Creation of indentation on weld surface of submerged arc welded steel pipe

Creation of indentation on weld surface of submerged arc welded steel pipe

Crater on weld surface is a common weld surface defect in submerged arc welding, which refers to the defect that the weld surface is partially lower than the normal weld surface.

Harm of crater on weld surface

In the quality standard of welding structure, there is no clear limit on the crater on the weld surface. As long as the crater on the weld surface is not lower than the base metal surface, it is generally not considered as a weld defect. The test shows that the mechanical properties of the weld will not be affected by the crater on the weld surface. Besides affecting the surface quality of the weld, the crater will also produce stress concentration at the crater, which has an impact on the corrosion resistance of the weldment, Under the same conditions, the weld pressure pit is more prone to corrosion.

Due to the application of steel pipe anti-corrosion technology and the influence of weld height on anti-corrosion coating thinning, the weld height of submerged arc welded steel pipe should be lower to save anti-corrosion cost. In production, the weld reinforcement should be controlled at ≤ 1 mm on the premise that it is not lower than the base metal surface. If the depth of pressure pit is greater than 1 mm, it will lead to weld defects. For submerged arc welded steel pipe, it is easy to have leakage and other defects at the pressure pit, It affects the anticorrosion quality of steel pipe.

Formation mechanism of weld indentation

The welding current of submerged arc welding is generally large, and the heat loss of arc is small, so the temperature of welding pool is relatively high and concentrated. The molten pool and molten metal react with the surrounding arc. For example, the oxidation of carbon will produce carbon monoxide gas. H, N and other elements dissolved in liquid metal at high temperature will precipitate gas due to the decrease of solubility during the process of molten pool temperature, Under the action of strong convection and electromagnetic stirring in the molten pool, the generated gas is brought to the surface of the molten pool and stays on the interface of liquid metal in the form of bubbles. If the resistance of slag layer cannot be overcome, the gas will stay on the interface of non solidified metal. Under the action of bubble pressure, pressure pits are produced. The size and depth of pressure pits are related to the size and pressure of bubbles between the welds, The larger the pressure pit area is, the larger the bubble pressure is and the deeper the pressure pit is. The shape of the pressure pit is mostly round bottom shape, and some are oval strip shape. When the pressure of the bubble is uniform in the horizontal direction, the round bottom shape pressure pit will be produced. When the pressure is uneven, the elliptical pressure pit will be produced.

It can be seen from the formation process of crater that the more gas is produced in the molten pool, the easier it is to produce crater, and the greater the resistance of slag and flux layer to gas escape, the more difficult it is for the gas to overflow, resulting in crater.