Causes of Slag Inclusion in Welded Steel Pipe

　　Slag inclusion in welded steel pipe is the slag left in the weld. Theoretically, there are three main reasons for slag inclusion in submerged arc welding seam:

　　1. There are many inclusions in raw materials (including base metal, welding wire and flux);

　　2. The interlayer is not cleaned during multi-layer welding;

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

　　According to the production characteristics of straight seam submerged arc welded steel pipe, it can be ruled out that the reason for slag inclusion in the weld seam due to the uncleanness of the interlayer during multi-layer welding.

　　In view of the slag inclusion in the weld caused by the large amount of inclusions in the raw materials, the proportion of slag inclusion in the weld fusion line only decreased slightly after taking measures such as pre welding inspection of the base metal, replacement of new welding wires and fluxes, which indicates that the inclusions in the raw materials are not the main reason for the slag inclusion.

　　Therefore, the main reason for slag inclusion in fusion line of thick wall straight seam submerged arc welded steel pipe is improper welding process parameters. The welding process parameters of thick wall longitudinal submerged arc welded pipe mainly include: line energy, welding current, welding voltage, welding speed, welding wire spacing, groove size, etc.

　　From the further analysis of welding metallurgy, it can be seen that the main reason for slag inclusion in the weld fusion line is that the temperature of the fusion line is too low, so that the liquid slag can not be separated in time; The reason why the temperature of the fusion line is too low is that the peak temperature of heating is too low or the cooling speed is too fast.