Page 199 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

184 Chapter Three

3.2.1 Centerline cracking
Centerline cracking is characterized as a separation in the center of a
given weld bead. If the weld bead happens to be in the center of the
joint, as is always the case on a single-pass weld, centerline cracks
will be in the center of the joint. In the case of multiple-pass welds,
where several beads per layer may be applied, a centerline crack may
not be in the geometric center of the joint, although it will always be
in the center of the bead (Fig. 3.1).
Centerline cracking is the result of one of the following three phe-
nomena: segregation-induced cracking, bead shape–induced cracking,
or surface profile–induced cracking. Unfortunately, all three phenom-
ena reveal themselves in the same type of crack, and it is often diffi-
cult to identify the cause. Moreover, experience has shown that often
two or even all three of the phenomena will interact and contribute to
the cracking problem. Understanding the fundamental mechanism of
each of these types of centerline cracks will help in determining the
corrective solutions.
Segregation-induced cracking occurs when low melting-point con-
stituents, such as phosphorus, zinc, copper, and sulfur compounds, in
the admixture separate during the weld solidification process. Low
melting-point components in the molten metal will be forced to the
center of the joint during solidification, since they are the last to
solidify and the weld tends to separate as the solidified metal con-
tracts away from the center region containing the low melting-point
constituents.
When centerline cracking induced by segregation is experienced,
several solutions may be implemented. Since the contaminant usually
comes from the base material, the first consideration is to limit the
amount of contaminant pickup from the base material. This may be
done by limiting the penetration of the welding process. In some
cases, a joint redesign may be desirable. The extra penetration afford-
ed by some of the processes is not necessary and can be reduced. This
can be accomplished by using lower welding currents.
A buttering layer of weld material (Fig. 3.2) deposited by a low-
energy process, such as shielded metal arc welding, may effectively
reduce the amount of pickup of contaminant into the weld admixture.

Figure 3.1 Centerline cracking. (Courtesy of The Lincoln
Electric Company.)

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