The generation of pits on the surface of submerged arc welded steel pipe welds

In the general quality standards for welded structures, there is no clear limit on surface pitting of welds and is not used as a basis for product acceptance. As long as the pits on the welding surface are not lower than the surface of the base metal, they are generally not considered welding defects, but they are considered to affect the appearance of the welding surface. Tests have shown that the weld surface pressure pit itself does not have a significant impact on the mechanical properties of the weld. However, in addition to affecting the surface quality of the weld, the pressure pit will also produce a certain stress concentration at the indentation of the weld, which will have a certain impact on the corrosion resistance of the welded product.

Under the same conditions, corrosion is more likely to occur at the weld pit. In recent years, as customers' requirements for the quality of submerged arc welded steel pipes have been further improved, the pitting on the surface of the weld has had an important impact on the quality and normal production of steel pipes in the following aspects.
At present, many submerged arc welded steel pipes are used for oil and gas transportation. The customer's steel pipe technical conditions require that the weld surface is not allowed to have an indentation greater than 1.5mm. Therefore, when the indentation depth on the weld surface is greater than 1.5mm, it must be processed Welding defects. At the same time, the current submerged arc welded steel pipe standards or customers' technical conditions have strict requirements on the length and number of steel pipe repair welds and the spacing between repair welds. Once the steel pipe appears several places lower than the base material, the welding seam pitting can easily lead to the degradation of the steel pipe or even scrapping it, which will at least increase the maintenance cost of steel pipe production.
Due to the widespread application of 3PE external anti-corrosion technology for steel pipes and the impact of welding height on the thinning of the anti-corrosion layer, current submerged arc welded steel pipes require the welding height to be as low as possible to save welding costs. Embalming costs as much as possible. In actual production, some steel pipe manufacturers control the external welding reinforcement to ≤1mm on the premise that the external weld seam is not lower than the surface of the steel pipe base material. At this time, if the indentation depth on the weld surface is greater than 1mm, the weld will be lower than the base material, resulting in serious weld defects.
For submerged arc welded steel pipes with internal powder coating and external 3PE anti-corrosion, leakage points or 3PE anti-corrosion bubbles and other anti-corrosion defects are likely to occur at the pits on the weld surface, thus affecting the performance of the steel pipe.
Therefore, in today's increasingly stringent requirements for the welding quality of steel pipes, full attention should be paid to the problems of the welding surface.
The formation mechanism of weld indentation
Since the welding current used in automatic submerged arc welding is generally large and the arc heat loss is small, the temperature of the welding pool is high and concentrated, making the molten pool and molten metal have severe physical interactions with the surrounding arc atmosphere. Chemical reactions will occur, such as carbon oxidation of CO gas. At the same time, during the process of lowering the temperature of the molten pool, elements such as H and N dissolved in the liquid metal at high temperatures will release gas due to the reduction in solubility.
Under the action of strong convection in the molten pool and electromagnetic stirring of the molten pool, the generated gas will be brought to the surface of the molten pool. These gases will stay in the form of bubbles at the interface between the liquid metal and the liquid slag. If the slag cannot be overcome, it will remain at the interface between the slag and unsolidified metal as the resistance of the slag layer breaks out of the slag layer. Under the action of bubble pressure, it will produce a shrinkage hole on the weld metal interface, and corresponding slag will also be produced in the A symmetrical hole. The size and depth of the pressure pit formed on the weld are related to the slag and the upcoming The size of the bubbles between solidified welds is related to pressure. The larger the bubble, the larger the indentation area formed on the weld; the greater the bubble pressure, the deeper the indentation pit left on the weld. The shape of the pressure pits is mostly round bottom, and some are oval and elongated. When the force in the horizontal direction of the bubble is uniform in all directions, a round-bottomed indentation pit is produced, while when the force in the horizontal direction is uneven, an elliptical indentation pit is produced.
In short, on the premise of ensuring the inherent quality of the weld, through reasonable selection of welding materials, especially appropriate flux. At the same time, reasonable selection and adjustment of welding process parameters can effectively reduce or prevent the occurrence of pressure pits on the surface of submerged arc welded steel pipe welds, and obtain submerged arc automatic welding welds with excellent intrinsic quality and appearance.