9
LONGITUDINAL CRACKING
Longitudinal or centerline cracking,
of the weld bead is not often encountered in mig welding. However, that
which does occur can be one of two types:
hot cracks and cold cracks. Typical hot cracks are shown in
Figure 10-5. Hot cracks are those that
occur while the weld bead is between the liquidus (melting) and
solidus (solidifying) temperatures.
In this temperature range the weld bead is ”mushy”. Hot cracks usually
result from the use of an incorrect
wire electrode (particularly in aluminum and stainless steel alloys). The
chemistry of the base plate can also
promote this defect (an example would be any high carbon stainless
steel casting). Any combination of the
joint design, welding conditions and welding techniques that results in
a weld bead with an excessively concave
surface can promote cracking. One
form of this defect which may often be encountered, particularly with any 5000
series aluminum, is called
a crater crack. These are small cracks which appear at the end of the weld where
the arc has been broken.
Although small, these cracks are troublesome since they can propagate into the
weld bead. A crater crack
is shown in Figure 10-6. The major reason for this defect is the incorrect technique
for ending the weld.
To properly end a weld, the crater should be filled. This is done by reversing
the arc travel direction before
breaking the arc. This technique is depicted in Figure 10-7. In addition, if the
welding control is designed
to supply gas for a short time after the arc is broken, the crater should be shielded
until it is completely
solidified.
Figure
10-5 - Example of Longitudinal
Cracking
Figure
10-6 - Example of Crater Cracking
