Reasons for slag inclusion in LSAW steel pipe

Reasons for slag inclusion in LSAW steel pipe

The welding process used in LSAW steel pipe is submerged arc welding technology, using filler welding and particle protected flux submerged arc. The diameter of production can reach 1500mm. LSAW is the English abbreviation of submerged arc welded straight seam steel pipe. The production process of submerged arc welded straight seam steel pipe includes JCOE forming technology and roll forming submerged arc welding technology. When the diameter is large, two steel plates may be used for rolling, which will form a double weld.

LSAW steel pipe welding temperature requirements:

The material is low carbon steel, and the welding temperature is controlled at 1250~1460°C, which can meet the welding penetration requirements of 3~5mm pipe wall thickness. The welding temperature is mainly controlled by adjusting the high-frequency eddy current thermal power and welding speed. When the input heat is insufficient, the edge of the heated weld cannot reach the welding temperature, and the metal structure remains solid, resulting in incomplete fusion or incomplete welding. When the input heat is too large, the edge of the heated weld exceeds the welding temperature, resulting in overburning or molten droplets, causing the weld to form a molten hole.

The production characteristics of LSAW steel pipe can eliminate the cause of slag inclusion in the weld due to unclean interlayer during multi-layer welding. In view of the large number of inclusions in the raw materials causing slag inclusions in the welds, after taking measures such as inspecting the base metal before welding and replacing the welding wire and flux, the proportion of slag inclusions in the weld fusion line was only slightly reduced, indicating that the inclusions in the raw materials Materials are not the main cause of slag inclusions.
Therefore, the main reason for fusion line slag inclusion in thick-walled straight seam submerged arc welded steel pipes is improper selection of welding process parameters. The welding process parameters of thick-walled straight seam submerged arc welded steel pipes mainly include: linear energy, welding current, welding voltage, welding speed, welding wire spacing, groove size, etc.

Slag inclusions are molten slag remaining inside the weld. From a theoretical analysis, the main reasons for slag inclusion in submerged arc automatic welding welds are the following three points:

① There are many inclusions in raw materials (including base metal, welding wire, and flux);
② During multi-layer welding, the layers are not cleaned cleanly;

③The selected welding process parameters are inappropriate, which is not conducive to the emergence of slag.