21 Welding
Circumferential Seams The
weld shape produced in circumferential welding can be controlled by varying the
position of the welding wire.
Figure 37 shows the effect of this variable - wire offset - on an outside seam
weld. Results range from a
deep, peaked weld to a shallow, flat weld. Depending on the work, any one of the
three results might be desirable
although the center one is most commonly required. The shallow, flat weld on the
right is unsatisfactory
for the joint shown since penetration was insufficient to fuse the bottom of the
Vee and a void exists.
This bead could be satisfactory on another job - a surfacing application, for
example. The effects
of wire movements in welding on an inside seam are the opposite of those illustrated
for an outside
seam in Figure 37. The
double-Vee groove butt joint is widely used in the fabrication of pressure vessels.
It has, however, one limitation
for making circumferential welds. The ratio of plate thickness to cylinder diameter
must be at least 1
to 25. If not, the large pool of molten metal will run and cause unstable welding
action as the cylinder rotates
under the welding head. If
normal welding conditions are used, a maximum recommended welding current exists
for every girth diameter.
This current is the highest that can be used without having a metal run-out. It
is affected by the welding
speed and fluidity of the weld metal and welding composition.
Figure 38 illustrates the relationship
between the welding speed and the outside diameter of the workpiece.
