Page 203 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

188 Chapter Three

colder base metal temperatures. Higher hardenability levels result
from higher carbon contents and/or alloy levels. For a given steel, the
most effective way to reduce the cooling rate is by raising the temper-
ature of the surrounding steel through preheat. This reduces the tem-
perature gradient, slowing cooling rates and limiting the formation of
sensitive microstructures. Effective preheat is the primary means by
which acceptable heat-affected zone properties are created, although
heat input also has a significant effect on cooling rates in this zone.
The residual stresses of welding can be reduced through thermal
stress relief, although for most structural applications, this is eco-
nomically impractical. For complex structural applications, temporary
shoring and other conditions must be considered as the steel will
have a greatly reduced strength capacity at stress-relieving temper-
atures. For practical applications, heat-affected zone cracking will
be controlled by effective low-hydrogen practice and appropriate
preheats.
When sufficient levels of hydrogen, residual stress, and material
sensitivity occur, hydrogen cracking will occur in the heat-affected
zone. For this cracking to occur, it is necessary for the hydrogen to
migrate into the heat-affected zone, an activity that takes time. For
this reason, the D1.1 code requires a delay of 48 h for the inspection
of welds made on A514 steel, known to be sensitive to hydrogen-
assisted heat-affected zone cracking.
With time, hydrogen diffuses from weld deposits. Sufficient diffu-
sion to avoid cracking normally takes place in a few weeks, although
it may take many months depending on the specific application. The
concentrations of hydrogen near the time of welding are always the
greatest, and if hydrogen-induced cracking is to occur, it will generally
occur within a few days of fabrication. However, it may take longer
for the cracks to grow to a sufficient size to be detected.
Although a function of many variables, general diffusion rates can
be approximated. At 450°F, hydrogen diffuses at the rate of approxi-
mately 1 in/h. At 220°F, hydrogen diffuses the same 1 in in approxi-
mately 48 h. At room temperature, typical diffusible hydrogen rates
are 1 in/2 weeks. If there is a question regarding the level of hydrogen
in a weldment, it is possible to apply a postweld heat treatment com-
monly called postheat. This generally involves the heating of the weld
to a temperature of 400 to 500°F, holding the steel at that tempera-
ture for approximately 1 h for each inch of thickness of material
involved. At that temperature, the hydrogen is likely to be redistrib-
uted through diffusion to preclude further risk of cracking. Some
materials, however, will require significantly longer than 1 h/in. This
operation is not necessary where hydrogen has been properly con-
trolled, and it is not as powerful as preheat in terms of its ability to

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