There are four basic types of weld passes

1.Primary pass or root pass, the first weld welded after spot fix.The root pass supports all subsequent passes and protects the back of the subsequent weld from gas erosion.The size of the root pass must meet the requirements of bearing the shrinkage stress of the root and not cracking of the weld.

2.Second pass, also known as hot pass, is welded to remove slag from the root pass.In addition, the hot pass ensures complete metal fusion between the root pass and the base metal.
There is little weld metal deposited in the hot pass compared to the subsequent fill pass.The weld heat input (or welding current) used for hot pass welding is slightly larger than for fill pass welding.Many welding operators say that "the hot pass burns off the slag" is not correct because the steel tube has already melted before the burning, and the correct welding operation of the hot pass only removes the slag.

3.Fill pass or intermediate pass, all passes after fixing and before cover pass.

4.Cover surface welding pass cover surface welding pass requirements of the weld surface is smooth, and there is a 2.5-5mm residual high, to strengthen the role of the weld.

Each pass shall be started and terminated at a different position to avoid accumulation of weak spots.If the root pass, hot pass, fill pass, and cover pass can be welded correctly in sequence, a weld with sufficient penetration can be obtained so that the joint is of equal strength to the pipe, or even stronger than the pipe itself.Multipass welding is not required for all pipes, for example, where the pipe is subjected to low pressure or the joint strength is not required, it may be dispensed with.The weld bead distribution is shown in figure.