• They conflict with penetration of the final weld
into the weld joint.
• They add to the crown dimension (height).
• They often crack during welding due to the heat
and expansion of the joint.
Expansion of the base metal during welding often
will cause a condition known as mismatch, Figure 6-23.
When mismatch occurs, the weld generally will not
penetrate completely through the joint. Many specifica-
tions limit highly stressed butt joints to a 10 percent
maximum mismatch of the joint thickness.
Whenever possible, butt joints should mate at the
bottom, Figure 6-24. Joints of unequal thickness should
be tapered in the weld area to prevent incomplete or
inadequate fusion. This is shown in Figure 6-25. When
this cannot be done, the heavier piece may be tapered on
the upper part of the joint as well.
Weld shrinkage. Butt welds always shrink across
the joint (transversely) during welding. For this reason, a
shrinkage allowance must be made if the “after welding”
overall dimensions have a small tolerance. Butt welds in
pipe, tubing, and cylinders also shrink on the diameter of
64 Gas Metal Arc Welding Handbook
Grooved Butt Welds
Pipe Welds
Flat 1G
Horizontal 2G
Vertical 3G
Rotated flat rolled 1G
Horizontal 2G
Vertical fixed 5G
Fixed 6G
Overhead 4G
Fillet Welds
Flat 1F
Horizontal 2F
Vertical 3F
Overhead 4F
Figure 6-22. American Welding Society definitions of welding positions.