Page 198 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

Welded Joint Design and Production 183

The requirements for fabrication of aluminum are contained in
AWS D1.2 Structural Welding Code—Aluminum.

3.2 Weld Cracking/Solutions
Weld cracking is a problem faced occasionally by the fabricator. This
section will discuss the various types of cracking and possible solu-
tions for steel alloys.
Several types of discontinuities may occur in welds or heat-affected
zones. Welds may contain porosity, slag inclusions, or cracks. Of the
three, cracks are by far the most detrimental. Whereas there are
acceptable limits for slag inclusions and porosity in welds, cracks are
never acceptable. Cracks in, or in the vicinity of, a weld indicate that
one or more problems exist that must be addressed. A careful analysis
of crack characteristics will make it possible to determine their cause
and take appropriate corrective measures.
For the purposes of this section, “cracking” will be distinguished
from weld failure. Welds may fail due to overload, underdesign, or
fatigue. The cracking discussed here is the result of solidification,
cooling, and the stresses that develop due to weld shrinkage. Weld
cracking occurs close to the time of fabrication. Hot cracks are those
that occur at elevated temperatures and are usually solidification
related. Cold cracks are those that occur after the weld metal has
cooled to room temperature and may be hydrogen related. Neither is
the result of service loads.
Most forms of cracking result from the shrinkage strains that
occur as the weld metal cools. If the contraction is restricted, the
strains will induce residual stresses that cause cracking. There are two
opposing forces: the stresses induced by the shrinkage of the metal
and the surrounding rigidity of the base material. The shrinkage
stresses increase as the volume of shrinking metal increases. Large
weld sizes and deep penetrating welding procedures increase the
shrinkage strains. The stresses induced by these strains will
increase when higher-strength filler metals and base materials are
involved. With a higher yield strength, higher residual stresses will
be presented.
Under conditions of high restraint, extra precautions must be uti-
lized to overcome the cracking tendencies which are described in the
following sections. It is essential to pay careful attention to welding
sequence, preheat and interpass temperatures, postweld heat treat-
ment, joint design, welding procedures, and filler material. The judi-
cious use of peening as an in-process stress relief treatment may be
necessary to fabricate highly restrained members.

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