7 Causes of Porosity in the Weld Seam of Steel Structure LSAW

The welding process of the submerged arc welding straight seam steel pipe is the submerged arc welding technology, which uses filler welding and particle protection flux submerged arc. The caliber produced can reach 1500mm. LSAW is the abbreviation of submerged arc welded longitudinal seam steel pipe. The production process of LSAW steel pipe includes JCOE forming technology and coil forming submerged arc welding technology.

When the caliber is large, two steel plates can be used for rolling, which will form a double weld phenomenon.

The pores of the LSAW steel pipe welds not only affect the tightness of the pipeline welds and cause pipeline leakage, but also become corrosion-inducing points, seriously reducing the weld strength and toughness. The factors that cause pores in the weld are:
1. Flux composition. When welding contains a proper amount of CaF2 and SiO2, it will react and absorb a large amount of H2, and generate HF that is highly stable and insoluble in liquid metal, thereby preventing the formation of hydrogen pores.

2. The bulk thickness of the flux is generally 25-45mm. The bulk thickness of the flux is large and the density is small, and the bulk thickness is taken, and vice versa; the bulk thickness of the high current and low welding speed is taken the big value, and vice versa. Or when the air humidity is high, the recycled flux should be dried before use.

3. Surface treatment of steel plate. In order to prevent sundries such as iron oxide scale falling off from uncoiling and leveling from entering the forming process, a surface cleaning device should be installed.

4. Steel plate edge treatment. The edge of the steel plate should be equipped with rust and burr removal devices to reduce the possibility of pores. The position of the cleaning device is well installed after the edge milling machine and the disc cutter. The structure of the device is two active steel wire wheels with adjustable clearance in the upper and lower positions on one side, and the board edge is pressed up and down.

5. Weld shape. The forming coefficient of the weld is too small, the shape of the weld is narrow and deep, the gas and inclusions are not easy to float, and it is easy to form pores and slag inclusions. Generally, the weld forming coefficient is controlled at 1.3-1.5, with thick-wall welded steel pipes taking the larger value, and thin-walling the smaller value.

6. Reduce the secondary magnetic field. In order to reduce the influence of magnetic bias, the connection position of the welding cable on the workpiece should only be far away from the welding terminal to avoid part of the welding cable from generating a secondary magnetic field on the workpiece.

7. Process. The welding speed should be appropriately reduced or the current should be increased to delay the crystallization speed of the weld pool metal to facilitate gas escape. At the same time, if the delivery position of the strip is unstable, it should be adjusted in time to prevent frequent fine-tuning of the front axle or rear axle. The bridge maintains its shape, making it difficult for gas to escape.

Tips:ASTM A53 covers seamless and welded steel pipe with nominal wall thickness. The surface condition is usually black and hot-dipped galvanized. ASTM A 53 is produced mainly for pressure and mechanical applications, and is also used for transport of steam, water, gas line pipes.

ASTM A53 Grade B is the material under the American steel pipe standard, API 5L Gr.B is also the American standard materialFree Web Content, A53 GR.B ERW refers to the electric resistance welded steel pipe of A53 GR.B; API 5L GR.B Welded refers to the material Welded steel pipe of API 5L GR.B.