7 Root Passes

A root pass is a weld pass made to produce a root bead. The root pass should penetrate completely into the root of the joint. The technique for welding the root bead varies slightly with welding position.Some undercutting is common along the toes of the face side of the weld. This undercutting is eliminated by the first intermediate pass (hot pass). However, undercutting along the root side of the weld (internal undercut) is not acceptable. It must be corrected, or the weld will be rejected.

Starting Techniques

There are two techniques for starting a root bead. One technique is to start in the root of the joint and “burn through” the tack welds. The other technique is to start on a tack weld and tie into successive tacks as welding progresses around the pipe.

The problem with burning through tacks, especially on large-diameter pipe, is that the root can close up as the root bead cools and contracts. Considering that tack welds on large-diameter pipe can be as long as 3” or more, burning through tacks also presents issues with productivity.

Stopping and Starting Root Beads

Depending on the diameter of the pipe, it may be necessary to stop several times to change electrodes while depositing the root bead. To extinguish the arc on the root pass, push the electrode through the keyhole. This maintains the proper keyhole size. Clean the weld thoroughly by chipping slag and brushing the weld with a wire brush, then feather the crater.

To restart the weld, strike the arc on the root bead about ½” behind the feathered edge. Move toward the feathered crater with a long arc. Shorten the arc to one electrode diameter at the crater, and hold it in position until the keyhole forms. Then continue welding.

Finishing Root Passes

Once the root bead is deposited, the slag is chipped and the bead is brushed with a wire brush, a hand grinder is used to grind the bead lightly to remove high spots and visible slag before depositing the first intermediate pass (hot pass) with an E7018 electrode.

Other Best Practices

Aside from the normal stops and restarts associated with changing electrodes, it is best to complete the root bead around the entire circumference of the pipe before stopping for an extended period. Root beads deposited with downhill travel are typically concave through the vertical portion of the weld. An additional bead is deposited on the vertical portion of the root bead to produce an even surface.

It is important to maintain a consistent weld pool and a keyhole about 5/32” to 3/16” for the entire root pass to ensure an even weld bead and complete penetration. Therefore, the welder must watch the weld pool and adjust travel speed and electrode angles as well as use different electrode manipulation techniques to maintain the weld pool and keyhole.

If there is poor or bad fit up, the welder must adjust travel speed and electrode angles to compensate. If the root opening is narrow, the travel speed should be reduced to ensure complete penetration. If the root opening is too wide, the travel speed should be increased to prevent excessive penetration. If weld quality is critical, the welder should bring poor joint fit-up to the attention of the supervisor before welding.