433). Furthermore, to improve the gas permeability of the flux—especially during high-current welding—a flux with a smaller particle size should be selected.
Flux particles should be as uniform as possible; avoid mixing coarse-grained flux with excessively fine-grained flux, as this reduces gas permeability. For flux that is continuously recirculated during the submerged arc welding of steel pipes, the recovery system must be capable of removing overly fine particles to maintain a relatively uniform particle size distribution. This is particularly critical for sintered or ceramic fluxes, which may have lower particle strength. Additionally, when welding steels that are not fully deoxidized—such as A3F or Q235B—a flux with higher manganese (Mn) and silicon (Si) content should be chosen to enhance the deoxidation process and minimize gas generation. Sintered fluxes should ideally have a higher sintering temperature to help reduce their moisture content.
Ensure proper flux usage: Flux must be kept clean and free from contaminants such as rust, dust, and iron oxides. Recirculated flux must undergo magnetic separation to remove any iron oxide particles. Before use, the flux should be cleaned and dried at a specified temperature to ensure the moisture content remains within limits—typically, the moisture content should be ≤0.1%. During the submerged arc welding process, the flux burden height (the depth of the flux layer) should be kept to the minimum necessary to maintain a stable arc. In the automatic submerged arc welding (SAW) of steel pipes, the flux is recovered and reused. Therefore, the welding system should be equipped with a flux recovery and supply unit that includes magnetic separation and screening capabilities. This ensures the removal of magnetic impurities (such as iron oxides) and the elimination of slag, dust, and fine flux particles, thereby maintaining a uniform particle size distribution. Ideally, the flux supply system should also feature heating and thermal insulation components to keep both the recovered and fresh flux at a temperature above 100°C, preventing the recycled flux from re-absorbing moisture.
Careful selection of welding wire and steel plate is essential. Materials with low concentrations of elements such as C, O, N, and H should be chosen whenever possible to minimize gas generation during the welding process. If the steel plate has a high C or O content, a welding wire with a higher concentration of deoxidizers—such as Mn and Si—should be used. Furthermore, all oil, rust, scale, and moisture must be thoroughly removed from the surfaces of the steel plate and the welding wire in the weld zone. Optimizing welding parameters is also critical: using a lower welding current can reduce gas generation and decrease pressure at the weld pool; increasing the arc voltage expands the arc cavity and thins the slag layer; and reducing the welding speed extends the time the metal remains in a liquid state and the slag remains at low viscosity, which facilitates the escape of gases and helps prevent the formation of depressions (craters).
Maintaining an appropriate welding inclination is also important. Increasing the downhill welding angle promotes the flow of slag, which thins the slag layer and helps reduce shrinkage cavities. In the SAW process for steel pipes, the welding eccentricity and the inclination of the wire can be adjusted to achieve an optimal welding angle, thereby minimizing or preventing the formation of shrinkage cavities in the weld.
By carefully selecting welding materials—particularly the flux—and appropriately adjusting the welding process parameters, it is possible to effectively reduce or prevent the occurrence of surface depressions in SAW steel pipe welds, resulting in welds with both excellent internal quality and a superior surface appearance.