17 TIG
Welding of Pipe The
chief advantage of TIG welding for the fabrication of piping
systems is that the welds are smooth, fully penetrated,
and free from obstructions or crevices on the inside. Also, TIG welds are stronger
and more resistant
to corrosion than welds made by any other process. The
brief instructions contained here are written for the standard vee joint with
a bevel of 37 degrees on each
side, a nose of 1/16 in., and a spacing of 1/16 in. to 3/32 inch. The operator
should practice welding in the
flat position, or on a rolled joint, until the technique has been acquired.
1. ROOT PASS WELDING
After the joint has been properly tacked
and put in position, the arc is struck on the side and carried down to
the bottom of the joint. Filler metal
is then added until the puddle bridges over the joint. Control of
penetration is the most important factor
in successful pipe welding. Such control can be obtained only by
repeated practice. After the puddle
bridges the joint, the arc is held on the puddle for a moment. Then the
puddle flattens out and becomes wedge
shaped, straight across the front, and with rounded corners at the
rear. When this takes place, the puddle
has fully penetrated the joint. In
general, the same technique is used for welding in the horizontal-fixed position.
However, the puddle must be
controlled more closely to prevent sag when welding in the overhead position.
In the vertical-fixed position,
the puddle is formed on the upper side and kept above the center line of the joint.
This will prevent undercutting
of the top side of the bead and avoid the tendency for the puddle to sag due to
gravity. 2. FILLER
PASS WELDING Weave
beads, produced by weaving the torch across the joint, can be used on carbon and
low-alloy steel in the
rolled or horizontal-fixed position. Stringer beads, laid parallel to the joint,
are used for welding stainless steel
pipe since there is less tendency toward carbide precipitation. Stringer beads
are recommended for welding
all carbon and low alloy pipe in the vertical-fixed position. The advantage of
using weave beads, in the
rolled or horizontal-fixed position, is that it requires less time to complete
the welds.
