798 Agricultural Mechanics and Technology Systems
Copyright Goodheart-Willcox Co., Inc.
• Cracks are severe, unacceptable discontinuities
that are sharp at the edges, concentrating points of
stress. See Figure 29-36. The damaging effect of a
crack is greater than that of most other discontinuities
because cracks have a tendency to spread, thus
multiplying their damage. Cracking is often caused
by stress concentration near discontinuities in welds
and base metal and near sharp edges in the design of
the structure. Hydrogen absorption can contribute to
crack formation in some metals. Cracks can be hot or
cold. Hot cracks form at high temperatures during the
solidifi cation of the weld. Cold cracks develop after the
weld cools somewhat as a result of stresses. Cracks in
the heat-affected zone, the area around the weld, most
often occur in base metals that have a high enough
carbon content that they can be hardened, Figure 29-37.
Cracking of the weld material is usually a result of
stresses external to the welded joint and compounded
by poor welding technique. Cracks must be completely
removed by grinding or gouging, and the void must
be fi lled with sound weld metal. Weld cracking is
best avoided by selecting the correct electrodes and
amperage as well as using approved techniques that
take into consideration heat input and the expansion
properties of the base metal.
Visual Inspection
Visual inspection, if performed correctly, is an
easy, inexpensive, convenient, and effective method
of evaluation for many welding applications. Welders
should be able to identify most of the signifi cant
welding discontinuities during visual inspection. They
should also be able to evaluate the severity of those
discontinuities to decide whether to accept or reject the
welding job prior to placing equipment in service.
Destructive Weld Testing
Destructive weld testing involves destruction of
the welded joint in order to establish weld integrity
or performance. Destructive tests include sectioning,
bending, or breaking the welded component in order to evaluate various
mechanical or physical properties of the welded metal. See Figure 29-38.
Examples of these tests are the guided bend test, tensile strength test, fracture
test, and weld separation test. These tests are most often done during welder
performance qualifi cation or certifi cation testing. A welding inspector is often
required to conduct, supervise, and evaluate these tests.
C
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Figure 29-36. Cracks are unacceptable weld
discontinuities. A—A crack formed due to inclusion.
B—Under-bead cracking. C—Weld cracking due
to overwelding, poor technique, and improper
electrode selection has resulted in a complete
failure of this joint over time.
B
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A
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